Diagnosis and Treatment of Ocular Pain: the Ophthalmologist's Perspective

Effect of Acupuncture Treatment on the Ocular Pain, Mental State and Ocular Surface Characteristics of Patients with Dry Eye Disease: A Non-Randomized Pilot Study

Background

Ocular pain is a prevalent symptom of dry eye disease (DED), which often accompanies potential psychological issues. The study aimed to explore whether acupuncture could improve ocular pain, mental state, and dry eye parameters in patients with DED.

Methods

The non-randomized pilot study included 48 patients divided into two groups: the acupuncture group (n=27) and the 0.3% sodium hyaluronate (SH) group (n=21). Participants in the acupuncture group underwent treatments on six bilateral acupuncture points (BL1, BL2, ST1, LI 20, SI1 and SI3) 3 times per week for 4 weeks. Patients in the SH group received 0.3% SH 4 times per day for 4 weeks. Ocular pain was assessed using the numerical rating scale (NRS), and mental state was evaluated through the self-rating anxiety scale (SAS) and self-rating depression scale (SDS). Ocular surface parameters, concentrations of inflammatory cytokines, and corneal nerve morphological indicators were measured at baseline, the first week, and the fourth week. Randomization procedures were not used in this study, and outcome assessors and statistical analysts were blinded.

Results

Compared with baseline, both NRS scores (from 5.91 ± 1.52 to 1.94 ± 1.57) and ocular surface discomfort index (OSDI) scores (from 49.75 ± 14.92 to 29.64 ± 18.79) were decreased after 1 and 4 weeks of treatment in both groups (all p < 0.05). At 4 weeks, the acupuncture group showed significant improvements, including increased tear break-up time (TBUT) and corneal perception, decreased SAS and SDS scores, and reduced concentrations of interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α concentration in tears (all p < 0.05). These changes were not observed in the SH group (all p > 0.05).

Conclusion

Acupuncture treatment could improve ocular surface characteristics in patients with DED, and more importantly, it alleviates their ocular pain and depressive state. The anti-inflammatory effect of acupuncture may be involved in this process. Future research with larger, randomized controlled trials (RCTs) is necessary to confirm these findings and clarify the mechanisms involved.

Corneal Neuropathic Pain: A Patient and Physician Perspective

Corneal neuropathic pain (CNP) is a debilitating condition characterized by pain in the absence of a noxious stimulus. Symptoms such as ocular stinging, burning, photophobia, irritation, and a deep aching pain can be severe despite a seemingly normal ocular surface on examination. CNP may develop due to either peripheral or central sensitization. Peripheral sensitization develops due to aberrant regeneration of corneal nociceptors and nerve fibers as a result of corneal injury or disease of peripheral corneal nerves. Whereas, central sensitization develops due to upregulation of excitatory neurotransmitters as a result of chronic inflammation, which leads to amplification of neuronal response to stimuli. Unfortunately, due to the disparity in severity of symptomology and the observable signs on examination, patients' symptoms are commonly thought to be "psychological" or "functional", and patients report feeling ignored and neglected. Additionally, diagnosis is often delayed which adversely affects patient outcomes. Research to date has focused on the scientific aspects of corneal neuropathic pain: its pathophysiology, epidemiology, investigations, and management. Research into the patient personal experience and the challenges faced by individual patients and their clinicians is lacking. We present the patient and physician perspective on the journey of both patients in order to provide insights into the challenges faced by patients and physicians in the diagnosis, assessment, and management of corneal neuropathic pain.

Qualitative Research to Understand the Patient Experience and Evaluate Content Validity of the Chronic Ocular Pain Questionnaire (COP-Q)

INTRODUCTION

Chronic ocular surface pain (COSP) is described as a persistent, moderate-to-severe pain at the ocular surface lasting more than 3 months. Symptoms of COSP have a significant impact on patients' vision-dependent activities of daily living (ADL) and distal health-related quality of life (HRQoL). To adequately capture patient perspectives in clinical trials, patient-reported outcome (PRO) measures must demonstrate sufficient evidence of content validity in the target population. This study aimed to explore the patient experience of living with COSP and evaluate content validity of the newly developed Chronic Ocular Pain Questionnaire (COP-Q) for use in COSP clinical trials.

METHODS

Qualitative, combined concept elicitation (CE) and cognitive debriefing (CD) interviews were conducted with 24 patients experiencing COSP symptoms in the USA. Interviews were supplemented with real-time data collection via a daily diary app task in a subset of patients (n = 15) to explore the day-to-day patient experience. Three healthcare professionals (HCPs) from the USA, Canada, and France were also interviewed to provide a clinical perspective. CE results were used to further inform development of a conceptual model and to refine PRO items/response options. CD interviews assessed relevance and understanding of the COP-Q. Interviews were conducted across multiple rounds to allow item modifications and subsequent testing.

