The Predictive Value of Pre-plus Disease in Retinopathy of Prematurity

Abstract

Objective

To determine the usefulness of pre-plus disease in predicting the need for laser treatment.

Methods

Posterior retinal video recordings were obtained during 710 indirect ophthalmoscopy examinations of 214 premature infants over a period of 5 years. Two masked experts reviewed short video recordings and determined whether there was plus, pre-plus, or neither. The primary analysis included results of one examination of the right eye at 33–34 weeks postmenstrual age (PMA). The primary outcome was a comparison of the proportion of eyes later requiring laser treatment between the group graded as pre-plus versus those graded as neither plus nor pre-plus disease.

Results

Of 10 eyes with pre-plus at 33–34 weeks PMA, 7 (70%) later required laser treatment; of 154 eyes without pre-plus or plus disease at 33–34 weeks, 14 (9%) later required laser treatment (risk ratio = 7.7; 95% CIs = 4.1, 14.8, p < 0.001). The mean time between the pre-plus examination and laser treatment was 1.6 (1.0–2.4) weeks. When adjusting for birth weight, gestational age, zone and stage, the presence of pre-plus at 33–34 weeks PMA independently predicted the need for laser treatment (adjusted odds ratio = 7.6, 95% CIs = 1.4, 42.3, p = 0.02).

Conclusions

Pre-plus disease observed early in the course of ROP is strongly associated with the development of severe ROP requiring laser treatment. Pre-plus has prognostic value beyond that already provided by birth weight, gestational age, zone and stage. Eyes with pre-plus should be followed closely to allow optimal timing of intervention.

Introduction

Retinopathy of prematurity (ROP) is the second leading cause of severe pediatric visual impairment in the United States. (1) Plus disease is an important marker of severe, potentially sight-threatening ROP, and it is characterized by severely abnormal dilation and tortuosity of central, posterior retinal blood vessels. (2–5) Plus disease is diagnosed when an eye has at least as much dilation and tortuosity of its posterior vessels as the standard photograph first used for the Cryotherapy for ROP clinical trial. (4) Another category of posterior pole vessel abnormality, pre-plus disease, was included in the revision of the International Classification of ROP. (6) Pre-plus disease is defined as vascular abnormalities of the posterior pole that are insufficient for the diagnosis of plus disease, but that demonstrate more arteriolar tortuosity and more venular dilatation than normal. However, there are very little data on the natural history or prognostic significance of pre-plus disease. Our aim was to investigate prospectively whether the presence of pre-plus disease predicts progression to severe ROP requiring laser treatment. We also sought to determine if the presence of pre-plus disease increases the predictive value beyond that which is already provided by the established ROP descriptors of location (zone) and severity (stage).

Subjects and Methods

The study protocol and HIPAA-compliant consent form were approved by the Duke Institutional Review Board (IRB). Parents of participants gave written informed consent for use of their infant's retinal video recordings. Some parents could not be contacted, and our IRB granted permission for use of their infants' data. Consecutive infants weighing less than 1250 grams were prospectively enrolled over a period of 5 years. As part of our routine screening for ROP, we videotaped every examination using the Heine video indirect ophthalmoscopy system and a 28-diopter condensing lens. All examinations were done by one of two authors. (DKW and SFF) Examinations began at 4–6 weeks of age, and follow-up examinations were done at least biweekly based on published guidelines. (7) Our criterion for laser treatment was type 1 ROP, based on the results of indirect ophthalmoscopic examination. (8)

Infants were eligible for inclusion in the study if they were followed at least biweekly until (1) retinal vascularization was complete or in zone III, (2) they had laser treatment, or (3) they were ≥40.5 weeks postmenstrual age. The age of 40.5 weeks was chosen as the minimum time for follow-up because it rounds up to 41 weeks, and by this time approximately 95% of eyes requiring treatment will have been treated. (9) Medical records were reviewed to obtain the following data: date of birth, estimated gestational age at birth, birth weight, race, inborn (at study center) vs. outborn status, and single vs. multiple birth. For each eye examination, the following data were collected as part of routine care: date of exam, zone (location of disease), stage (severity of disease), circumferential extent of highest stage (clock hours), and presence or absence of plus disease.

