Alex T. Legocki,Aaron Y. Lee,Leona Ding,Yasman Moshiri,Emily M. Zepeda,Thomas B. Gillette,Laura E. Grant,Ayesha Shariff,Phanith Touch,Cecilia S. Lee,Kristina Tarczy-Hornoch,Michelle T. Cabrera
Multivariate Models to Diagnose Early Referral-Warranted Retinopathy of Prematurity With Handheld Optical Coherence Tomography
retinopathy of prematurity (ROP),referral-warranted retinopathy of prematurity,retina,optical coherence tomography (OCT),pediatric ophthalmology
Purpose The purpose of this study was to create multivariate models predicting early referral-warranted retinopathy of prematurity (ROP) using non-contact handheld spectral-domain optical coherence tomography (OCT) and demographic data. Methods Between July 2015 and February 2018, infants ≤1500 grams birth weight or ≤30 weeks gestational age from 2 academic neonatal intensive care units were eligible for this study. Infants were excluded if they were too unstable to participate in ophthalmologic examination (2), had inadequate image quality (20), or received prior ROP treatment (2). Multivariate models were created using demographic variables and imaging findings to identify early referral-warranted ROP (referral-warranted ROP and/or pre-plus disease) by routine indirect ophthalmoscopy. Results A total of 167 imaging sessions of 71 infants (45% male infants, gestational age 28.2+/−2.8 weeks, and birth weight 995.6+/−292.0 grams) were included. Twelve of 71 infants (17%) developed early referral-warranted ROP. The area under the receiver operating characteristic curve (AUC) was 0.94 for the generalized linear mixed model (sensitivity = 95.5% and specificity = 80.7%) and 0.83 for the machine learning model (sensitivity = 91.7% and specificity = 77.8%). The strongest variables in both models were birth weight, image-based Vitreous Opacity Ratio (an estimate of opacity density), vessel elevation, and hyporeflective vessels. A model using only birth weight and gestational age yielded an AUC of 0.68 (sensitivity = 77.3% and specificity = 63.4%), and a model using only imaging biomarkers yielded 0.88 (sensitivity = 81.8% and specificity = 84.8%). Conclusions A generalized linear mixed model containing handheld OCT biomarkers can identify early referral-warranted ROP. Machine learning produced a less optimal model. Translational Relevance With further validation, this work may lead to a better-tolerated ROP screening tool.
Multivariate Models to Diagnose Early Referral-Warranted Retinopathy of Prematurity With Handheld Optical Coherence Tomography

The purpose of this study was to create multivariate models predicting early referral-warranted retinopathy of prematurity (ROP) using non-contact handheld spectral-domain optical coherence tomography (OCT) and demographic data.

Between July 2015 and February 2018, infants ≤1500 grams birth weight or ≤30 weeks gestational age from 2 academic neonatal intensive care units were eligible for this study. Infants were excluded if they were too unstable to participate in ophthalmologic examination (2), had inadequate image quality (20), or received prior ROP treatment (2). Multivariate models were created using demographic variables and imaging findings to identify early referral-warranted ROP (referral-warranted ROP and/or pre-plus disease) by routine indirect ophthalmoscopy.

A total of 167 imaging sessions of 71 infants (45% male infants, gestational age 28.2+/−2.8 weeks, and birth weight 995.6+/−292.0 grams) were included. Twelve of 71 infants (17%) developed early referral-warranted ROP. The area under the receiver operating characteristic curve (AUC) was 0.94 for the generalized linear mixed model (sensitivity = 95.5% and specificity = 80.7%) and 0.83 for the machine learning model (sensitivity = 91.7% and specificity = 77.8%). The strongest variables in both models were birth weight, image-based Vitreous Opacity Ratio (an estimate of opacity density), vessel elevation, and hyporeflective vessels. A model using only birth weight and gestational age yielded an AUC of 0.68 (sensitivity = 77.3% and specificity = 63.4%), and a model using only imaging biomarkers yielded 0.88 (sensitivity = 81.8% and specificity = 84.8%).

A generalized linear mixed model containing handheld OCT biomarkers can identify early referral-warranted ROP. Machine learning produced a less optimal model.

With further validation, this work may lead to a better-tolerated ROP screening tool.