RESULTS

Eye pain, eye itch, burning sensation, eye dryness, eye irritation, foreign body sensation, eye fatigue, and eye grittiness were the most frequently reported symptoms impacting vision-dependent ADL (e.g., reading, using digital devices, driving) and wider HRQoL (e.g., emotional wellbeing, social functioning, work). COP-Q instructions, items, and response scales were understood, and concepts were considered relevant. Feedback supported modifications to instruction/item wording and confirmed the most appropriate recall periods.

CONCLUSIONS

Findings support content validity of the COP-Q for use in COSP populations. Ongoing research to evaluate psychometric validity of the COP-Q will support future use of the instrument in clinical trial efficacy endpoints.

[Chronic eye pain]

Numerous conditions in the field of ophthalmology are associated with pain in or around the eye. Chronic pain associated with the eye is a common finding in the daily routine of ophthalmologists and can be associated with primary ocular or extraocular diseases as well as with other conditions. Appropriate diagnostic assessment and management of people with chronic pain requires an understanding of the condition based on the biopsychosocial model in which the interactions of biological/somatic, psychological and social factors are determining pain and suffering. Beyond the ophthalmological findings,  close interdisciplinary cooperation and assessment are required. Therefore, if eye pain is insufficiently responsive to treatment or if symptoms of chronic pain are evident, pain medicine expertise should be involved. The management of chronic ocular pain is based on interdisciplinary multimodal approaches, in addition to the ophthalmologist-specific approaches. These focus on self-efficacy, patient competence and acceptance of pain as central goals of treatment rather than pain relief. Patient information, education and the development of a suitable concept by the interdisciplinary team are essential therapeutic aspects in this context.

Persistent socket pain in a dog after the enucleation of the eye and its clinical management

Persistent socket pain is a condition described in humans after enucleation of the eye. This report aims at describing this condition in dogs. A 10-year-old male-neutered crossbreed was presented to the referral veterinary surgeon for enucleation of the right ocular globe. Anaesthesia and surgery were uneventful although during the postoperative period the dog was reluctant to open the mouth and to be explored by the referral veteterinary surgeon. Despite treatment with meloxicam, paracetamol and tramadol, no improvements were observed. Ten weeks after surgery, the dog was referred to the Dick White referrals for further investigations. Ophthalmic examination was normal, though palpation of the wound triggered an avoidance response. Magnetic resonance imaging showed changes compatible with orbital cellulitis. The area of interest was evaluated with the use of the mechanical Von Frey filaments. A response, characterised by sudden turning of the head and attempts to withdraw it, was evoked with filament 4.93 (8.0 g) during stimulation of the periorbital area. After induction of anaesthesia, an ultrasound-guided injection containing levobupivacaine 0.5% and methylprednisolone was performed within the retrobulbar area. Three hours after recovery from anaesthesia, no discomfort was observed during palpation of the area. Re-evaluation was performed with the Von Frey filaments; no response could be evoked during testing with all 20 filaments (from 2.36 to 6.65) applied on either side of the face. The patient was discharged with a course of gabapentin and, 3 weeks after the intervention, the dog showed no clinical signs of pain. Persistent socket pain is an unpleasant sensation at the level of the enucleated orbit, and it should be regarded as a challenging condition to diagnose and treat. The MRI findings appeared to be essential to select the most appropriate interventional treatment. The injection of local anaesthetic and steroid into the retrobulbar space was useful for both confirming the diagnosis and treating pain by reducing the peripheral signalling and decreasing the residual inflammation.

Topical formulations of Aprepitant are safe and effective in relieving pain and inflammation, and drive neural regeneration

PURPOSE

To test long-term ocular toxicity and analgesic/anti-inflammatory efficacy of two novel ocular formulations of neurokinin 1 receptor (NK1R) antagonist Aprepitant.

METHODS

for toxicity studies, two Aprepitant formulations (X and Y) were tested on C57BL/6 N mice. Gold standards were 0.4% Oxybuprocaine, 0.1% Diclofenac, or saline. For efficacy studies, C57BL/6 N mice underwent corneal alkali burn, and then received Aprepitant formulation X, Dexamethasone or saline. Eye-drops were applied 3 times/day for 90 days (toxicity) and 14 days (efficacy). Stromal opacity, corneal epithelial damage, nociception and sensitivity were assessed in vivo. The eye-wiping test and corneal sensitivity were assessed to evaluate analgesic efficacy and nerve function. At the end of the experiments mice were euthanized, and corneas were dissected for immunohistochemistry and RT-PCR analyses.