A research assistant (RA) transferred video clips to a computer using a video capture board. The RA then selected the best digital image frame for each eye. Only one eye of each infant was included in the study to allow use of standard statistical tests without adjustment for intereye correlation. The right eye was included unless images from the left eye were clearly superior. At least 2 months (and in most cases years) after the images were collected, the RA presented batches of images in random order to 2 of the 3 graders (DKW, SFF, and MEH), all of whom have extensive experience examining and treating infants with ROP. Images from the same infant were never presented in sequential order and were randomly mixed with images from other infants. The graders were given no information about the results of the examination, and they independently inspected each image and graded each quadrant for dilation and tortuosity separately using a scale of 0–9. (Table 1) When determining whether there was plus or pre-plus or neither, the masked graders relied on the standard definitions of plus disease (4) and pre-plus disease (6) detailed in the introduction section. In addition, all graders were given color images of the standard photograph of plus disease and examples of pre-plus disease from the International Classification of ROP Revisited manuscript. (6) Using the individual quadrant grades, an overall grade was assigned to each eye. This overall assessment was based on the guideline that for an eye to have plus disease, at least 2 quadrants must have tortuosity sufficient for plus disease and at least 2 quadrants must have dilation sufficient for plus disease (the same rule was used for pre-plus disease). Each image was examined by 2 graders, and if they disagreed, then the 3^rd grader examined the image, and the final grade was based on majority vote. All 3 graders served as one of the 2 primary graders for some images and as the tiebreaking grader for other images. The 2 authors who performed the diagnostic examinations and who also served as graders agreed to report if they recognized any of the images as being taken from a particular infant, but this did not occur.

Table 1.

10-pomt scale used for grading (separately) retinal blood vessel tortuosity and dilation

0	Straighter / thinner than normal
1	Normal, almost straighter / thinner than normal
2	Normal vessel tortuosity / dilation
3	Normal, almost pre-plus tortuosity / dilation
4	Pre-plus, almost normal tortuosity / dilation
5	Pre-plus tortuosity / dilation
6	Pre-plus, almost plus tortuosity / dilation
7	Plus, almost pre-plus tortuosity / dilation
8	Plus tortuosity / dilation
9	Severe plus tortuosity / dilation

Based on date of exam and date of birth, postmenstrual age at each exam was calculated. One exam from each infant was included for analysis in each of the following postmenstrual age groups: 31–32 weeks, 33–34 weeks, 35–36 weeks, 37–38 weeks, 39–40 weeks, and 41–42 weeks. Weeks of age were rounded to the nearest whole number, so that the category of 33–34 weeks, for example, included examinations done between 32 weeks plus 4 days and 34 weeks plus 3 days. If two or more exams for one infant were done during a single time period, then the one examination closest to the midpoint for the category (e.g. 33.5 weeks) was included. The age group of 33–34 weeks was chosen a priori for the primary analysis because, in one of our pilot studies, this age stratum had the highest proportion of eyes with pre-plus disease that later needed laser treatment. (Wallace DK, Ells AL. Natural progression and significance of pre-plus vascular changes in retinopathy of prematurity. Poster presented at: AAPOS Annual Meeting, March, 2003; Waikoloa, Hawaii.) In addition, infants at this age were young enough to expect that few of them would have to be excluded based on already having received laser treatment. For the primary analysis (33–34 weeks) and for each age-specific analysis (e.g. 35–36 weeks), infants were excluded if the need for laser was determined at the same time as the examination, if plus disease was present (as determined by masked graders), or if video recordings had insufficient quality to allow determination of plus or pre-plus disease. For the primary analysis, if the video recording from the examination closest to 33.5 weeks had inadequate quality for grading, then another examination done between 32.5 and 34.5 weeks was used if available.

