Abstract
Predictive validity of each word from the McGill Pain Questionnaire (MPQ) has not been investigated independent of pain etiology. The purpose of this study was to explore differences in the words used to describe nociceptive and neuropathic pain. Patients with lung cancer (N = 123) selected words from the 78 MPQ pain quality descriptors and indicated the corresponding pain site for each word. Using only the MPQ pain location and the disease and treatment data abstracted from medical records, each pain site was classified as nociceptive, neuropathic (etiology). Pain etiology and quality descriptors were tested for proportional differences. Of the 457 pain sites, 343 were classified as nociceptive (75%), 114 as neuropathic (25%). Lacerating, stinging, terrifying and suffocating were selected for a significantly larger proportion of nociceptive sites whereas throbbing, aching, numb, tender, punishing, pulling, tugging, pricking, punishing, miserable, and nagging were selected for a larger proportion of neuropathic sites. Interestingly, several pain quality descriptors (burning, shooting, flashing, tingling, itching, and cold) previously associated with neuropathic pain did not distinguish between neuropathic and nociceptive pain etiologies in this lung cancer sample. Infrequent selection of most MPQ words and lack of neurological exam data in the categorizing scheme are possible explanation for inconsistency with previous literature. Prospective investigations are needed to validate pain quality descriptors for nociceptive and neuropathic types of pain.
Keywords: Classification of Pain, Lung Neoplasms, Pain Quality Descriptors, Human
INTRODUCTION
Empirical data from subjects with diseases known for somatic, visceral or nerve damage indicate that clusters of words discriminate among nociceptive and neuropathic pain syndromes. Predictive validity of each distinctive word, however, has not been investigated independent of pain etiology. Despite this limitation, clinical wisdom and recent practice guidelines suggest that verbal quality descriptors can distinguish between nociceptive and neuropathic pain syndromes and therefore should be used to guide pain therapy (1). The purpose of this study was to explore the discriminate power of verbal pain quality descriptors. The specific aim was to compare the proportion of nociceptive and neuropathic pain sites that were described by each of the 78 pain quality descriptors from the widely used, valid and reliable McGill Pain Questionnaire (2). The resulting lists of words would be indicative of nociceptive or neuropathic pain, which would greatly facilitate diagnosis and treatment of pain in general practice.
BACKGROUND
Pain experts and researchers suggested that pain due to peripheral or central neural damage or aberration (neuropathic pain) may differ in clinical features and responsiveness to pharmacological therapy, from pain caused by activation of primary afferents in somatic or visceral tissues (nociceptive pain) (1, 3–10). Research on the analgesic effects of opioids indicated that neuropathic pain may be resistant (10) or less responsive (4, 9) to opioid therapy, suggesting the necessity of using alternative analgesics or larger opioid doses to achieve pain relief in patients with neuropathic pain. Studies on analgesic effects of adjuvants supported the use of tricyclic antidepressants (11–16), anticonvulsants (17), and local anesthetics (5, 9, 18) in managing neuropathic pain conditions. Thus recognition of the underlying pathophysiology of pain is essential for clinicians to make appropriate therapy decisions. Any measures that aid in differentiating neuropathic pain from nociceptive pain would facilitate effective clinical management of pain.
Substantial evidence supports the diagnostic value of pain descriptors to distinguish among varying pain syndromes (19–23). Quantification of word descriptors in the McGill Pain Questionnaire (MPQ) (2) or its variants may provide added clues for differentiating subtypes of headache (21, 22), subgroups of toothache (24), neuropathic and non-neuropathic pain (19, 25), and groups of medical diagnoses (20). Discriminant analysis of word descriptor scores (19, 20, 24, 25) or clusters of words (22, 23) demonstrated that 71% to 91% of the patients would be correctly distinguished among pain syndromes, with 66% to 90% accuracy in predicting the diagnoses. These findings suggest that word descriptors can be cogent tools to differentiate between nociceptive and neuropathic pains. Application of these analytic methods, however, may be cumbersome in clinical setting. A greater potential of using word descriptors in clinical practice may be achieved by providing a list of distinct pain quality descriptors to dichotomize pain as either nociceptive or neuropathic.