Retinopathy of prematurity (ROP) is a major cause of childhood blindness worldwide, characterized by abnormal vasoproliferation in the developing retina. The diagnosis of ROP is made by clinical examination in premature infants, traditionally by binocular indirect ophthalmoscopy (BIO) with use of an eyelid speculum and scleral depressor. This examination is known to cause significant agitation and discomfort for the infants, demonstrated by deviations of cardiorespiratory metrics and pain scores, and this stress is particularly associated with the speculum and scleral depressor use.– Wide-field digital retinal imaging (e.g. Retcam, Natus Medical Incorporated, Middleton, WI, USA) is a well-established alternative to BIO, but also requires contact with the globe. Unfortunately, studies have shown that Retcam photography is also associated with significant alterations in pain scores and vital signs, in some cases greater than BIO.–, Handheld optical coherence tomography (OCT) is an imaging tool used to capture high-resolution cross-sectional retinal and optic nerve images in the supine position. One direction of current research is imaging of the vascular-avascular junction in the retinal periphery, which once again requires a combination of eyelid speculum placement and often scleral depression use or direct contact between the eye and OCT probe.– However, Maldonado et al. introduced a technique in 2010 to acquire handheld OCT images in awake infants without the use of an eyelid speculum, allowing the infant to remain comfortable during image acquisition. Preterm infants have been shown to have diminished behavioral and cardiovascular manifestations of stress during such handheld OCT imaging compared to standard binocular indirect ophthalmoscopy. In these cases, imaging does not capture the peripheral vascular-avascular junction and is instead limited to the posterior pole. In spite of limited field of view, recent work has associated awake handheld OCT vitreoretinal findings in these posterior areas with clinical ROP severity. These potential markers of ROP severity include elevated retinal vessels, scalloped retinal layers, hyporeflective vessel lumina, retinal spaces, punctate hyper-reflective vitreous opacities, and vitreous bands (Fig. 1).– Nonetheless, handheld OCT has never been considered as a potential ROP screening tool because none of these individual findings alone can predict an infant's risk of blindness. The purpose of this study is to use statistical and machine learning tools to identify early referral-warranted ROP (eRW-ROP) from demographic and handheld spectral domain OCT (SD-OCT) imaging findings of the posterior retina.

This prospective, observational study enrolled neonates undergoing routine ROP examinations at the University of Washington and Seattle Children's Hospital (Seattle, WA, USA) neonatal intensive care units (NICUs). The study was approved by the University of Washington and Seattle Children's Hospital institutional review boards. All study procedures adhered to regulations of the Health Insurance Portability and Accountability Act and the tenets of the Declaration of Helsinki. The infants’ parents or legal guardians gave informed consent prior to the initial handheld OCT imaging session.

Infants were eligible for participation if their gestational age was ≤30 weeks or if birth weight was ≤1500 grams. Pupils were dilated with 1% phenylephrine hydrochloride and 0.2% cyclopentolate ophthalmic solution (Cyclomydril; Alcon Laboratories, Geneva, Switzerland) prior to the imaging and clinical examinations. Each infant's medical record was reviewed for race/ethnicity, sex, gestational age, birth weight, clinical examination results (stage, zone, and presence of plus disease), and history of ophthalmic injections or laser treatment. Infants were excluded if they were deemed too unstable by the NICU medical team for safe BIO examination and/or handheld OCT imaging, if they had inadequate macular SD-OCT image quality, as determined by the image graders, or if they received ROP treatment prior to enrollment.

Imaging was performed on the same day as each routine ROP examination using an Envisu C2300 handheld SD-OCT system (Leica Microsystems, Wetzlar, Germany) as previously described. Infants were positioned supine without sedation, and handheld SD-OCT images were acquired by one of three trained imagers (authors A.S., L.E.G., and T.B.G.). Sucrose and/or a pacifier were given to infants as needed to aid compliance with the examination. Fingers were used to retract the eyelids; eyelid specula were not used during examinations, and no contact was made with the ocular surface. Attempts were made to acquire high-quality images of the fovea and optic nerve of both eyes for no longer than 15 minutes per imaging session per infant.