A secondary analysis determined whether pre-plus that was present at any study examination (i.e. present at one or more exams for a given eye) predicted the need for laser treatment. For this analysis, “study” examinations included only those exams closest to the midpoint for the category (e.g. 33.5 weeks) and usually did not include every inpatient examination for a given infant. If pre-plus was not observed during any study exams, then it was necessary to have at least one exam at 35 weeks PMA or later to be included in this analysis of whether pre-plus was present at any one of an infant's multiple study exams. Those without a study exam after 35 weeks were excluded for this analysis only because they were more likely to have pre-plus that was not observed and to be misclassified as never having pre-plus disease.

Sample Size Calculations and Statistical Analyses

Based on pilot data, we expected that 35% of those classified as pre-plus at 33–34 weeks PMA would eventually require laser treatment, (10) while 6% of those classified as neither plus nor pre-plus would need laser treatment. (4) Using a two-sided Fisher Exact Test, with a significance level of 0.05, we calculated that 150 total subjects for the primary analysis would provide 91% power to find a difference between groups.

Comparisons between proportions were done using Fisher's Exact Test. Logistic regression models were used to determine if pre-plus independently predicted the need for laser treatment when also considering concurrent examination findings of zone and stage. Additional models were constructed that included pre-plus disease, zone, stage, birth weight, and gestational age as predictors of laser treatment. All analyses were done using SAS version 9.1 (Cary, North Carolina).

Results

Description of Cohort

Seven hundred ten total video recordings from one eye of 214 infants were included. No parents of eligible infants refused consent, and all eligible infants were included. Of the 710 video recordings, 81 (11%) from 64 infants could not be graded due to inadequate image quality (typically poor focus or hazy media). At least one recording could be graded for all but one of 214 infants. Online Video 1, Video 2, and Video 3 are examples of study patients with plus disease, pre-plus disease, and neither, respectively. Table 2 shows the characteristics of the infants in our cohort. Those infants with pre-plus in the right eye at 33–34 weeks had a lower mean birth weight and younger mean gestational age than those infants without pre-plus disease. Those infants with pre-plus were also more likely to be Caucasian, to be the product of a multiple birth, to have ROP in zone I, and to have stage 3 disease.

Table 2.

Characteristics of cohort

	Pre-plus at 33–34 weeks PMA	Neither pre-plus nor plus at 33–34 weeks PMA	Pre-plus at any study exam	Neither pre-plus nor plus at any study exam	All infants
Number *	10	154	40	123	214
Mean birth weight	688 grams	836 grams	729 grams	847 grams	838 grams
Mean gestational age	24.2 weeks	26.3 weeks	25.3 weeks	26.6 weeks	26.5 weeks
Race / Ethnicity
Caucasian	8 (80%)	50 (32%)	21 (53%)	42 (34%)	73 (34%)
African-American	1 (10%)	85 (55%)	12 (30%)	67 (54%)	113 (53%)
Other	1 (10%)	19 (12%)	7 (18%)	14 (11%)	28 (13%)
Inborn (vs. Outborn)	8 (80%)	130 (84%)	37 (93%)	100 (81%)	180 (84%)
Single (vs. Multiple)	3 (30%)	113 (73%)	26 (65%)	89 (72%)	154 (72%)
Lowest zone of ROP
Zone I	9 (90%)	38 (25%)	22 (55%)	26 (21%)	54 (25%)
Zone II	1 (10%)	89 (58%)	18 (45%)	71 (58%)	114 (53%)
Zone III	0	4 (3%)	0	5 (4%)	5 (2%)
N/A – No ROP	0	23 (15%)	0	21 (17%)	41 (19%)
Highest stage of ROP
No ROP	0	23 (15%)	0	21 (17%)	41 (19%)
Stage 1	0	24 (16%)	0	22 (18%)	32 (15%)
Stage 2	5 (50%)	82 (53%)	21 (53%)	65 (53%)	101 (47%)
Stage 3	5 (50%)	25 (16%)	19 (47%)	15 (12%)	40 (19%)

^*The total numbers of infants differ because infants were excluded from the 33–34 week primary analysis if there was no video during that time, and they were excluded from the “any study exam” analysis if there was no videotaped inpatient exam at 35 weeks PMA or later.