A number of studies indicated that patients with certain pain syndromes frequently select certain words to describe their pain (19–23, 25, 26). For example, patients with cancer pain consistently characterized their pain as shooting, sharp, gnawing, burning, heavy, and constant in several studies (20, 26–28). However, only a few studies provided distinctive words that described exclusively more subjects from one pain syndrome group than from others. Melzack and his colleagues (23) reported that patients with trigeminal neuralgia, a neuropathic pain syndrome, were likely to describe their pain as flashing, terrifying, blinding, torturing, and constant whereas patients diagnosed with atypical facial pain described their pain as vicious, diffuse, and excruciating. Clinical anecdotes also suggested that neuropathic pain may be characterized as burning, tingling, shooting, lancinating, shock-like, pricking, cold, itching, dysesthesia, or paroxysmal (29–33). In contrast, nociceptive somatic pain is often characterized as well localized, sharp, aching, throbbing, squeezing, stabbing, gnawing, or pressure-like and nociceptive visceral pain as diffuse, poorly localized, cramping, gnawing, aching, sharp, tender, throbbing, deep, pressure, colicky, or squeezing (30–33). Predictive validity of each of the words, however, has not been examined adequately.
Only one published study systemically examined the predictive validity of each distinctive word descriptor in differentiating nociceptive and neuropathic pain. Using the French Adjective Questionnaire (QDSA) which consisted of 61 descriptors in 17 subclasses, Boureau et al. (19) examined the differences in the selection of each pain descriptor between patients with neuropathic pain and those with nonneuropathic pain. Patients with neuropathic pain selected significantly more frequent on burning, electric shock, tingling, pricking, itching, and cold; whereas patients with non-neuropathic pain selected significantly more frequent on beating, crushing, heavy, dull, tiring, exhausting, nauseating, fearful, frightful, obsessive, and wretched. The findings are consistent with previous literature for words selected by patients with nociceptive and neuropathic pain etiologies. The researchers, however, did not identify criteria used to categorize subjects as experiencing neuropathic or non-neuropathic pain and it is not clear that the patient's diagnosis was made devoid of the pain quality descriptors verbalized prior to the study.
It is clear that differentiating the type of pain based on its underlying mechanism can facilitate decision making about pain therapy, but the challenge exists on how to do so. Previous studies suggest that word descriptors may be cogent tools to aid the differential diagnosis. Predictive validity of each distinctive word, however, needs further exploration. Although numerous pain researchers and clinicians have categorized pain in cancer patients as nociceptive or neuropathic etiology (4, 5, 7, 9, 19, 34–40), the criteria they used to diagnose the pain were usually based on disease group and may not have been made devoid of patient-verbalized pain quality descriptors. The purpose of our study was to explore the discriminate power of verbal quality descriptors in a sample of 123 patients with lung cancer pain.
METHODS
Subjects
Secondary analysis was conducted on a pre-existing data set consisting of 149 patients with lung cancer from three Western states. These patients were recruited for previous studies on lung cancer pain between 1988 and 1996 (41–43). Inclusion criteria required subjects to 1) be English speaking adults diagnosed with lung cancer; 2) have had at least one episode of pain within one week prior to data collection; and 3) be physically able to ambulate for two minutes on their own (42).
Of the 149 patients, 26 patients were excluded from the analysis due to either missing tumor location data (n = 23) or report of no pain on the data collection day (n = 6). The final sample (N = 123) consisted of 80 men and 43 women who were primarily white (81%); married (56%); and had 8 to 12 years of formal education (57%). Their ages ranged from 39 to 85 years (M = 62 years, SD = 10), and 82% of them were diagnosed with lung cancer within a year (Mdn = 3 months, M = 10, SD = 21; min = 0.5, max = 168). About 7% of the lung cancers were stage I, 7% stage II, 35% stage III, and 50% stage IV. Histologically, 10% of the tumors were small cell (oat) and 90% were non-small cell. Metastatic tumor location was local or regional in 41% of the subjects, distant in 45% and non-metastatic in 15%. Most of the subjects (89%) received radiation therapy, 34% had surgery, and 45% had chemotherapy as single or multimodality treatments for the lung cancer.
Instruments
Demographic Data Form (DDF)
The demographic data form (DDF) documented oncologic variables and demographics through review of medical records and patient self-report. All available objective evidence of cancer lesions and antineoplastic therapies for the primary or metastatic lung cancer were recorded in summary format.