RESULTS

In normal mice, formulation X was not toxic when topically administered for 90 days. Formulation Y was associated with increased leukocyte infiltration in the cornea (p < 0.001). X1 and X2 formulations significantly reduced corneal pain, as Diclofenac and Oxybuprocaine, but did not reduce corneal sensitivity. Formulation Y, instead, was not analgesic at any time point. In the alkali burn model, X1 and X2 formulation enhanced epithelial damage recovery, and reduced inflammation both at day 7 and 14. Moreover, formulation X showed a stronger analgesic effect when compared to the saline and Dexamethasone groups (p < 0.01). Finally, formulation X1 and X2 restored corneal sensitivity by promoting corneal nerve regeneration.

CONCLUSIONS

Aprepitant X formulation is a promising candidate for the treatment of pain associated with inflammation of the ocular surface.

Ocular Pharmacology and Toxicology of TRPV1 Antagonist SAF312 (Libvatrep)

Purpose

To evaluate the pharmacology and toxicology of SAF312, a transient receptor potential vanilloid 1 (TRPV1) antagonist.

Methods

TRPV1 expression in human ocular tissues was evaluated with immunohistochemistry. Inhibition of calcium influx in Chinese hamster ovary (CHO) cells expressing human TRPV1 (hTRPV1) and selectivity of SAF312 were assessed by a fluorescent imaging plate reader assay. Ocular tissue and plasma pharmacokinetics (PK) were assessed following a single topical ocular dose of SAF312 (0.5%, 1.0%, 1.5%, 2.5%) in rabbits. Safety and tolerability of SAF312 were evaluated in rabbits and dogs. Effects of SAF312 on corneal wound healing after photorefractive keratectomy (PRK) surgery were assessed in rabbits.

Results

TRPV1 expression was noted in human cornea and conjunctiva. SAF312 inhibited calcium influx in CHO-hTRPV1 cells induced by pH 5.5 (2-[N-morpholino] ethanesulfonic acid), N-arachidonoylethanolamine, capsaicin, and N-arachidonoyl dopamine, with IC50 values of 5, 10, 12, and 27 nM, respectively, and inhibition appeared noncompetitive. SAF312 demonstrated high selectivity for TRPV1 (>149-fold) over other TRP channels. PK analysis showed highest concentrations of SAF312 in cornea and conjunctiva. SAF312 was found to be safe and well tolerated in rabbits and dogs up to the highest feasible concentration of 2.5%. No delay in wound healing after PRK was observed.

Conclusions

SAF312 is a potent, selective, and noncompetitive antagonist of hTRPV1 with an acceptable preclinical safety profile for use in future clinical trials.

Translational Relevance

SAF312, which was safe and well tolerated without causing delay in wound healing after PRK in rabbits, may be a potential therapeutic agent for ocular surface pain.

Botulinum toxin A decreases neural activity in pain-related brain regions in individuals with chronic ocular pain and photophobia

Introduction

To examine the effect of botulinum toxin A (BoNT-A) on neural mechanisms underlying pain and photophobia using functional magnetic resonance imaging (fMRI) in individuals with chronic ocular pain.

Methods

Twelve subjects with chronic ocular pain and light sensitivity were recruited from the Miami Veterans Affairs eye clinic. Inclusion criteria were: (1) chronic ocular pain; (2) presence of ocular pain over 1 week recall; and (3) presence of photophobia. All individuals underwent an ocular surface examination to capture tear parameters before and 4-6 weeks after BoNT-A injections. Using an event-related fMRI design, subjects were presented with light stimuli during two fMRI scans, once before and 4-6 weeks after BoNT-A injection. Light evoked unpleasantness ratings were reported by subjects after each scan. Whole brain blood oxygen level dependent (BOLD) responses to light stimuli were analyzed.

Results

At baseline, all subjects reported unpleasantness with light stimulation (average: 70.8 ± 32.0). Four to six weeks after BoNT-A injection, unpleasantness scores decreased (48.1 ± 33.6), but the change was not significant. On an individual level, 50% of subjects had decreased unpleasantness ratings in response to light stimulation compared to baseline ("responders," n = 6), while 50% had equivalent (n = 3) or increased (n = 3) unpleasantness ("non-responders"). At baseline, several differences were noted between responders and non-responders; responders had higher baseline unpleasantness ratings to light, higher symptoms of depression, and more frequent use of antidepressants and anxiolytics, compared to non-responders. Group analysis at baseline displayed light-evoked BOLD responses in bilateral primary somatosensory (S1), bilateral secondary somatosensory (S2), bilateral anterior insula, paracingulate gyrus, midcingulate cortex (MCC), bilateral frontal pole, bilateral cerebellar hemispheric lobule VI, vermis, bilateral cerebellar crus I and II, and visual cortices. BoNT-A injections significantly decreased light evoked BOLD responses in bilateral S1, S2 cortices, cerebellar hemispheric lobule VI, cerebellar crus I, and left cerebellar crus II. BoNT-A responders displayed activation of the spinal trigeminal nucleus at baseline where non-responders did not.