Primary Analysis

One-hundred sixty-four eyes of 164 infants were examined at 33–34 weeks PMA and therefore were included in the primary analysis. Of 10 eyes with pre-plus at 33–34 weeks postmenstrual age, 7 (70%) later required laser treatment; of 154 eyes without pre-plus at 33–34 weeks, 14 (9%) later required laser treatment (risk ratio = 7.7; 95% CIs = 4.1, 14.8; odds ratio = 23.7, 95% CIs = 5.5, 101.9, p < 0.001). For those eyes with pre-plus disease at 33–34 weeks PMA that progressed to severe disease requiring laser treatment, the mean time to laser was 1.6 weeks (1.0 – 2.4 weeks; median = 1.3 weeks). For those eyes without pre-plus disease at 33–34 weeks PMA that progressed to disease requiring laser treatment, the mean time to laser was 3.1 weeks (1.0 – 7.0 weeks; median = 2.6 weeks). Four patients (2%) were excluded from the primary analysis because plus disease was present and/or the need for laser was determined at the time of that examination, and 5 patients (3%) were excluded because the video recordings had insufficient quality to allow determination of plus or pre-plus disease. There was disagreement between 2 graders for 5 of the images (3%) included in the primary analysis, requiring a tie-breaking assessment by the other grader.

Secondary Analyses

Table 3 shows results for each postmenstrual age stratum. Of 14 eyes with pre-plus at 35–36 weeks postmenstrual age, 6 (43%) later required laser treatment; of 120 eyes without pre-plus at 35–36 weeks, 5 (4%) later required laser treatment (risk ratio = 10.3; 95% CIs = 3.6, 29.4; odds ratio = 17.3, 95% CIs = 4.3, 69.0, p < 0.001). For other postmenstrual age strata, the presence of pre-plus disease did not predict the need for laser treatment.

Table 3.

Proportion of infants with and without pre-plus disease who eventually required laser treatment, stratified by postmenstrual age at time of videotaped study examination.

Post-menstrual Age at Examination	Total number of infants	Number with pre-plus	Number (%) with pre-plus requiring laser	Number without pre-plus	Number (%) without pre-plus requiring laser	p value
31–32 weeks*	115	2	1 (50%)	113	20 (18%)	0.3
33–34 weeks	164	10	7 (70%)	154	14 (9%)	<0.001
35–36 weeks	134	14	6 (43%)	120	5 (4%)	<0.001
37–38 weeks	94	12	0	80	3 (4%)	1.0
39–40 weeks	68	8	0	60	1 (2%)	1.0
41–42 weeks	38	1	0	37	0	-
Any study exam**	163	40	15 (38%)	123	8 (7%)	<0.001

^*Each row is a separate analysis, and one eye from one infant could be included in any or all rows if there was a videotaped inpatient examination of sufficient quality at the corresponding age. Numbers in cells represent the number of infants (one eye per infant) meeting the criteria listed in columns.

^**Pre-plus was considered to be present at “any study exam” if it was present at one or more exams for a given eye. If pre-plus was never observed, then it was necessary to have at least one exam at 35 weeks PMA or later to be included in this analysis.

Of 40 eyes with pre-plus at any study examination, 15 (38%) later required laser treatment. Of 123 eyes that never had pre-plus disease on a study exam (and had at least one study examination at 35 weeks PMA or later), 8 (7%) later required laser treatment (risk ratio = 5.8; 95% CIs = 2.6, 12.6; odds ratio = 8.6, 95% CIs = 3.3, 22.6, p < 0.001).

Logistic Regression Models

Table 4 shows the results of logistic regression models that were used to determine if pre-plus disease had predictive value beyond that already provided by concurrent examination findings of zone and stage. When adjusting for zone and stage, the presence of pre-plus disease at 33–34 weeks PMA was associated with the need for laser treatment (adjusted odds ratio = 7.8, 95% CIs = 1.5, 41.5, p = 0.02). Lower zone and higher stage of ROP at 33–34 weeks PMA were also associated with the need for laser treatment. (adjusted odds ratios = 0.1 (0.04–0.3) for zone and 3.0 (1.3–6.8) for stage) When adjusting for zone and stage, there was not a statistically significant association between pre-plus and the need for laser treatment for any other PMA strata.

Table 4.