McGill Pain Questionnaire (MPQ)
The McGill Pain Questionnaire (2), a validated pain assessment tool (26, 27) with documented construct validity (44) and test-retest reliability (42), elicited information regarding pain location, depth, quality, pattern, and intensity as marked by the patient. The 1970 version of the MPQ measured pain location with an anterior/posterior body outline on which the patient marked the location and the depth of each pain site as either internal, external, or both. Pain quality was measured by a 78-word list that represented sensory, affective, evaluative, and miscellaneous aspects of pain. Pain pattern was measured by a nine-descriptor list and pain intensity by a six-word categorical scale ranging from none to excruciating. We followed the published guidelines in the initial administration of the MPQ (2), but the subjects were not given definitions of the pain quality descriptors. After completing their selection, patients were asked to indicate which pain site marked on the body outline was described by each of the 78 pain quality descriptors.
Lung Cancer Etiology Tool (LCET)
The Lung Cancer Etiology Tool was developed specifically for this study. It consisted of a data tabulation form and a list of 11 criteria defining nociceptive pain sites and 14 criteria defining neuropathic pain sites (Table 1). By definition, the pain sites were considered nociceptive unless at least one neuropathic criterion applied. If nociceptive and neuropathic criteria both applied, the site was categorized as mixed nociceptive and neuropathic. A consensus method was used to assign each of the mixed sites as either a nociceptive or neuropathic type of pain. The criteria for categorizing pain etiology were based on an extensive literature review and focused on common lung cancer pain syndromes and the location and distribution indicators of nociceptive and neuropathic types of pain (45–49). Two of the authors were trained and independently scored each pain site as nociceptive, neuropathic or mixed and indicated their sense of confidence in categorizing each pain site (e.g., rated as either strong confidence or some confidence) based on the presence or absence of supporting objective evidence (e.g., pain location and tumor involvement, cancer treatment to area, or pre-existing condition totally consistent with neuropathic criteria was rated as neuropathic, strong confidence). A random sample representing 20% of the pain sites was scored by both raters. The average inter-rater reliability was 86% agreement for categorization of pain etiology and 70% agreement on degree of confidence in that categorization.
Table 1.
Criteria for Defining Pain Etiology as Nociceptive, Neuropathic or Mixed Nociceptive and Neuropathic
| Nociceptive Pain Etiology |
| Definition- Activation of primary afferent nociceptors located in the peripheral nervous system in somatic (skin, muscle, or bone) or visceral (body organs) tissue. |
|
|
| Criteria for Nociceptive Pain |
|
|
| -pain that is not transmitted, transferred, or trajected (54). |
|
|
| -pain that is confined to site of origin without radiation as evidenced by a pain area with clearly defined margins or an area that is marked as an isolated point (54). |
|
|
| -evidence of hypertrophic osteoarthropathy consistent with patient reported pain in shoulders, wrists, elbows, knees or ankles (55). |
|
|
| -pain externally located that is consistent with radiation port location (55). |
|
|
| -pain that is referred from visceral tissue to a body area consistent with spinal innervation of visceral primary afferent and cutaneous afferent in put convergence (54). |
|
|
| -demographic data form indicates tumor involvement of lung pleura consistent with report of pain located in the chest wall anterolaterally or posteriorly overlying the process, localized pain in the shoulder or ridge of the trapezius, or pain in upper 2/3 of the abdomen and upper lumbar region (54) (This criteria may be a source of error due to its similarity to neuropathic criteria 1. Therefore it is suggested that this criteria be excluded in future research). |
|
|
| -localized area of pain consistent with radiographic evidence of bony metastases (ribs for example) or soft tissue involvement on the demographic data form (54). |
|
|
| -intermittent cramping or colicky abdominal pain in periumbilical region or right hypogastric area (54). |
|
|
| -bilateral or unilateral head pain involving the entire hemicranium (54). |
|
|
| -demographic data form indicates recent withdrawal of corticosteroids consistent with patient report of pain in joints and/or muscles (45, 47, 54). |
|
|
| -demographic data form indicates long term steroid use (> 6 weeks) consistent with patient report of unilateral or bilateral pain in the leg, shoulder, or knee (45, 47, 54). |
|
|