Routine BIO examinations were performed by one of three trained pediatric ophthalmologists (including authors M.T.C. and K.T.-H.) on the same day as the handheld SD-OCT image acquisition. BIO examinations, performed with an eyelid speculum and scleral depressor, were used to make all clinical decisions. Clinical examination findings were determined by the ophthalmologist examining the patient on that date. If clinically warranted according to routine care, subsequent examinations for an infant were performed 1 to 3 weeks apart. The pediatric ophthalmologists were masked to handheld SD-OCT imaging results.

Only right eye findings prior to treatment for type 1 ROP were included in the analysis. The OCT volume scans of each right eye were reviewed by two trained graders (authors A.S. and E.M.Z.) using the system's software (Envivovue; Leica Microsystems, Wetzlar, Germany) for vitreoretinal findings (see Fig. 1). Vitreous bands were defined as coalescing linear opacities above the retina that were visible in at least three consecutive frames with intensity above the background, and were classified as tractional (straight bands at a steep angle that made a connection to the retinal surface) and non-tractional (bands that did not appear to have a connection to the retina captured by the scan, and hovered parallel to the retina). Two additional trained graders (authors A.T.L. and Y.M.) evaluated all OCT volume scans of each right eye for findings previously defined by Maldonado et al. as predictive of clinical plus disease: vessel elevation (spiked vessels emerging from the inner retinal surface, graded as mild or severe), scalloped retinal layers (mild = scalloping of the inner plexiform layer and severe = scalloping of the outer plexiform layer), hyporeflective vessels, and retinal spaces (hyporeflective spaces located between retinal vessels or adjacent to vessels). As described by Maldonado et al., all regions of all available OCT volumes were included for assessment of the above findings, without specifying number of involved quadrants. Disagreement among the graders for any OCT finding was mediated by a third trained grader (author M.T.C.), and all graders were masked to the patient's clinical examination results. A custom, open-sourced web browser-based application developed by one of the authors (A.Y.L.) was used to quantify the density of punctate hyperreflective vitreous opacities, a calculation known to be associated with ROP stage. As previously described, five adjacent B-scan frames centered on the fovea (approximate width = 256 microns) were selected in the highest quality macular scan for a given imaging session based on subjective assessment. These foveal and parafoveal frames were chosen due to their easily identifiable anatomy, which would allow for analysis of comparable vitreous volume and location among infants. Although opacities may have been seen outside these foveal frames, these were not included in this quantitative analysis in order to maintain uniformity in regions and volumes sampled. Two independent, masked, trained graders (authors T.B.G. and M.T.C.) manually segmented the vitreous opacities and vitreoretinal border in these frames, excluding vitreous above the optic nerve where hyaloid remnants are frequently seen. Punctate opacities were defined as having markedly higher signal than background with clear separation from the retina. All completed segmentations were reviewed by an independent, trained grader (author P.T.), and inaccurate segmentations were sent back for repeat segmentation. To estimate vitreous opacity density (vitreous opacity ratio, shown to be associated with ROP stage) in the setting of variable captured vitreous area, the number of vitreous opacities per unit vitreous area in pixels, as determined by the software, was averaged between the 2 graders and multiplied by 100,000,000.

A multivariate model was developed to predict eRW-ROP, which was defined as same-day BIO-determined plus disease, zone I ROP, or ROP stage 3 or greater, with the addition of any pre-plus disease to better capture at-risk infants. As multiple examinations per infant were included, a generalized linear mixed model approach was used to adjust for this. The model included variable OCT findings and postmenstrual age at imaging that were time-specific – that is, on the same date as the clinical examination – however, the birth weight and gestational age were infant-specific regardless of the date of examination. Variable selection was performed by backward elimination using the Akaike information criterion for model evaluation to predict eRW-ROP using candidate demographic variables (gender, birth weight in grams, gestational age in weeks, race/ethnicity, and postmenstrual age at imaging in weeks) and OCT variables (presence of tractional and non-tractional vitreous bands, vitreous opacity ratio, vessel elevation [none, mild, and severe], scalloped retinal layers [none, mild, and severe], presence of hyporeflective vessels, and presence of retinal spaces). Recognizing the known correlation between gestational age and birth weight, each of these two variables were individually excluded to assess impact on the model, ultimately creating a final model excluding one of these variables. To understand whether OCT alone, without demographic variables, could feasibly diagnose referral-warranted ROP, a modified mixed model was created that incorporates only the OCT variables above (before backward elimination). Conversely, to assess the predictive power of infant demographics that are currently used for ROP screening criteria, a modified mixed model was created including only birth weight and gestational age.