Discussion

BoNT-A injections modulate light-evoked activation of pain-related brain systems and photophobia symptoms in some individuals with chronic ocular pain. These effects are associated with decreased activation in areas responsible for processing the sensory-discriminative, affective, dimensions, and motor responses to pain.

A novel combination of corneal confocal microscopy, clinical features and artificial intelligence for evaluation of ocular surface pain

OBJECTIVES

To analyse various corneal nerve parameters using confocal microscopy along with systemic and orthoptic parameters in patients presenting with ocular surface pain using a random forest artificial intelligence (AI) model.

DESIGN

Observational, cross-sectional.

METHODS

Two hundred forty eyes of 120 patients with primary symptom of ocular surface pain or discomfort and control group of 60 eyes of 31 patients with no symptoms of ocular pain were analysed. A detailed ocular examination included visual acuity, refraction, slit-lamp and fundus. All eyes underwent laser scanning confocal microscopy (Heidelberg Engineering, Germany) and their nerve parameters were evaluated. The presence or absence of orthoptic issues and connective tissue disorders were included in the AI. The eyes were grouped as those (Group 1) with symptom grade higher than signs, (Group 2) with similar grades of symptoms and signs, (Group3) without symptoms but with signs, (Group 4) without symptoms and signs. The area under curve (AUC), accuracy, recall, precision and F1-score were evaluated.

RESULTS

Over all, the AI achieved an AUC of 0.736, accuracy of 86%, F1-score of 85.9%, precision of 85.6% and recall of 86.3%. The accuracy was the highest for Group 2 and least for Group 3 eyes. The top 6 parameters used for classification by the AI were microneuromas, immature and mature dendritic cells, presence of orthoptic issues and nerve fractal dimension parameter.

CONCLUSIONS

This study demonstrated that various corneal nerve parameters, presence or absence of systemic and orthoptic issues coupled with AI can be a useful technique to understand and correlate the various clinical and imaging parameters of ocular surface pain.

Developing a Measure to Quantify Ocular Pain Postoperatively: The Adaptation of the Ocular Pain Assessment Survey

Purpose

Since quantification and communication of ocular pain is important for a healthier patient follow-up and postoperative guidance, reliable measures like the Ophthalmic Pain Assessment Survey (OPAS) are needed to assess the outcome and management of different operations. To address that need, we carried out the adaptation of OPAS into Turkish to reach different age groups and backgrounds, widening the use of OPAS on patients who underwent an ophthalmic operation.

Methods

We used back-translation method and achieved cultural adaptation through content validity scoring by 5 independent ophthalmologists. The survey is then administered three times: preoperatively, postoperatively within 24 hours, and finally a week later in the follow-up visit. Validity is measured in comparison to Visual Analog Scale using Spearman's correlation coefficient and reliability is measured using Cronbach's alpha. Factor analysis is performed by principal component analysis and rotation is performed using Varimax method when necessary.

Results

We reached a total of 132 patients with a mean age of 64.2 years. Most of them underwent phacoemulsification (n = 83), followed by PRK (n = 37). Overall, the T-OPAS demonstrated good reliability (mean C. alpha: 0.830) and its correlation with the VAS was especially high (S. coeff. >0.5) in the first three sections in all three surveys. Factor analysis yielded 5 subscales, allowing us to shape the final form of T-OPAS.

Conclusion

Through this adaptation of OPAS into a foreign language, we present a reliable and valid tool for postoperative pain quantification, allowing objective measurement of pain in different populations such as the elderly.

Differentiating primary dry eye disease from ocular neuropathic pain: implications for symptom management

CLINICAL RELEVANCE

Eyecare practitioners' management of ocular surface disease is essential in managing increasing dry eye disease (DED) presentations including ocular neuropathic pain (ONP). Topical Proxymetacaine offers a simple, readily available and practical method of detecting ONP in practice and can be used to differentiate ONP from DED by eyecare practitioners, when accompanied with an anterior segment examination.