| Neuropathic Pain Etiology |
| Definition- Pain initiated or caused by a primary lesion, or transitory pertubation in the peripheral or central nervous system |
|
|
| Criteria for Neuropathic Pain |
|
|
| -demographic data form indicate the presence of tumor in the apex of the lung consistent with patient report of trajectory on the cervical 5, 6, 7, and 8 thoracic 1 spinal dermatomes (45–47, 54, 55). |
|
|
| -projection or transmission of pain along a course of nerves with either segmental or peripheral distribution (54). |
|
|
| -localized, periodic pain, aggravated by contact, consistent with surgical incision site greater than 1 month post operative (45–47, 54). |
|
|
| -localized pain located midline in the back with or without radiation that increases with lying down, straining , neck flexion, or straight leg raising and decrease with sitting and standing (54). |
|
|
| -segmental or nonsegmental pain predominantly in the anterior or lateral thigh or in the posterior aspect of the leg extending below the knee (45, 46, 54). |
|
|
| -demographic data form indicates the following antineoplastic chemotherapy and patient reports localized pain in hands and feet (vinca alkaloids -especially vincristine, cis-platinum, procarbazine, misonidazole, and hexamethylmelamine) (45, 47, 54). |
|
|
| -multifocal pain located in the legs, aggravated by pressure consistent with evidence of fibromyalgia (54). |
|
|
| -radicular, polyradicular, glove, or stocking distribution of pain consistent with neural damage (4, 54). |
|
|
| -unilateral, radicular pain in cervical and lumbosacral regions (45, 54). |
|
|
| -bilateral, radicular pain in the thorax (45, 47, 54). |
|
|
| -bilateral sacroiliac and iliac crest pain (45). |
|
|
| -midscapular pain or bilateral shoulder pain (45). |
|
|
| -pain located in thoracic dermatome, especially in patients who have a prior history of radiation therapy to the thoracic region, or those who had completed chemotherapy, 1 month prior to completion of the demographic data form and the MPQ (45, 54). |
|
|
| unilateral, intermittent pain located in the distribution of the trigeminal nerve. |
|
|
| Mixed Nociceptive and Neuropathic Pain Etiology |
| Definition- Pain that possesses both nociceptive and neuropathic pain indicators and can not be categorized as nociceptive or neuropathic. |
Procedures
The 123 subjects' DDFs and MPQs were gathered for review of available information about the pain sites and disease information. The MPQ pain quality descriptor page was separated from the existing data set. Therefore, the raters had no knowledge of the specific pain quality descriptors chosen for each pain site. The MPQ pain location and pattern were reviewed in conjunction with DDF tumor location and cancer treatment data. Pain sites were numbered in order of patient report and were considered in relation to other pain sites when determining the etiology category.
CRUNCH4 statistic software was used for data entry and statistical analyses based on each reported pain site, including descriptive statistics on sample demographics, and frequency analysis for each pain quality descriptor. The Fisher's exact statistic was applied to a 2 × 2 contingency table containing etiology classifications (nociceptive or neuropathic) and each of the 78 quality word descriptors from the MPQ (selected or not selected). Although an alpha level of p < .10 would have been appropriate for statistical significance in this exploratory study, p < .05 was set a priori because multiple comparisons were planned.
RESULTS
Univariate Analyses
Pain cause, intensity, and pattern
Subjects reported the cause of their pain was related to cancer (44%), surgery (9%), radiation (8%), non-cancerous causes (9%), combination of causes (18%), or unknown causes (13%). Present pain intensity (PPI) was reported as none (18%), mild (31%), discomforting (33%), distressing (15%), horrible (2%), and excruciating (2%). The majority of patients rated their worst pain intensity as distressing (29%), horrible (23%), or excruciating (31%). Pain pattern was reported as continuous by 54% of the subjects, intermittent by 41%, and brief by 5%.
Pain quality
Among the 78 pain quality descriptors, at least 30% of the subjects described their pain quality sore, aching, tender, tiring, sharp, and annoying (Table 2). No pain site was described as lancinating or drawing. The MPQ pain rating index (PRI) total score ranged from 1 to 53 (M = 19, SD = 12), with an average of 8 words chosen (SD =4, min =1, max = 18) by the subjects. The PRI sensory (M =12, SD = 7, min =1, max =32), PRI affective (M =2, SD =2, min =0, max =12), PRI evaluative (M =2, SD =2, min =0, max =5), and PRI miscellaneous (M =3, SD =3, min =0, max =13) scores are similar to normative scores previously reported for cancer samples (26).
Table 2.