Extreme gradient boosting, a machine learning tool (Python Software Foundation, version 3.8.1, http://www.python.org; XGBoost, https://github.com/dmlc/xgboost; scikit-learn), was used to develop a regression model to similarly predict eRW-ROP based on the same candidate explanatory variables that were used for the generalized linear mixed model (gender, birth weight, gestational age, race/ethnicity, postmenstrual age at imaging, presence of tractional and non-tractional vitreous bands, vitreous opacity ratio, vessel elevation, scalloped retinal layers, presence of hyporeflective vessels, and presence of retinal spaces). Shapley additive explanations (SHAP) values were generated to provide an estimate of the impact of each observation on the model's output.

A receiver operating characteristic (ROC) curve was created for each model, using clinical eRW-ROP as the reference standard. Areas under the curve (AUC) were calculated for each model. Sensitivity and specificity values were reported based on the maximum Youden index (J) for each model. The unweighted Cohen's kappa (k) statistic was used to evaluate interobserver agreement for the presence or absence of all OCT vitreoretinal findings (tractional vitreous bands, non-tractional vitreous bands, vessel elevation, scalloped retinal layers, hyporeflective vessels, and retinal spaces). For the vitreous opacity ratio, interobserver agreements for the manual counting of punctate hyper-reflective vitreous opacities were assessed by F1 score and agreements for the vitreoretinal border segmentation were assessed using a Dice coefficient (Python Software Foundation, version 3.7.4, http://www.python.org; scikit-learn; NumPy; OpenCV).– Other than vitreous opacity ratio calculations, all statistical analyses were performed using SAS version 9.4 (SAS Institute, Inc., Cary, NC, USA).

A total of 111 premature infants were recruited for the study, of which 40 were excluded. Twenty were excluded due to poor image quality (attributable to difficult alignment of the probe with the infant's direction of gaze). Poor image quality was defined as lack of visualization of all retinal laminations and/or inadequate signal strength to identify all OCT findings of interest, or absence of five high-quality B-scan frames centered at the fovea for calculation of the vitreous opacity ratio. Eight infants were withdrawn by their guardians due to concern for infant stress, two were determined to be too hemodynamically unstable by the NICU medical team, two were withdrawn by the parents who did not want to pursue follow-up, one was withdrawn after death due to unrelated respiratory failure, five were excluded due to lost imaging data, and two were excluded due to prior treatment for type 1 ROP. The remaining 71 infants underwent 176 imaging sessions. Infants were generally visibly comfortable for all handheld OCT imaging sessions, however, these data were not recorded as part of this study. After excluding an additional 9 right eye imaging sessions among 4 infants that occurred following treatment for type 1 ROP, the remaining 167 right eye imaging sessions from 71 infants were included in the study. Of these infants, 32 (45%) were boys. Mean birth weight was 995.6 (±292.0) grams, mean gestational age was 28.2 (±2.8) weeks, and mean postmenstrual age at imaging was 36.4 (±3.5 weeks). Thirty-nine of the 71 infants (55%) were White, 12 (17%) were Hispanic, 2 (3%) were Native American, 5 (7%) were Black, 1 (1%) was Pacific Islander, 4 (6%) were Asian, and 8 (11%) were other. The mean number of imaging sessions per infant was 2.4 (±1.6, range = 1-8) over a mean of 4.6 (±4.2) weeks. Twelve of the 71 infants (17%) developed eRW-ROP in the right eyes at mean 1.7 (±4.2) weeks after their first examination, and 4 (5.6%) developed type 1 ROP in the right eyes which required treatment. All infants that required treatment received laser photocoagulation. Demographic and clinical data are summarized in the Table.