Odds ratios and 95% confidence intervals from logistic regression analysis of predictors of laser treatment, stratified by post-menstrual age at examination.

Postmenstrual Age	Zone	Stage	Pre-plus
31–32 weeks	0.5 (0.2–1.2)	1.4 (0.7–2.6)	4.5 (0.2–82.5)
33–34 weeks	0.1 (0.04–0.3)*	3.0 (1.3–6.8)*	7.8 (1.5–41.5)*
35–36 weeks	0.4 (0.07–2.2)	1.3 (0.5–3.6)	2.3 (0.9–7.6)

^*95% confidence intervals exclude 1.0

When adjusting for birth weight, gestational age, zone and stage, the presence of pre-plus disease at 33–34 weeks PMA was associated with the need for laser treatment (adjusted odds ratio = 7.6, 95% CIs = 1.4, 42.3, p = 0.02). In this model, lower zone and higher stage of disease at 33–34 weeks PMA were also associated with the need for laser treatment. (adjusted odds ratios = 0.1 (0.04–0.4) for zone and 2.9 (1.2–7.4) for stage) At 35–36 weeks PMA, pre-plus was associated with the need for laser treatment when adjusting for birth weight, gestational age, zone and stage. (adjusted odds ratio = 9.8, 95% CIs = 1.8, 53.0, p = 0.008). When adjusting for these variables, pre-plus disease was not associated with need for laser treatment at 31–32 weeks or 37–38 weeks PMA.

Discussion

The International Classification of Retinopathy of Prematurity Revisited paper expanded the classification of posterior pole vascular appearance to include pre-plus disease, an intermediate grade of vascular abnormality between plus disease and normal posterior pole vessels. The prognostic significance of plus disease has been firmly established; (2–4) however, the prognostic value of pre-plus disease has previously not been established. When planning this study, we reasoned that if pre-plus disease had prognostic significance, then its presence would aid clinicians in predicting disease progression and choosing appropriate follow-up intervals. Indeed, we found that pre-plus disease noted between 33 and 36 weeks PMA was predictive of progression to severe ROP requiring laser treatment. We had determined a priori based on pilot data that the primary analysis would include only examinations done at 33–34 weeks. We found that 70% of eyes with pre-plus disease at 33–34 weeks PMA eventually needed laser treatment, whereas only 9% of eyes without pre-plus disease at that time progressed to need for laser. These prospective data agree with pilot data from a small, retrospective study. In that study, 5 of 8 eyes with mild vascular dilation and tortuosity insufficient for plus disease progressed to laser treatment, whereas none of 24 eyes without mild vascular changes progressed to laser treatment (p<0.001). (10)

Clinicians consider many factors beyond posterior pole appearance when planning and performing serial diagnostic examinations of premature infants, including birth weight, gestational age, and concurrent examination findings of zone and stage. Assuming that these factors are known and considered in an infant without plus disease, we sought to understand if there is any prognostic value added by distinguishing between pre-plus and no pre-plus disease. Results of logistic regression showed that pre-plus disease at 33–34 weeks PMA is independently associated with the future need for laser treatment, even after adjusting for zone and stage in one model and for birth weight, gestational age, zone and stage in a second model. These results reaffirm that pre-plus vascular changes early in the disease course should alert examiners to the possibility of progression to severe disease. Pre-plus noted later in the disease course (i.e. 37 weeks or beyond) was not associated with the need for laser treatment. One possible explanation is that eyes that first develop pre-plus at 37 weeks or beyond, or eyes that have persistent pre-plus that has not worsened to plus disease by 37 weeks, have a more indolent course than those that develop pre-plus earlier.

There is value to using the term “pre-plus” beyond its prognostic significance. “Plus disease” refers specifically to severe vascular dilation and tortuosity, requiring at least 2 quadrants with vascular abnormalities that meet or exceed that of a standard photograph used for multiple clinical trials. (3,11) There is a spectrum of posterior pole vascular appearance from normal to mild dilation and/or tortuosity to the more severe changes characteristic of plus disease. Confusion can arise when clinicians use phrases such as “mild plus,” since it is not clear whether that expression refers to plus disease that barely meets severity requirements or to vascular abnormalities that are insufficient for true plus disease. The former requires laser treatment, whereas the latter does not. The term “pre-plus” fills this void and allows more accurate written and verbal communication between ophthalmologists and neonatologists.