Pain Quality Descriptors and Frequency of Selection by Pain Site Etiology
| Selected | Pain Site Categorized As | Word Cited in Literature asb | ||||
|---|---|---|---|---|---|---|
| Descriptors | By Patient (N = 123) | By Site (N = 457) | NOC (n = 343) | NP (n = 114) | ||
| f (%) | f (%) | f (%) | f (%) | NOC | NP | |
| SENSORY | ||||||
| Aching | 57 (46) | 149 (33) | 90 (26) | 59 (52) *** | (29–33) | (25, 29, 32, 39) |
| Sharp | 50 (41) | 106 (23) | 76 (22) | 30 (26) | (25, 30, 31) | (25) |
| Tender | 39 (32) | 83 (18) | 53 (16) | 30 (26) * | (25, 32) | (29) |
| Sore | 37 (30) | 77 (17) | 59 (17) | 18 (16) | ||
| Throbbing | 35 (28) | 74 (16) | 48 (14) | 26 (23) * | (25, 30, 31, 33) | (25) |
| Shooting | 34 (28) | 84 (19) | 67 (20) | 17 (15) | (25, 29, 39) | |
| Stabbing | 32 (26) | 69 (15) | 55 (16) | 14 (12) | (33) | (39) |
| Hurting | 29 (24) | 78 (17) | 62 (18) | 16 (14) | ||
| Burning | 29 (24) | 47 (10) | 35 (10) | 12 (11) | (19, 25, 29–33, 39) | |
| Dull | 26 (22) | 58 (13) | 48 (14) | 10 (9) | (19, 29) | (32) |
| Itchy | 26 (21) | 50 (11) | 35 (10) | 15 (13) | (19) | |
| Cramping | 25 (20) | 56 (12) | 38 (11) | 18 (16) | (30, 31, 33) | (25) |
| Pressing | 23 (19) | 44 (10) | 35 (10) | 9 (8) | ||
| Pricking | 16 (13) | 33 (7) | 19 (6) | 14 (12) * | (19, 25, 31) | |
| Pulling | 16 (13) | 39 (9) | 23 (7) | 16 (14) * | ||
| Hot | 16 (13) | 34 (7) | 24 (7) | 10 (9) | ||
| Tingling | 16 (13) | 39 (9) | 27 (8) | 12 (11) | (19, 25, 30, 31) | |
| Heavy | 15 (12) | 32 (7) | 29 (9) * | 3 (3) | (19) | |
| Gnawing | 13 (11) | 39 (9) | 27 (8) | 12 (12) | (30–33) | |
| Pinching | 13 (11) | 20 (4) | 13 (4) | 7 (6) | ||
| Wrenching | 13 (11) | 28 (6) | 23 (7) | 5 (4) | ||
| Pulsing | 12 (10) | 28 (6) | 24 (7) | 4 (4) | ||
| Stinging | 12 (10) | 35 (8) | 32 (9) * | 3 (3) | (39) | |
| Pounding | 9 (7) | 21 (5) | 17 (5) | 4 (4) | ||
| Smarting | 9 (7) | 22 (5) | 14 (4) | 8 (7) | ||
| Flashing | 8 (7) | 13 (3) | 7 (2) | 6 (5) | (23) | |
| Tugging | 8 (7) | 9 (2) | 4 (1) | 5 (4) * | ||
| Cutting | 8 (7) | 20 (4) | 15 (4) | 5 (4) | ||
| Quivering | 7 (6) | 17 (4) | 13 (4) | 4 (4) | ||
| Taut | 7 (6) | 13 (3) | 10 (3) | 3 (3) | ||
| Crushing | 7 (6) | 14 (3) | 13 (4) | 1 (1) | ||
| Jumping | 6 (5) | 13 (3) | 10 (3) | 3 (3) | ||
| Flickering | 5 (4) | 18 (4) | 13 (4) | 5 (4) | ||
| Boring | 5 (4) | 8 (2) | 4 (1) | 4 (4) | ||
| Lacerating | 5 (4) | 20 (4) | 20 (6) ** | 0 (0) | ||
| Splitting | 5 (4) | 6 (1) | 4 (1) | 2 (2) | ||
| Rasping | 5 (4) | 5 (1) | 5 (2) | 0 (0) | ||
| Beating | 4 (3) | 10 (2) | 8 (3) | 2 (2) | (19) | |
| Scalding | 3 (2) | 5 (1) | 2 (1) | 3 (3) | ||
| Searing | 2 (2) | 14 (3) | 13 (4) | 1 (1) | ||
| Drilling | 1 (1) | 2 (<1) | 0 (0) | 2 (2) | ||
| Lancinating | 0 (0) | 0 (0) | 0 (0) | 0 (0) | (29–31, 33) | |
| AFFECTIVE | ||||||
| Tiring | 41 (33) | 138 (30) | 102 (30) | 36 (32) | (19) | (25) |
