Prolonged aftersensations to thermal noxious stimulation as a screening tool for central contributions to chronic ocular pain

Abstract

Painful aftersensations (AS) are sensations that persist after termination of a noxious stimulus, and prolonged AS may reflect abnormal somatosensory processing. We aimed to validate AS as indicators of central nervous system processing abnormalities in individuals with chronic ocular pain (COP). We examined two cohorts (Groups 1; n=278 and 2; n=64) who underwent an ocular surface examination, quantitative sensory testing (QST), and pain symptom assessment. Individuals were asked to rate pain intensity and unpleasantness (0–100) of AS at 15 s and 30 s after termination of a repeated stimulus protocol at two sites (forehead and forearm). Ratings >0 were considered positive for AS. Using Group 1 data, exploratory regression analyses determined which AS metrics most strongly related to central neuropathic-like or nociplastic ocular pain (NOP). The presence of unpleasant AS at 15 and 30 s after hot pain on the forearm accounted for the most variability in indicators of NOP (i.e., scores on the Neuropathic Pain Symptom Index modified for the Eye (NPSI-Eye) and response to anesthetic eyedrops). We assessed the robustness of these AS metrics as indicators of NOP using data from a separate cohort (Group 2), by comparing NPSI-Eye scores, response to anesthetic eyedrops, and ocular signs (Schirmer’s scores, tear film, and corneal fluorescein staining) between those with AS and those who did not report AS. The presence of AS was related to greater NPSI-Eye scores and persistence of eye pain after local anesthetic, but not to tear dysfunction signs, suggesting AS can differentiate patients with potential central mechanisms of COP.

Perspective:

The presence of unpleasant aftersensations at 15 and 30 s after removal of a noxious hot stimulus was associated with other indicators of central somatosensory abnormalities in two disparate populations. This suggests that AS could be a clinical screening tool for central contributors to ocular pain.

Introduction

Chronic ocular pain (COP) is a frequent presenting complaint to eye clinics that can severely impact daily function and quality of life.¹ Individuals with COP characterize their pain using many descriptors (e.g., aching, burning, sharp, dry, gritty, or itchy), report different temporal patterns (e.g., constant or periodic), and identify various triggers (e.g., light and wind).² COP has numerous causes and thus a thorough evaluation is needed to determine pain contributors in individual patients. Ocular surface disturbance and tear abnormalities are peripheral causes of COP and thus a slit lamp examination, fluorescein staining, Schirmer strips, and InflammaDry (Quidel, San Diego) tests are needed to identify nociceptive sources of pain including anatomic abnormalities, aqueous tear deficiency, tear instability, and local inflammation.^3,4 However, in a large subgroup of patients, there is a disconnect between pain symptoms and ocular surface signs of disease, indicating that other mechanisms may be driving symptoms, including neuropathic and nociplastic sources.^5–8

Differentiating between neuropathic or nociplastic pain types is difficult given symptom overlap, a lack of established criteria, and similar treatment approaches for these two entities with respect to ocular pain. In the current manuscript we use the combined term neuropathic/nociplastic ocular pain (NOP) to capture the idea of ocular pain driven by neural dysfunction, with or without the presence of a known lesion or disease.⁹ Few diagnostic tools are available to detect NOP, though previous studies provide preliminary support for certain assessments. For example, the presence of microneuromas in vivo confocal microscopy (IVCM) has been suggested as a diagnostic marker for peripheral neuropathic mechanisms in individuals with COP.¹⁰ On the other hand, self-reported light hypersensitivity^11,12 and persistent ocular pain after anesthetic eye drop placement^13,14 have been suggested as indicators of central pain processing abnormalities in COP. Other tests thought to suggest neuropathic or nociplastic mechanisms include hypersensitivity to airpuff stimuli delivered to the cornea,^15,16 cutaneous allodynia (e.g., pain to light touch) around the eye,¹⁷ abnormal activation patterns on functional magnetic resonance imaging (fMRI),¹⁸ and co-morbidities such as migraine¹⁹ and fibromyalgia.^7,20,21 However, many of these measurement methods are not ideal for use in the clinical arena given the need for specialized equipment or expertise. Thus, there is a need for clinically feasible screening tools that can help identify when central neuropathic or nociplastic mechanisms contribute to COP.

Quantitative sensory testing (QST) may provide such a strategy. QST applies psychophysical principals to measure somatosensory function in experimental conditions²² and has been used to investigate peripheral and central somatosensory abnormalities in conditions such as painful diabetic neuropathy,^23–25 post-spinal cord injury pain,^26,27 and migraine.²⁸ We have previously reported associations between certain QST metrics (e.g., pain thresholds, temporal summation (TS) of pain) and quality and severity of COP.²⁹ For example, in a cohort of 118 veterans, the severity of NOP symptoms, quantified with the Neuropathic Pain Symptom Inventory modified for the Eye (NPSI-Eye²¹), correlated with TS to a noxious hot stimulus (r = 0.26, p = 0.004) and with intensity ratings of painful aftersensations (AS; lingering painful or unpleasant sensations after the termination of a noxious stimulus) (r = 0.28, p = 0.002) on the forearm.²⁹ Further supporting our findings, in a separate study of 235 veterans, self-reported severity of photoallodynia (i.e., heightened painful or unpleasant sensations evoked by visual light stimuli) at ≥2 (on 0–10 numerical rating scale, NRS), was related to the presence of 30-second AS from repeated thermal stimulation at the forearm, with 85% sensitivity and 48% specificity.¹²

Based on our previous analyses,^12,29 the presence of AS is of potential interest as a diagnostic tool for detection of central contributors to COP. Adding to our findings, evidence from studies of conditions such as fibromyalgia,^30,31 temporomandibular disorders,³² and post-surgical pain^33,34 have similarly reported links between AS and these chronic pain syndromes. In this study we examine data from two cohorts^35,36 to determine which of several AS metrics may be best suited as a diagnostic indicator of central mechanisms contributing to COP.