Among 71 infants included in the study, right eye handheld SD-OCT imaging revealed that 6 infants (8%) had evidence of tractional vitreous bands, 20 (28%) had non-tractional vitreous bands, and 17 (24%) had macular punctate hyper-reflective vitreous opacities (within the five B-scans centered at the fovea). Among the 17 infants with macular vitreous opacities, the maximum vitreous opacity ratio reached by each infant was mean 1.12 (±1.13). Agreement between 2 graders in counting opacities revealed an F1 score of 0.82 (±0.36) and a Dice coefficient for vitreous segmentation of 0.97 (±0.04). With regard to the VASO criteria outlined by Maldonado et al,, 53 (75%) demonstrated vessel elevation (43/53 [81%] mild and 10/53 [19%] severe), 38 (54%) had scalloped retinal layers (31/38 [82%] involving inner plexiform layer, and 7/38 [18%] involving outer plexiform layer), 11 (15%) had hyporeflective vessels, and 12 (17%) had retinal spaces. Cohen's kappa coefficient for interobserver agreement was 0.61, rated as “substantial” agreement. Agreement among all OCT findings was 79.8%.

The logistic mixed model consisted of the following nine predictors from demographic and handheld OCT imaging data: birth weight, gestational age, postmenstrual age at imaging, non-tractional vitreous bands, vitreous opacity ratio, vessel elevation, scalloped retinal layers, hyporeflective vessels, and retinal spaces. The model eliminated gender, race/ethnicity, and tractional vitreous bands as part of the backward elimination process. The AUC was 0.96, with a sensitivity of 90.9% and a specificity of 96.6% (J = 0.88). Due to the known correlation between birth weight and gestational age, each of these two variables were manually excluded from the model in turn, which resulted in minimal change to the predictive power (AUC = 0.91 after birth weight removed, and 0.94 after gestational age removed). Therefore, the model without gestational age was selected as the “best” model to avoid this correlation (Fig. 2). This final model eliminated race/ethnicity and tractional vitreous bands as part of the backward elimination process. The strongest predictors of eRW-ROP in that model were severe vessel elevation (P < 0.001), vitreous opacity ratio (P = 0.002), hyporeflective vessels (P = 0.004), retinal spaces (P = 0.006), and birth weight (P = 0.006). The AUC was 0.94 (Fig. 3A), with a sensitivity of 95.5% and a specificity of 80.7% (J = 0.76). The mixed model consisting of only OCT variables included vitreous opacity ratio, vessel elevation, hyporeflective vessels, and retinal spaces. Tractional vitreous bands, non-tractional vitreous bands, and scalloped retinal layers were removed by backward elimination. This model had an AUC of 0.88 (Fig. 3B), with a sensitivity of 81.8% and a specificity of 84.8% (J = 0.67). The mixed model that included only the explanatory variables birth weight and gestational age had a reduced AUC of 0.68 (Fig. 3C), with a sensitivity of 77.3% and a specificity of 63.4% (J = 0.41).

The extreme gradient boosting approach yielded a multivariate model with AUC 0.83 (Fig. 3D), sensitivity 91.7%, and specificity 77.8% (J = 0.69) for predicting eRW-ROP. The most impactful variables based on SHAP values were birth weight, vessel elevation, gestational age, postmenstrual age at the time of imaging, hyporeflective vessels, and vitreous opacity ratio (Fig. 4).