| Exhausting | 33 (27) | 126 (28) | 87 (25) | 39 (34) | (19) | |
| Frightening | 19 (15) | 71 (16) | 59 (17) | 12 (11) | (19) | |
| Fearful | 18 (15) | 50 (11) | 41 (12) | 9 (8) | (19) | |
| Sickening | 16 (13) | 40 (9) | 32 (9) | 8 (7) | ||
| Suffocating | 13 (11) | 41 (9) | 38 (11) ** | 3 (3) | ||
| Grueling | 9 (7) | 27 (6) | 16 (5) | 11 (10) | ||
| Terrifying | 7 (6) | 28 (6) | 26 (8) * | 2 (2) | (23) | |
| Wretched | 6 (5) | 18 (4) | 14 (4) | 4 (4) | (19) | |
| Punishing | 4 (3) | 16 (4) | 8 (2) | 8 (7) * | ||
| Blinding | 4 (3) | 9 (2) | 6 (2) | 3 (3) | (23) | |
| Cruel | 3 (2) | 9 (2) | 5 (2) | 4 (4) | ||
| Vicious | 2 (2) | 4 (1) | 4 (1) | 0 (0) | (23) | |
| Killing | 2 (2) | 1 (<1) | 1 (<1) | 0 (0) | ||
| EVALUATION | ||||||
| Annoying | 57 (46) | 181 (40) | 132 (39) | 49 (43) | (25) | |
| Troublesome | 30 (24) | 107 (24) | 74 (22) | 33 (29) | ||
| Miserable | 30 (24) | 111 (24) | 74 (22) | 37 (33) * | ||
| Intense | 22 (18) | 73 (16) | 54 (16) | 19 (17) | ||
| Unbearable | 12 (10) | 34 (7) | 23 (7) | 11 (10) | ||
| MISCELLANEOUS | ||||||
| Nagging | 32 (26) | 88 (19) | 57 (17) | 31 (27) * | (25) | |
| Nauseating | 29 (24) | 88 (20) | 70 (20) | 18 (16) | (19) | |
| Tight | 28 (23) | 57 (13) | 37 (11) | 20 (18) | (29) | |
| Numb | 19 (15) | 31 (7) | 12 (4) | 19 (17) *** | (29) | |
| Radiating | 16 (13) | 35 (8) | 25 (7) | 10 (9) | ||
| Penetrating | 16 (13) | 26 (6) | 15 (4) | 11 (10) | ||
| Piercing | 13 (11) | 24 (5) | 19 (6) | 5 (4) | ||
| Agonizing | 12 (10) | 37 (8) | 27 (8) | 10 (9) | ||
| Squeezing | 9 (7) | 17 (4) | 15 (4) | 2 (2) | (32, 33) | (32) |
| Cold | 9 (7) | 27 (6) | 20 (6) | 7 (6) | (19, 25) | |
| Spreading | 7 (6) | 14 (3) | 10 (3) | 4 (4) | ||
| Torturing | 6 (5) | 13 (3) | 10 (3) | 3 (3) | (23) | |
| Cool | 6 (5) | 21 (5) | 18 (5) | 3 (3) | ||
| Freezing | 6 (5) | 23 (5) | 19 (6) | 4 (4) | ||
| Dreadful | 5 (4) | 29 (6) | 26 (8) | 3 (3) | ||
| Tearing | 4 (3) | 10 (2) | 7 (2) | 3 (3) | ||
| Drawing | 0 (0) | 0 (0) | 0 (0) | 0 (0) | ||
Key: Fisher's exact<0.05
Freq = Frequency NOC = Nociceptive NP = Neuropathic
Fisher's exact of 2 by 2 analysis with two tailed test
p < .05
p < .01
p < .001
Bolded descriptors were selected by 20% or greater of the pain sites in NC, NP
(Number) indicates reference list number
Pain sites
A total of 457 pain sites was reported by the 123 subjects, with a mean of 4 pain sites per subject (SD = 3). Of these sites, 325 were categorized as nociceptive (71%), 101 as neuropathic (22%). The other 31 (7%) sites were mixed pain sites and for the final analyses were re-classified as nociceptive (n = 18) or neuropathic (n = 13) pain using consensus of two authors. Of the 123 subjects, sixty-two (50%) had pain sites all classified as nociceptive (NOC) and 12 (10%) had pain sites all classified as neuropathic (NP). The other 40% of the subjects had both nociceptive and neuropathic pain sites (NOC/NP).