Materials and methods

Data from two study samples were used in the analyses. Data from the first study sample were used to examine which of a number of AS metrics was most associated with NOP symptoms. Data from a second study cohort were used to evaluate the repeatability and generalizability of the selected AS metrics derived from the first study sample. Both studies were approved by the Miami VA Institutional Review Board (study 1: IRB #3011.02 and #3011.04; study 2: IRB #3011.07), and were performed in compliance with the principles of the Declaration of Helsinki.³⁷ Written informed consent was obtained from all subjects and financial compensation was provided for participation.

Populations and procedures

Group 1: Exploratory sample

Study population.

In the first study, a convenience sample of 278 subjects with normal external eye anatomy were enrolled from the Miami VA eye clinic between October 2013 to October 2017. Individuals with a range of ocular pain symptoms (ranging from none to severe) were included. We excluded individuals with ocular pain in the setting of an acute ocular process (e.g., infection), recent ocular surgery, medical or surgically treated glaucoma, contact lenses wear, or due to a systemic condition (e.g., sarcoidosis, Sjögrens, human immunodeficiency virus, graft-vs-host disease). As such, sources of pain in our population, when present, were presumably due to tear film (e.g., reduced tear production, stability) or NOP etiologies.

Participants enrolled in this study completed a single visit with the primary goal of assessing relationships among the presence and severity of ocular symptoms, and corneal and cutaneous evoked sensitivity measures.^2,12,13,29 Secondary analysis of data from these patients was performed in the present study to identify the AS metric most related to chronic NOP symptoms.

Questionnaires.

Participants completed validated questionnaires regarding ocular pain symptoms, including a numerical rating of average ocular pain intensity over the past week (numerical rating scale (NRS); range 0–10),² and neuropathic-specific symptom severity using the NPSI-Eye.²¹ Individuals also completed questionnaires previously validated for dry eye symptoms, including the 5-item Dry Eye Questionnaire (DEQ5, range 0–22)³⁸ and the Ocular Surface Disease Index (OSDI, range 0–100).³⁹ The dry eye questionnaires include both questions related to pain (e.g., intensity and frequency of eye “discomfort” and frequency of “painful or sore eyes”), and several others not directed to capturing pain symptoms (e.g., intensity and frequency ratings of dryness, tearing, visual disturbances). Furthermore, pain symptoms within dry eye questionnaires are thought to capture components of COP,^2,39 but are not geared towards capturing NOP-specific symptoms. Mild or greater dry eye symptoms was defined as DEQ5 ≥ 6 based on prior DEQ-5 cut-off values.^38,40 Self-reported questionnaires for demographics and comorbidities were also completed.

Ocular surface evaluation.

Tear function and ocular surface health were measured using standard techniques as guided by the Tear Film and Ocular Surface Dry Eye WorkShop II (TFOS DEWS II).⁴ This included tear stability measured by tear breakup time (TBUT) – the amount of time until the first observed black area in the tear film after fluorescein placement - and calculated by averaging three readings for each eye. Values ≤ 5 s are considered abnormal, as a more rapid TBUT is indicative of tear film instability. Fluorescein was also used to highlight corneal epithelial cell disruption, graded to the National Eye Institute scale,⁴ with the cornea visually sectioned into 5 areas, each scored on a range of 0–3 and the scores then summed (range 0–15). A higher score indicates more severe epithelial disruption, with a score ≥ 2 considered abnormal. Tear production was measured using anesthetized Schirmer’s strips (1 drop (~10μl) of proparacaine 0.5%) by placing the strip in the temporal corner of the eye for five minutes. Lower scores indicate lower tear production with a score < 5 mm considered abnormal. Finally, meibum quality was graded on a scale of 0–4 (0=clear, 1=cloudy, 2=granular, 3=toothpaste, and 4=no extraction).⁴ Higher values indicated more abnormal quality, and a value ≥ 2 was considered abnormal. Values for these measurements were recorded for both eyes, and the more abnormal value was used for analyses.

Persistent ocular pain after topical anesthetic.

Using a verbal NRS (range 0–10), individuals rated their current eye pain intensity immediately before and 30 s after administration of l drop of proparacaine 0.5% in each eye. Individuals were grouped into three groups: (1) those who reported having current eye pain prior to anesthesia whose pain completely resolved after anesthesia (“complete response”), indicating a peripheral etiology of pain^13,41 (nociceptive or peripheral neuropathic), (2) those with eye pain prior to anesthesia whose pain did not completely resolve after anesthesia (“incomplete response”), indicating a central or non-ocular surface contributor to pain,^13,41 and (3) those who did not have current eye pain prior to anesthetic administration (NRS = 0), indicating an indeterminant mechanism of pain using this paradigm.