Utilizing 167 imaging sessions from 71 premature infants at a single site, we developed models to identify eRW-ROP using only posterior retinal handheld SD-OCT findings and demographic data, with the best being the generalized linear mixed model (AUC = 0.94, sensitivity = 95.5%, and specificity = 80.7%). To our knowledge, this study is the first to comprehensively diagnose early high-risk ROP through handheld OCT (PubMed search terms: optical coherence tomography AND retinopathy of prematurity), although further validation is needed. The gold standard ROP screening examination includes BIO and, more recently, wide-field retinal photography. These approaches involve interpretation of both central and peripheral retinal pathology using techniques that are distressing to the infants. This stress has been associated with the use of an eyelid speculum or scleral depressor.– Investigations into the utility of OCT in diagnosing ROP have focused either on the posterior vitreoretinal interface or the peripheral vascular-avascular junction. In the latter, techniques are limited by use of eyelid speculum, scleral depression, and/or direct contact between the imaging probe and the eye, leading to infant distress.– In contrast, handheld OCT of the posterior retina requires neither ocular contact nor an eyelid speculum. In this study and others, infants generally remained comfortable throughout OCT image acquisition. Ultimately, OCT imaging of the posterior vitreoretinal interface may reduce the number of distressing, traditional examinations currently necessary to screen preterm infants for ROP. Other studies have also sought to reduce the number of infants receiving such disruptive examinations by improving on existing screening criteria for ROP. Recently, the Postnatal Growth and Retinopathy of Prematurity study (G-ROP) demonstrated that consideration of postnatal growth may reduce the number of infants that require ophthalmoscopy by as much as 15% to 30%.– In contrast, instead of creating screening criteria, as in G-ROP, we propose that OCT imaging could be analogous to digital wide-field retinal imaging, similarly identifying a subset of infants who would require ophthalmoscopy, but with the advantage of significantly reduced cardiorespiratory disturbance to the infants and potentially greater quantitative data. New screening criteria, such as G-ROP, have the potential to work synergistically with OCT, by creating a reduced number of infants requiring OCT screening. OCT then provides quantitative evaluation of the posterior segment in these infants, allowing for more specific information on ROP risk to reduce the number of ophthalmoscopy examinations even further. Such an approach requires further validation, however. Machine learning has served as an exciting new tool for data analysis in ophthalmology, however, this study's extreme gradient boosting yielded a less optimal model (AUC = 0.83, sensitivity = 91.7%, and specificity = 77.8%) compared to the generalized linear mixed model. Limitations of machine learning include skewed data analysis when based on smaller, less representative datasets or inadequate representation from rare pathologies in the dataset. Our results highlight the importance of model exploration using traditional statistical approaches in addition to machine learning, particularly when datasets are limited. The OCT variables used for our models were previously identified potential predictors of ROP risk located in the posterior pole. Vessel elevation, scalloped retinal layers, hyporeflective vessels, and retinal spaces were described by Maldonado et al. to collectively diagnose plus disease by imaging the posterior retina. We previously established the vitreous opacity ratio (an estimate of density of punctate hyperreflective vitreous opacities) and vitreous bands as OCT findings correlating with ROP stage and plus disease, respectively. Future exploration into other OCT-based biomarkers of ROP using emerging technologies is warranted, such as handheld OCT angiography, choroidal measurements, and wide-field OCT imaging of the ROP ridge., Further automation in image analysis would decrease subjectivity and improve efficiency for OCT analysis of all such biomarkers. As in previous weight-based models of ROP risk,,– demographic factors, such as birth weight or gestational age, were included to improve the performance of the models in this study. Nonetheless, demographics were relatively weak drivers of these models (see Figs. 2, 4). Excluding the demographics entirely with a model using only OCT variables resulted in a good although lower AUC of 0.88 (see Fig. 3B), with a sensitivity of 81.8% and a specificity of 84.8%. In contrast, an algorithm containing birth weight and gestational age alone resulted in a significant drop in predictive power (AUC = 0.68, with a sensitivity of 77.3% and a specificity of 63.4%; see Fig. 3C). This study is limited by a single site population without a separate validation set for the models. A validation set could not be achieved with adequate sampling of severe ROP in the timeframe of the current study. The AUC is therefore not indicative of the models’ performance in any dataset other than that used to derive the models. Nonetheless, the primary purpose of this work is to serve as a proof-of-concept for the utility of handheld OCT of the posterior retina to assess ROP risk in preterm infants, rather than to create a final model validated for clinical use. Once validated, future clinical use could involve reducing the number of patients that require distressing screening examinations. A cutoff for the predictive formula would be set at 100% sensitivity (53% specificity in the model presented here) to avoid missing any patients with vision-threatening disease. Limitations in handheld SD-OCT image quality in awake preterm infants should also be acknowledged. Twenty of 111 (18%) infants were excluded due to inadequate image quality (attributable to difficult alignment of the probe with the infant's direction of gaze), suggesting that this device would face challenges in real-life clinical application in awake infants. However, use of this device can still reduce how many infants receive ophthalmoscopic examinations, and recent advances in newer handheld OCT technology have resulted in consistently high image quality for awake infant vitreoretinal analysis., Validation of these algorithms with such newer devices are needed. Finally, although this study is limited by lack of quantitative vitreous opacity analysis outside the macula, this previously validated approach allows for uniform anatomic sampling among infants. In summary, this study demonstrated that multivariate models using demographic and handheld OCT data can identify eRW-ROP in premature infants. The use of handheld OCT as a better-tolerated early ROP screening tool would reduce examination morbidity in this vulnerable population. Validation of future models in large multicenter studies is necessary before considering clinical use.