Bivariate Analyses
Sixteen of the 78 words were chosen for greater than 20% of the pain sites in one of the pain etiology categories (Table 2, Bolded Words). The Fisher's 2×2 analysis of each of the 78 descriptors revealed that 15 words differed statistically by pain type (Table 2). Lacerating, heavy, stinging, suffocating, and terrifying were selected more often for sites categorized as nociceptive whereas throbbing, aching, numb, tender, pulling, tugging, pricking, punishing, miserable, and nagging were selected more often for sites categorized as neuropathic.
A significantly higher proportion of women selected gnawing (21% vs 5%), dull (31% vs 15%), and heavy (23% vs 6%) to describe their pain whereas a significantly higher proportion of men selected shooting (34% vs 16%), sharp (48% vs 28%), and tingling (20% vs 0%) (Fisher's Exact Test, p< .05). There were no statistically significant differences in MPQ pain rating index or PPI scores between men and women or between adult (<65) or older adult (≥65) subjects. There were no statistically significant differences in present pain intensity (PPI), worst pain intensity, and age among patients with NOC, NP, or both NOC/NP types of pain etiology. Patients with NOC/NP pain types had trends for higher mean scores on all MPQ rating indices than patients with NP or NOC pain (Table 3). Pair-wise comparisons revealed that patients with NOC/NP types of pain had significantly higher scores (p < .05).
Table 3.
McGill Pain Questionnaire Pain Rating Index Scores among Subjects with Nociceptive (NOC) Pain Only, Subjects with Neuropathic (NP) Pain Only, and with Both (NOC/NP) Types of Pain.
| Pain Rating Index Score | NOC n = 62 | NP n = 12 | NOC/NP n = 49 | F |
|---|---|---|---|---|
| M (SD) | M (SD) | M (SD) | ||
| PRI-S | 10.5 (6.5) | 12.1 (6.7) | 13.7 (7.6) | 3.0 |
| PRI-A | 1.6 (2.1) | 2.5 (3.3) | 2.6 (2.7) | 2.5 |
| PRI-E | 1.8 (1.7) | 2.1 (1.6) | 2.3 (1.6) | 1.2 |
| PRI-M | 2.6 (2.9) | 2.5 (3.3) | 4.3 (3.4) | 4.4* |
| PRI-T | 16.5 (11.0) | 18.9 (13.7) | 23.0 (12.9) | 3.9* |
| NWC | 6.8 (3.9) | 8.3 (4.9) | 9.1 (4.5) | 3.9* |
p < .05
Underlined scores are statistically different using Bonferroni multiple comparisons (p <.05).
DISCUSSION AND CONCLUSION
In this exploratory study, we found 10 neuropathic pain descriptors and 5 nociceptive pain descriptors that were selected for a statistically larger proportion of lung cancer pain sites classified respectively as neuropathic or nociceptive. Throbbing (25, 39), pricking (25, 29, 31), aching (25, 39), tender (29), numb (29), and nagging (25) are consistent with previous descriptors of neuropathic pain. Tugging, pulling, punishing, and miserable are not typically considered to be neuropathic pain descriptors. Heavy (19, 29) and terrifying (23) are consistent with previous research findings related to nociceptive pain descriptors. Published literature is scant to document lacerating, stinging and suffocating to be nociceptive pain descriptors. Our significant findings and trends provide some clarification about individual MPQ words as descriptors for neuropathic or nociceptive pain. Findings also demonstrate the need for large samples and multivariate analyses in order to provide a definitive list of descriptors, perhaps along with other evidence that will be predictive of neuropathic and nociceptive types of pain.
Interestingly, the quality descriptors most frequently associated with neuropathic pain in the literature (burning, shooting, lancinating, tingling, itching, cold, flashing, blinding, torturing) (4, 20, 27, 39) were not found to be statistically significant in our study. One explanation for the inconsistency with previous research lies in the categorization criteria. All pain sites were assumed to be nociceptive in nature unless there was evidence to the contrary that was consistent with a neuropathic criterion. Therefore, an inflated number of pain sites may have been categorized as nociceptive. In previous studies reviewed, the researchers did not identify specific criteria used to determine etiology other than that of a neurological exam or clinical evaluation. In addition, it was not clear whether the etiology classification was made devoid of any verbal descriptors (19, 20). In one study (20), pain intensity and pain pattern descriptors were used in addition to pain quality descriptors to reliably discriminate between pain syndromes. Our dataset lacked physical exam data and actual scans for detailed review of cancer sites. Therefore, our neuropathic categorization criteria may not have been specific enough to correctly categorize all of the pain sites that actually were neuropathic.