Cutaneous quantitative sensory testing.

Two sites were chosen for somatosensory testing, one area near the site of chronic pain (forehead, approximately 1 cm over the right brow) and one remote from the area of pain (right ventral forearm, at the midpoint between the cubital fossa and the wrist). Participants in Group 1 completed a battery of QST procedures, including vibratory and thermal detection thresholds, thermal pain thresholds, hot and cold pain temporal summation (TS) protocols and associated AS measurements, as well as a conditioned pain modulation protocol.^13,29 For the purposes of the present analyses, we only used the data from reports of TS and lingering sensations (i.e., AS) after the termination of the repeated thermal stimuli, using the methods described in detail below.

Temporal summation (TS).

The TS protocol was performed at the forehead followed by the forearm, using a 0.5 Hz train of 10 repeated one-second stimuli manually touched to the skin using the 30 mm × 30 mm contact thermode of the Medoc TSA-II device (Medoc Ltd.,). Separate TS protocols were completed using a noxious hot stimulus, and using a noxious cold stimulus, at each site (forehead and forearm). The temperature of the stimuli used for the TS protocols in Group 1 were determined from an initial group of 20 veterans, using a temperature 1°C above their average hot pain threshold (for the hot pain TS protocol), and 1°C below their average cold pain threshold (for the cold pain TS protocol). The stimuli were 47.1°C (hot) and 1.4 °C (cold) for the forehead, and 45.5°C (hot) and 6.3°C (cold) for the forearm. TS values were obtained by subtracting the rating of the first, singular stimulus from the rating of peak pain during the repetitive stimulus series.

Aftersensations (AS).

Painful AS are lingering feelings of pain and unpleasantness that are experienced after a noxious stimulus has been removed. To measure AS, participants were asked to rate the current intensity of pain and unpleasantness at the test site 15 and 30 s after the TS protocol was completed. For the current analyses, we dichotomized these ratings to indicate the presence (rating > 0) or absence (rating = 0) of AS under each condition. Thus, for Group 1, we examined 16 AS metrics covering two test sites (forehead and forearm), two modalities (hot and cold), two sensory quality types (pain and unpleasantness), and two time points (15 s and 30 s after stimulus termination). If AS ratings were unable to be captured within a window of approximately ±5 s of the 15 s or 30 s timepoints for a particular condition, these data were treated as missing and not included in analyses under that condition.

Group 2: Confirmatory sample

Study population.

In a second study, 64 individuals were recruited into three groups a) those presenting with no ocular pain; b) those with COP and photoallodynia; and c) those with COP without photoallodynia. Participants were recruited from the Miami VA eye clinic and the University of Miami Bascom Palmer Eye Institute between November 2020 and April 2024. The primary aim of this study was to examine differences among the cohorts with regard to blood-oxygenation-level-dependent responses to visual light stimuli using functional MRI of the brain.¹¹ For the present paper, secondary analyses were conducted using a subset of measures from this database, utilizing a targeted analysis to investigate the repeatability/generalizability of results from the exploratory sample/Group 1.

Questionnaires, ocular surface evaluation, persistent ocular pain after topical anesthetic, and cutaneous quantitative sensory testing.

Tests in Group 2 were conducted similarly as in Group 1, with the exceptions of parameters of the TS protocol and AS as detailed below.

Temporal summation (TS).

Similar procedures were used to capture TS as in Group 1, with two exceptions: 1) only the hot TS protocol was used; and 2) the temperature presented at the forehead and the forearm were set as 1°C above hot pain threshold for each individual subject (determined by averaging three trials of threshold using the method-of-limits procedure measured approximately ten minutes before the AS protocol was conducted). All other procedures were identical to those used in Group 1, including the thermode size (30 mm by 30 mm), the subject instructions, the number (10) and duration (1 s) of the stimulus presentations, the test sites used (forehead and forearm), and AS timing (15 s and 30 s after stimulus termination).

Aftersensations (AS).

For this study group, we recorded 8 AS metrics covering two test sites (forehead and forearm), one modality (hot), two sensory quality types (pain and unpleasantness), and two time points (15 s and 30 s after stimulus termination). All other procedures were identical to those used in Group 1.

Statistical analysis

SPSS 29.0.1 (SPSS, Chicago, Illinois, USA) statistical package was used to perform the statistical analyses. Data for all variables of interest were assessed for normality, and statistical analyses were conducted accordingly. All AS measurements were considered primary “predictor” variables of interest, as the presence of AS is thought to indicate prolonged excitation within the central nervous system³⁴ and may be a marker of neuropathic or nociplastic-like mechanisms. Pain intensity and unpleasantness AS ratings were highly positively skewed, with an imbalance of “0” for both 15 s and 30 s AS. Thus, we converted all AS ratings into dichotomous variables (present vs absent) for the purpose of the present analysis.

Descriptive statistics were calculated for demographics, co-morbidities, and ocular signs and symptoms for participants from each of the two study cohorts. T-tests and Chi square tests were used as appropriate.