We found that a large proportion of eyes with pre-plus disease later required laser treatment, especially those eyes with pre-plus at 33–36 weeks PMA. However, some eyes with pre-plus disease never progress to plus disease and never require laser treatment. Is it possible to predict which eyes with pre-plus are most likely to progress to laser? Ghodasra et al. measured vessel tortuosity and width using Computer-Assisted Image Analysis of the Retina (CAIAR), and they reported that eyes with pre-plus that progressed had more vessel tortuosity and dilation than did eyes with pre-plus that regressed. They concluded that since vascular abnormalities in ROP are a continuum and clinical diagnosis is subjective, quantitative measurement of retinal vessel tortuosity and width by image analysis algorithms like CAIAR may improve risk stratification of eyes with ROP. (12)

Our study should be viewed in light of some limitations. First, the establishment of “truth” using judgment of 3 experts is somewhat subjective, and studies have shown that experts frequently disagree when diagnosing plus or pre-plus disease from digital images. (13,14) Instead of still images, we used videos from indirect ophthalmoscopy, and judgment by examiners performing indirect ophthalmoscopy is a standard in ROP diagnosis. We acknowledge that we cannot know with certainty that overall eye grades based a quadrant scale of 1–10 and given after review of video clips would match eye grades of plus, pre-plus, or neither given during a bedside examination; however, such a study is planned for these same images. If video grading of plus disease by masked examiners is considerably less accurate than bedside exam, then pre-plus could have been misclassified. However, the bias introduced in this scenario (even if pre-plus is “overcalled”) would be in the direction of finding no association between pre-plus and need for laser treatment. It will also be useful in future studies to substantiate our findings using a more objective measure of vascular width and tortuosity such as computer-assisted analysis of images. (15–18) A second limitation is that we could enroll only those infants who had follow-up sufficient (to at least 40.5 weeks PMA) to be reasonably sure that they did or did not develop type 1 ROP and require laser treatment. Nevertheless, we believe that our findings are generalizable to infants with ROP. The infants who are discharged home or transferred early to a local nursery tend to be healthy and are unlikely to develop severe ROP, and these infants likely have a very low incidence of pre-plus disease. Even when these generally healthy infants do develop pre-plus, we have no reason to believe that they would be any more or less likely to progress from pre-plus to severe ROP than infants in our nursery of the same age. A third limitation is that some of our secondary analyses were limited by sample size or by our study's practical design to videotape only inpatient examinations. Finally, a possible limitation is that we required at least 2 quadrants of pre-plus dilation and 2 quadrants of pre-plus tortuosity to designate an eye as having pre-plus disease. This level of detail of not included in the published definition of pre-plus disease, which is simply “vascular abnormalities of the posterior pole that are insufficient for the diagnosis of plus disease, but that demonstrate more arteriolar tortuosity and more venular dilatation than normal.” (5) We chose to require 2 quadrants of abnormality to make assessment of pre-plus analogous to that of plus disease, and also to make it unlikely that mild abnormalities in one quadrant alone, either real or perceived by a masked examiner viewing a video segment, would drive the diagnosis of pre-plus disease.

In conclusion, pre-plus disease observed between 33 and 36 weeks PMA is associated with the eventual need for laser treatment, with the strongest association at examinations between 33 and 34 weeks PMA. Pre-plus disease at this age has prognostic value beyond that which is already provided by birth weight, gestational age, and zone and stage of ROP. Infants with pre-plus early in the course of ROP have increased risk to need laser treatment and should be followed closely to allow optimal timing of intervention, particularly if other ROP risk factors are present.

Supplementary Material

video clips

Video 1. Posterior retina of an eye judged by the masked graders to have plus disease.

Video 2. Posterior retina of an eye judged by the masked graders to have pre-plus disease.

Video 3. Posterior retina of an eye judged by the masked graders to have neither plus nor preplus disease.