Clearly, further clarification and refinement of the criteria defining nociceptive and neuropathic etiology and study replication are necessary to verify whether or not the MPQ quality descriptors previously associated with neuropathic pain distinguish between neuropathic and nociceptive pain syndromes. Future scrutiny of the refined etiology categorization criteria by pain specialists and neurologists would aid in the specificity and precision of defining pain as nociceptive or neuropathic etiology. In addition, clinical exam data such as the report of allodynia, hyperalgesia, and hyperaethesia may be necessary to improve the diagnostic capability of the etiology categorization tool. Future studies involving the refined criteria in larger sample sizes will be necessary to further determine the validity and specificity of the categorization criteria.
Another explanation for the inconsistency may be due to the lack of statistical power to detect significance. Except for shooting, burning, and itchy, other words were used to describe less than 10% of the pain sites.
Our findings indicated that 50% of the patients with lung cancer had at least one neuropathic pain site and 25% of the overall pain sites were neuropathic. Given the need for well established defining criteria, it is not surprising that limited epidemiological data on the pain etiology in lung cancer population is available in the literature. Compared to studies in other cancer populations, our study had a higher percentage of patients with neuropathic pain but had a similar proportion of neuropathic pain over all pain sites. Previous studies showed that 16% to 34% of the cancer patients (6, 36, 40, 50–52) had neuropathic pain and that 20% to 33% of the pain sites in cancer patients were neuropathic (36, 53). Grond et al. (36) reported that 32% of their patients with respiratory cancer (N = 218) had neuropathic pain and that 20% of their pain sites (n = 438) were neuropathic. For 12% of these patients and 8% of the pain sites, the etiology was unknown. The remainder were reported as non-neuropathic.
The higher prevalence of neuropathic pain in our study may be due to population characteristics or methodologic differences. In our sample, 85% of the patients had advanced disease, whereas 70% was noted by other investigators (36). Lung cancer is often diagnosed in late stage and with loco-regional or distant. The higher prevalence of neuropathic pain may also be due to the fact that we examined each of the pain sites while other studies focused on only main pain syndromes (36). It is clear that patients with lung cancer often have more than one pain site and more than one pain syndrome (53). Furthermore our classification scheme requires additional validation before assuming our findings are representative of the population. Previous investigators provided limited information about how they categorized pain as nociceptive or neuropathic, which also raises questions about generalizability of their findings. We provide detailed criteria and rules for how to classify the pain etiology all of which can be replicated in the future prospective studies in which additional criteria for neurologic exam data should be included.
Comparison of the Present Pain Intensity (PPI) score in the current sample with previous samples revealed that previous samples had on average a higher pain intensity than the current sample (PPI = 1.6, SD = 1.1) (Graham et al.(27) PPI = 2.0, SD = not reported and McGuire (28) PPI = 2.7, SD = not reported). A comparison of the average Pain Rating Index (PRI) scores between this sample and previous research indicates the PRI scores of this lung cancer sample were similar to but lower than scores of previous cancer populations (27). These findings suggest that our sample had slightly less pain than in other studies of cancer patients, which may account for some but not all of the differences noted in our study and previous research.
Although further research is necessary to validate the results and clinical implications of our study, the provide a foundation for categorizing lung cancer pain as nociceptive, neuropathic, or mixed based on pain location, medical record data for primary and metastatic tumor sites, and anti-tumor therapies. Additional refinement of the criteria used to categorize pain as neuropathic or nociceptive will aid clinicians in determining the etiology of pain and appropriate treatments for the numerous pain syndromes experienced by patients with lung cancer.
ACKNOWLEDGEMENTS
This study includes data from samples reported in Wilkie, Keefe, Dodd, and Copp (1992); Wilkie and Keefe (1991); and Wilkie, Williams, Grevstad, and Mekwa (1995). This secondary analysis and data collection for 86 patients were supported by the National Cancer Institute R29CA-62477 and the American Cancer Society Professorship program.
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