Exploratory analysis to identify best candidate AS metrics indicating central mechanism of ocular pain (Group 1)

Using data from Group 1 (the “exploratory” cohort), we performed two separate regression models with forward selection. The dependent variables for the regression analyses were total scores on the NPSI-Eye questionnaire (via linear regression) and response to the peripheral anesthetic challenge (via logistic regression - complete response or no pain before vs incomplete response), as these assessments have been associated with NOP severity in previous work.⁴² The independent, or predictor, variables were the 16 dichotomized AS measures obtained in Group 1. The purpose of conducting these two regression analyses was to evaluate which of the AS metrics was most highly related to NOP-associated symptoms (NPSI-Eye) and signs (peripheral anesthetic challenge). Follow-up regression analyses were then conducted to include demographics and comorbidities, to examine whether the relationship between the AS metrics and the outcomes of interest would be substantially altered by the addition of these potential confounding variables. The selected AS metrics were then further examined in our validation cohort (Group 2). An independent statistician used the Bolasso algorithm,⁴³ as well as forward and backward selection using the BIC criterion, in R to address potential issues with multicollinearity among predictors.

Construct validity

Construct validity evaluates whether a measure (i.e., AS) accurately assesses the intended concept (i.e., central NOP) and not other concepts (i.e., nociceptive pain).⁴⁴ We examined convergent validity using data from Group 2 by assessing the relationship between the presence of AS 15 s and 30 s after removal of a hot noxious stimulus from the forearm (as determined via the regression models from the exploratory cohort data) and NPSI-Eye total scores. To evaluate divergent validity, we assessed the relationships between the presence of AS 15 s and 30 s after removal of a hot noxious stimulus from the forearm and outcomes that are indicators of non-specific ocular pain (NRS of eye pain), dry eye-related symptom severity (DEQ5 and OSDI total scores), or nociceptive pain (tear production (Schirmers strips), tear instability (TBUT), and corneal staining). We hypothesized that individuals reporting AS would have higher NPSI-Eye total scores than those without AS. Further, we hypothesized that general ocular pain intensity, dry eye symptom severity, and tear parameters would be less related to AS responses than NOP-specific symptom report.

Concurrent validity

Concurrent validity measures the degree to which results on a new test (AS) are associated with scores on another, established test (anesthetic challenge and TS) that is presumed to reflect the same underlying construct (i.e., central nociceptive system dysfunction). To evaluate concurrent validity, we compared TS ratings and the anesthetic challenge responses based off the presence of AS across the two groups.^13,41 We hypothesized that TS ratings would be greater in those with AS, and that individuals with a “complete response” to anesthetic eye drops would be significantly less likely to report AS than those who had an “incomplete response.”

Expanded details needed for replication of any/all study procedures can be obtained by contacting the corresponding author.

Patient and/or the public were not directly involved in the design or conduct of the studies and analyses reported here.

Results

Participant characteristics

In total, 278 individuals comprised Group 1 (mean [SD] age, 59 [9] years, range 27–87), the majority being male (89%), Black (63%), and non-Hispanic (76%); and 64 people comprised Group 2 (mean [SD] age, 56 [10] years, range 34–77), the majority being male (67%), White (70%), and non-Hispanic (52%). Participant characteristics significantly differed between Groups 1 and 2 across various categories, including age, sex, and ethnicity (Supplementary Table 1). However, most individuals in both groups reported mild or greater dry eye symptoms (Supplementary Table 2). In Group 1, there was a mean DEQ5 score of 11 (SD 5) and Group 2 had the same mean score of 11 (SD 6). In Group 1, 164 (59%) participants had at least one of 16 AS present (painful or unpleasant aftersensations at either test site, thermal modality, or time post-TS). In Group 2, 43 (67%) participants had at least one of 8 AS present (painful or unpleasant aftersensations at either test site or time post-TS).

Regression models

We first ran two forward stepwise regression models, using data from the exploratory group (Group 1), to determine which of 16 AS metrics most closely aligned to indicators of NOP. NPSI-Eye total scores was the outcome/dependent variable for the linear regression model, and response to anesthetic eye drops (complete relief of eye pain vs. incomplete relief or no relief) was the outcome/dependent variable for the forward stepwise logistic regression model. AS presence (+/−) for each of the 16 AS measures were included as independent variables in the forward stepwise regression models (Table 1). The AS metric accounting for the greatest variance in NOP symptom severity (NPSI-Eye total score) was the presence of an unpleasant sensation 30 s after the hot pain TS protocol on the forearm (HotArm30Unpl). The AS metric accounting for the greatest variance in the NOP sign (failure to relieve eye pain with anesthetic eye drops) was the presence of unpleasant sensations at 15 s after the hot pain TS protocol on the forearm (HotArm15Unpl). These two AS metrics were thus examined in subsequent validation analyses in Group 2. Examination of stability of these findings were conducted by running a second set of stepwise regression models with the inclusion of demographics and comorbidities in addition to the AS metrics. Beta values for HotArm30Unpl and HotArm15Unpl remained significant in these models and were only slightly changed in all cases (from 0.233 to 0.218 with demographics, and from 0.233 and 0.267 with comorbidities in the model of NPSI-Eye scores; and from 1.491 to 1.493 with demographics, and from 1.68 to 1.74 with comorbidities in the model of local anesthetic challenge).

Table 1.

Forward stepwise linear and logistic regression models examining the aftersensations (AS) parameters contributing to neuropathic ocular pain (NOP) presentation in Group 1.

Characteristic	Value
Symptoms – NPSI-Eye, total (0–100)
Type of regression	Linear
Predictor	HotArm30Unpl
Unstandardized Coefficient (B)	13.44
Standardized Coefficient	0.23
P-value	0.002*
95% Confidence Interval	(5.15, 21.73)
Adjusted R2	0.05
Signs – Anesthetic Challenge (complete response or no pain before vs. incomplete response)
Type of regression	Logistic
Predictor	HotArm15Unpl
Unstandardized Coefficient (B)	1.49
Standard Error (SE)	0.46
Wald Statistic	10.45
Degrees of Freedom (df)	1
P-value	0.001*
Exp(B)	4.44

Neuropathic Pain Symptom Inventory modified for the Eye (NPSI-E). HotArm30Unpl, aftersensations unpleasantness ratings reported 30 s after removal of a hot noxious stimuli from the forearm; HotArm15Unpl, aftersensations unpleasantness ratings reported 15 s after removal of a hot noxious stimuli from the forearm

^*statistically significant, p<0.05

To address potential issues with multicollinearity among predictors, an independent statistician obtained the same two binary predictor variables for NPSI-Eye total score and response to anesthetic eye drops, respectively, using the Bolasso algorithm⁴³ as well as with forward and backward selection using the BIC criterion in R where the selection was performed after including demographic and comorbidity variables in addition to the 16 dichotomized AS variables. Moreover, the two binary predictor variables (the presence (AS+) or absence (AS−) of unpleasant AS at 15 and 30 s after hot pain TS on the forearm - HotArm15Unpl and HotArm30Unpl) were shown to have statistically significant and clinically relevant differences in the outcomes of interest according to whether they were AS+ or AS− in the confirmatory (Group 2) data, which had no subjects in common with the training data. Thus, the results of our variable selection from the exploratory cohort were generalizable to the exploratory cohort data.

Participant characteristics by AS metrics

Based on our regression analyses, we focused on the presence of unpleasant AS to a hot pain stimulus on the forearm, which was found in 25% of participants at 15 s and in 14% of participants at 30 s in Group 1; and in 52% and 37%, respectively, in Group 2. Demographics, comorbidities, and medication use for the exploratory and confirmatory samples grouped by the presence (AS+) or absence (AS−) of unpleasant AS at 15 s after hot pain TS on the forearm (HotArm15Unpl) are presented in Supplementary Table 3, and for AS+ and AS− of unpleasant AS at 30 s after hot pain TS on the forearm (HotArm30Unpl) are presented in Supplementary Table 4. Demographics and co-morbidities were similar between the AS+ and AS− groups with the exception of age and smoking status in Group 1 (for HotArm15Unpl, Supplementary Table 3), and ethnicity in Group 2 (HotArm15Unpl and HotArm30Unpl, Supplementary Table 4).

Confirmatory analyses

Questionnaire data assessing ocular symptoms, measures of DE signs, and response to corneal peripheral nerve block are grouped by AS status in Tables 2 and 3 for both Groups 1 and 2.

Table 2.

Ocular symptoms and signs in the study population grouped by presence versus absence of unpleasant aftersensations 15 s after removal of a hot noxious stimulus from the forearm (HotArm15Unpl).

	Group 1				Group 2
	AS+ (n=70)	AS− (n=202)	p-value	Cohen’s d (95% CI)	AS+ (n=33)	AS− (n=31)	p-value	Cohen’s d (95% CI)
Ocular symptoms mean±SD
Worst eye pain past week, NRS, range 0–10	4.7±3.3	4.1±3.0	0.17	−0.19 (−0.46, 0.08)	5.4±3.2	3.1±3.5	0.01*	−0.66 (−1.16, −0.42)
Average eye pain past week, NRS, range 0–10	3.9±2.7	3.1±2.5	0.04*	−0.29 (−0.57, −0.02)	4.6±3.0	1.9±2.5	<0.001*	−0.95 (−1.47, −0.43)
NPSI-Eye, range 0–100	33.7±22.4	25.4±19.0	0.003*	−0.42 (−0.69, −0.14)	29.8±22.1	13.2±19.4	0.002*	−0.80 (−1.4, −0.34)
DEQ5, range 0–22	12.4±5.0	10.8±5.2	0.02*	−0.32 (−0.59, −0.05)	13.7±5.3	9.0±5.5	<0.001*	−0.87 (−1.38, −0.35)
OSDI, range 0–100	43.8 ±24.1	34.2 ± 24.4	0.005*	−0.40 (−0.61, −0.05)	53.1±26.6	28.1±24.8	<0.001*	−0.97 (−1.49, −0.45)
Ocular signs† mean±SD
Tear break up time (seconds), range 0–30	9.4±5.3	9.9±5.2	0.57	0.08 (−0.19, 0.35)	6.2±3.7	8.4±4.1	0.03*	0.58 (0.07, 1.09)
Corneal staining, range 0–15	1.8±2.7	2.0±2.3	0.67	0.06 (−0.21, 0.33)	3.5±3.4	1.8±3.7	0.07	−0.47 (−0.97, 0.04)
Schirmer (mm), range 0–35	14.9±8.7	12.7±7.7	0.051	−0.27 (−0.55, 0.001)	11.7±8.4	12.2±9.5	0.82	0.06 (−0.44, 0.55)
Meibum quality, range 0–4	1.8±1.3	1.9±1.2	0.44	0.11 (−0.17, 0.38)	1.1±1.5	0.7±1.3	0.30	−0.27 (−0.77, 0.24)
Markers of central contribution to eye pain
TS (hot pain at forearm)	20.8±30.8	8.4±17.7	<0.001*	−0.57 (−0.84, −0.29)	25.2±24.2	9.8±24.2	0.007*	−0.64 (−1.14, −0.13)
Topical anesthetic challenge n (%)			0.002*				0.002*
Complete response	10 (5.7%)	13 (6.4%)			4 (12.1%)	0
Incomplete response	15 (21.4%)	13 (6.4%)			20 (60.6%)	10 (34.5%)
No pain before	51 (72.9%)	176 (87.1%)			9 (27.3%)	19 (65.5%)

Confidence interval (CI), Standard deviation (SD), continuous positive airway pressure (CPAP), number in group (n), 5 Item Dry Eye Questionnaire (DEQ5), Ocular Surface Disease Index (OSDI), Numerical Rating Scale (NRS), Neuropathic Pain Symptom Inventory modified for the Eye (NPSI-Eye), Meibomian glands (MG).

^*statistically significant p<0.05

^†More abnormal value used from right or left eye in analysis.

Table 3.

Ocular symptoms and signs in the study population grouped by presence versus absence of unpleasant aftersensations 30 s after removal of a hot noxious stimuli from the forearm (HotArm30Unpl).

	Group 1				Group 2
	AS+ (n=39)	AS− (n=221)	p-value	Cohen’s d (95% CI)	AS+ (n=23)	AS− (n=40)	p-value	Cohen’s d (95% CI)
Ocular symptoms mean±SD
Worst eye pain past week, NRS, range 0–10	5.0±3.3	4.1±3.1	0.13	−0.27 (−0.61, 0.08)	5.8±3.1	3.4±3.7	0.01*	−0.69 (−1.22, −0.16)
Average eye pain past week, NRS, range 0–10	4.3±2.8	3.2±2.6	0.02*	−0.42 (−0.76, −0.08)	4.9±2.8	2.2±2.6	<0.001*	−1.02 (−1.56, −0.47)
NPSI-Eye, range 0–100	38.0±24.1	25.9±19.2	<0.001*	−0.60 (−0.95, −0.26)	32.5±20.6	14.2±18.9	<0.001*	−0.94 (−1.47, −0.40)
DEQ5, range 0–22	13.2±5.3	10.9±5.1	0.01*	−0.45 (−0.80, −0.11)	14.3±5.6	9.7±5.4	0.002*	−0.85 (−1.37, −0.30)
OSDI, range 0–100	46.8±26.4	35.2±24.2	0.007*	−0.47 (−0.82, −0.13)	52.4±26.2	32.6±26.5	0.006*	−0.75 (−1.28, −0.21)
Ocular signs† mean±SD
Tear break up time (seconds), range 0–30	8.5±4.7	9.8±5.3	0.16	0.24 (−0.10, 0.58)	6.7±4.2	7.6±4.0	0.39	0.23 (−0.29, 0.75)
Corneal staining, range 0–15	1.9±2.3	1.9±2.3	0.87	0.03 (−0.31, 0.37)	3.6±3.8	2.0±3.4	0.10	−0.44 (−0.96, 0.08)
Schirmer (mm), range 0–35	15.9±9.3	12.8±7.6	0.03*	−0.39 (−0.73, −0.05)	13.3±9.1	11.3±8.8	0.40	−0.22 (−0.74, 0.30)
Meibum quality, range 0–4	1.9±1.3	1.9±1.2	0.94	0.01 (−0.33, 0.36)	1.0±1.4	0.8±1.4	0.66	−0.12 (−0.64, 0.41)
Markers of central contribution to eye pain
TS (hot pain at forearm)	24.1±28.9	9.2±20.4	<0.001*	−0.68 (−1.03, −0.34)	26.8±24.4	12.3±24.7	0.013*	−0.59 (−1.11, −0.07)
Topical anesthetic challenge n (%)			0.10				0.006*
Complete response	2 (5.1%)	11 (5.0%)			4 (17.4%)	0
Incomplete response	8 (20.5%)	20 (9.0%)			13 (56.5%)	16 (40.0%)
No pain before	29 (74.4%)	190 (86.0%)			6 (26.1%)	22 (55.0%)

Standard deviation (SD), continuous positive airway pressure (CPAP), number in group (n), 5 Item Dry Eye Questionnaire (DEQ5), Ocular Surface Disease Index (OSDI), Numerical Rating Scale (NRS), Neuropathic Pain Symptom Inventory modified for the Eye (NPSI-Eye), Meibomian glands (MG).

^*statistically significant p<0.05

^†More abnormal value used from right or left eye in analysis.

Construct validity

To evaluate convergent validity, we assessed relationships between the AS metrics identified in the exploratory analyses (unpleasant AS at 15 s and 30 s after a prolonged noxious hot stimulus on the forearm in Group 1) and the severity of neuropathic ocular pain symptoms (NPSI-Eye scores). As shown in Tables 2 and 3, NPSI-Eye scores were significantly higher in the AS+ group compared to the AS− group, across both the exploratory and confirmatory cohorts. Dry eye symptom scores (DEQ5, OSDI) and global eye pain ratings (NRS) were also significantly higher in the AS+ versus AS− groups. Effect size calculations (Tables 2 and 3) across Groups 1 and 2, and across AS metrics (15 s and 30 s), were highest for NPSI-Eye (mean = −0.69; median = −0.60; range = −0.42 to −0.94) compared to the non-specific ocular symptom reports (means ranging from −0.45 to −0.67; medians ranging from −0.27 to −0.47; total range from −0.19 to −1.02). Relationships between AS and DE signs (TBUT, staining, Schirmer, meibum quality) were even less robust, with AS+ and AS− groups primarily displaying similar levels of DE signs, with the exceptions of TBUT in Group 2 (15 s AS) and Schirmer in Group 1 (30 s AS).

Concurrent validity

To evaluate concurrent validity, we examined degree of temporal summation by the presence or absence of AS. We found that those with AS+ had significantly higher TS scores than those who were AS−. We also examined pain levels after topical anesthetic placed in the eye. We found that a higher frequency of individuals in the AS+ groups (15 s for Groups 1 and 2; 30 s for Group 1) reported an “incomplete response” to proparacaine (Tables 2 and 3).

Discussion

Prolonged sensations of pain or discomfort that persist after the removal of a noxious stimulus are known as aftersensations (AS). In this study, we examined the utility of AS as a potential indicator of central processing abnormalities in those with chronic ocular pain, using data from two cohorts with a wide range of ocular symptoms and signs. Our investigation sought to identify which aspects of AS (pain intensity vs unpleasantness; short duration/15 s vs longer duration/30 s; site near affected area/forehead vs remote area/forearm) would be most associated with other indicators of NOP. We found that the presence of unpleasant AS to a repeated hot stimulus (TS protocol) applied to the forearm at 15 and 30 s after stimulus termination, were the most strongly related markers associated with symptoms and signs of NOP (NPSI-Eye score, “incomplete response” to anesthetic challenge). These AS metrics were generally not related to DE signs (TBUT, staining, and Schirmer’s score), with a few exceptions (TBUT (Group 2 at 15 s) and Schirmer’s (Group 1 and 30 s)). These findings support the premise that AS tested at a site remote from the eye can be a useful indicator of central processing contributors to COP.

Our findings are important as ocular pain may arise from several etiologies, broadly grouped into nociceptive, neuropathic, and nociplastic.⁴² Evidence supporting this statement is that some individuals with ocular pain have tear and ocular surface abnormalities,³ yet others have symptoms of pain that far outweigh these signs of peripheral ocular disease.² Supportive of a neuropathic or nociplastic etiology in the latter group, is that a subgroup of individuals with COP have evidence of peripheral (corneal) and systemic (forearm) sensory dysfunction.^15,29,45 For example, studies utilizing the Belmonte aesthesiometer have found both increased^15,46–48 and decreased^49,50 corneal sensation in a wide range of individuals with DE compared to controls, suggesting heterogeneity of somatosensory phenotypes within the DE population, many of whom have ocular pain symptoms.^2,39 Ocular pain severity has been found to relate to corneal sensitivity, with our group previously noting a negative relationship between report of ocular pain intensity and corneal mechanical detection thresholds (r between −0.17 and −0.20, P ≤ 0.05).¹⁵ These findings suggest that corneal somatosensory dysfunction is linked to ocular pain.

Only a few groups have examined relationships between ocular pain and evoked pain sensitivity outside the region of the eye. One United Kingdom study looked at females (n=689) with DE (defined as being diagnosed by a clinician, use of artificial tears or gel, and/or self-reported presence of DE in the past 3 months).⁴⁵ Those with ocular pain symptoms (OSDI sum score of ≥3 on pain-specific questions, n=118) had decreased heat pain thresholds on the forearm compared to those with no ocular pain symptoms.⁴⁵ Our previous findings in a primarily male veteran population (n=118) supported these results, showing significant correlations between ocular pain symptom severity and²⁹ multiple evoked pain sensitivity metrics on the forearm.²⁹ The findings indicate that heightened pain sensitivity on the forearm, as evidenced by decreased heat pain thresholds in individuals with dry eye pain symptoms, may be linked to broader central pain processing dysfunction. In another study of 235 veterans, self-reported severity of photoallodynia was also related to the presence of 30-second AS from repeated thermal stimulation at the forearm.¹² In the current study, we build on these findings and demonstrate repeatability of this relationship across two cohorts that were recruited with different inclusion criteria. Individuals with NOP were specifically recruited in the later (Group 2) study but not the earlier (Group 1) study, which likely explains the resulting higher proportion of AS+ participants in Group 2 compared to Group 1. Despite differences in demographics, comorbidities, medication use, inclusion criteria, and differences in the specific temperatures used in the hot pain TS protocol which evoked the AS, a consistent pattern was noted with respect to NOP symptoms and signs and AS presence. This pattern of results suggests that unpleasant AS at least 15 s after noxious heat stimulus termination according to our protocol may indicate central pain processing dysfunction in individuals across a continuum of ocular pain symptoms.

Our current findings are supported by prior studies that examined the utility of measuring AS in other chronic pain conditions. Specifically, fibromyalgia (FM), temporomandibular disorders (TMD), and irritable bowel syndromes (IBS) are chronic pain entities that are often co-morbid with ocular pain.^42,51 In FM, a cross-sectional study (n=74) found that clinical pain intensity was significantly correlated with painful AS ratings 30 s after removal of a hot stimulus on the thenar eminence of the palm (r=0.24, p=0.04).⁵² Systemic hypersensitivity has also been noted in IBS with a case (n=40) control (n=20) study reporting higher pain intensity ratings after a 120 s-cold pressor test in those with diagnosed IBS compared to controls (51 vs. 27, p=0.02).⁵³ Further, patients diagnosed with both tension-type headache and TMD (n=13) had a longer AS duration after removal of a hot stimulus on the right arm compared to controls (n=20), p<0.05.⁵⁴ Our results similarly found greater severity of neuropathic/nociplastic features of eye pain, namely higher NPSI-Eye scores and persistent pain after topical anesthesia, in individuals with, versus without, AS at the forearm, suggesting that the presence of AS can identify central somatosensory dysfunction across several pain conditions involving the head and face, including in those with COP.

The pathophysiology underlying the presence of AS may involve several mechanisms. Some studies have linked AS to abnormal activity in the spinal cord dorsal horn and brain.^55,56 In one study, functional magnetic resonance imaging (fMRI) was utilized to examine CNS activation during an AS protocol in individuals with FM (n=14) versus controls (n=15).⁵⁵ Heat pain stimuli were applied at the thenar eminence of the right hand (C6 dermatome) during fMRI sessions. The group with FM had increased AS pain ratings at 15- and 30-seconds after stimulus termination (0–100 NRS) compared to controls (13.86 ± 4.8 vs. 9.17 ± 5.35, p<0.05).⁵⁵ Additionally, greater sustained blood oxygen level dependent (BOLD) responses after removal of the heat stimulus were seen in the ipsilateral dorsal horn region of the C6 segment in FM versus controls, demonstrating a central mechanism of painful AS. A separate FM study⁵⁶ found that brain activity in the insula, anterior middle cingulate cortex, and medial prefrontal cortex (MPFC), key structures of the central pain network,^57–59 was correlated with the severity of 15 s AS ratings (scale 0–100) to a pressure pain stimulus induced by a blood pressure cuff around the left leg.⁵⁶ The study also found reduced deactivation in the medial temporal lobe (MTL) during the AS period in participants with FM (n=53) compared to controls (n=17), suggesting altered cortical processing activity and the importance of these regions in contributing to the phenomena of AS. Specifically, the parahippocampus has reduced gray matter density,^60,61 reduced cerebral bloodflow,⁶² and reduced binding potential to dopamine tracers⁶³ in FM patients. Additionally, anatomical and functional changes in the amygdala have been observed in FM patients.^64–66 Further studies are needed to examine neural correlates of AS as they relate to COP.

Study limitations include the predominantly male population from two institutions (Miami VA and the University of Miami) in a restricted geographic location, the use of specific tests to examine tear and nerve dysfunction, and potential unaccounted confounders (e.g., diet, environmental considerations). Additionally, in both Study 1 and Study 2, the assessment of the presence of AS was immediately following a temporal summation protocol, thus these two measures (AS and TS) were not independent. Use of this protocol was modeled after similar paradigms in the literature,^67,68 but necessarily limits the interpretation of the association of TS and AS responses. However, additional convergent validity of AS as an indicator of central sensitization is supported by the relationship between presence/absence of AS and peripheral anesthetic challenge.

Of note, all individuals who reported AS (pain intensity/unpleasantness) at 30 s also reported corresponding AS at the 15 s time point. We collected responses at both time points and analyzed them separately in order to investigate whether the presence of 15 s AS would be sufficient to differentiate between individuals with clinically-relevant central nociceptive dysfunction, or if a 30 sec duration would be required. Our findings, though generalizable across two study cohorts with a difference in stimulus intensity of the evoking stimulus, are necessarily limited by our specific AS protocol parameters.

Despite these limitations, our data suggest that unpleasant AS recorded 15 or 30 s after a hot pain TS protocol at a site remote from the eye may be a useful marker to identify when central processing abnormalities are contributing to symptoms of COP. This is important because, while we have several tests that assist with the identification of nociceptive contributors to ocular pain (e.g., tear film parameters, ocular surface epithelial disruption), we have fewer tests to detect central somatosensory processing abnormalities.

There is a need for an expanded toolbox of diagnostic tests, as the treatment of nociceptive and neuropathic/nociplastic pain are different. Nociceptive pain can be addressed by targeting observed abnormalities. However, in people who exhibit features indicative of NOP, such as increased presence of AS at sites remote from the eye, persistent eye pain after topical anesthesia, visual light hypersensitivity, and cutaneous allodynia, it is important to consider targeting these central abnormalities for NOP management. Approaches can include oral neuromodulators, trigeminal neurostimulation techniques, and cognitive behavioral therapies.⁷ Several studies described in our review⁶⁹ have noted that oral neuromodulators, transcutaneous electrical stimulation, and botulinum toxin injections can all provide some benefit in individuals with presumed NOP. While the field is still in its infancy, these findings suggest that targeted treatment of central mechanisms in the appropriate individual can reduce ocular pain and enhance quality of life. Future studies are thus needed to further evaluate AS as a screening tool for central NOP, including its prognostic accuracy for identifying those most likely to respond to strategies that target central neural function, with the hope that this approach will ultimately lead to the development of an in-office clinical tool and precision-based, tailored algorithms that improve quality of life.

Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jpain.2025.105561.

Data availability

The raw data supporting the conclusions of this article will be made available by the authors upon request.