BACKGROUND

Differentiating DED from ONP presents a significant opportunity to eyecare providers, allowing appropriate treatment choices for more adequate symptom control, greater patient satisfaction, and reduced emergent re-presentations. This study aims to differentiate patients presenting with DED symptoms into DED or ONP using a simple diagnostic tool, which can be used in practice to allocate appropriate treatment options for the patient's respective condition. A comparison of the prevalence of presentations of ONP presenting with DED symptoms in hospital ophthalmology settings and in optometric primary care settings will also be made.

METHODS

Patients with symptoms of DED were opportunistically recruited as they presented to ophthalmology outpatient clinics and primary care optometric services. Patients were then categorised as DED, ONP, or mixed DED/ONP based on their subjective response 30 seconds post-Proxymetacaine Hydrochloride 2% instillation.

RESULTS

Twenty-one patients were recruited, including 12 patients from ophthalmologic outpatient clinics and nine patients from primary care optometric services. Twelve patients were identified to have primary DED, while nine patients were identified to have ONP. 43% of patients presenting with DED symptoms had features of neuropathic pain in ophthalmologic outpatient presentations compared to 44% of patients in primary care optometric services.

CONCLUSION

Categorising DED and ONP patients by their response to Proxymetacaine can be used as a simple diagnostic tool in guiding future patient management and can be indicative of their potential response to topical therapies. The use of topical Proxymetacaine and the resultant change in ocular pain score can facilitate selection of patients who may benefit from centrally acting neuropathic pain agents over topical ocular therapy.

Initial experiences using plasma rich in growth factors to treat keratoneuralgia

Keratoneuralgia, a clinical diagnosis of sensitized corneal pain without visible ocular surface damage, generally has minimal response to conventional therapies. Causes include refractive surgery and chronic dry eye. We evaluated the efficacy of Plasma Rich in Growth Factors (PRGF), a novel treatment prepared using a commercially available kit, in patients with keratoneuralgia. A retrospective chart review identified patients who had the clinical diagnosis of keratoneuralgia and were treated with PRGF for at least 3 months from October 2015 to April 2020 at a single academic institution. Both objective eye exam findings and concurrent treatments were obtained at baseline, 3 months, and final visit (if available). A questionnaire was administered to identified patients, including symptoms scores measured with a visual analog scale. The results of this survey and other objective findings were compared before and after PRGF treatment. 16 out of 32 patients (50%) with a mean follow-up period of 33 ± 26 months answered the questionnaire. Refractive surgeries were the cause of keratoneuralgia in 14 patients (87.5%), with LASIK the most common procedure (11 patients, 69%). There were no adverse events recorded or reported. Symptom scored by VAS in a modified Symptoms Assessment in Dry Eye questionnaire significantly decreased after PRGF use (85 ± 16 to 45 ± 33, p = 0.0002). Ten patients (63%) reported PRGF is superior to other therapy and would recommend to others. There were no significant trends in visual acuity, objective exam findings, or concurrent treatments after PRGF treatment. PRGF is safe and can potentially alleviate symptoms in patients with keratoneuralgia, a rare but devastating complication after refractive surgery. Prospective trial is indicated to explore PRGF as a potentially useful treatment for keratoneuralgia.

Understanding chronic ocular surface pain: An unmet need for targeted drug therapy

Chronic ocular surface pain (COSP) may be defined as a feeling of pain, perceived as originating from the ocular surface, that persists for >3 months. COSP is a complex multifactorial condition associated with several risk factors that may significantly interfere with an individual's daily activities, resulting in poor quality of life (QoL). COSP is also likely to have a high burden on patients with substantial implications on global healthcare costs. While patients may use varied terminology to describe symptoms of COSP, any ocular surface damage in the ocular sensory apparatus (nociceptive, neuropathic, inflammatory, or combination thereof) resulting in low tear production, chronic inflammation, or nerve abnormalities (functional and/or morphological), is typically associated with COSP. Considering the heterogeneity of this condition, it is highly recommended that advanced multimodal diagnostic tools are utilized to help discern the nociceptive and neuropathic pain pathways in order to provide targeted treatment and effective clinical management. The current article provides an overview of COSP, including its multifactorial pathophysiology, etiology, prevalence, clinical presentation, impact on QoL, diagnosis, current management, and unmet medical needs.

Corneal Confocal Microscopy Features and Tear Molecular Profile in Study Participants with Discordance between Ocular Surface Disease Clinical Signs and Discomfort

Various ocular surface conditions such as dry eye disease can present with severe discomfort and pain. However, it is clinically challenging to establish etiology and prescribe correct treatment in patients who have a lot of discordance between symptoms and signs. To understand the basis of such discordance, we stratified subjects with ocular surface pain based on concordance between the severity of signs and symptoms and evaluated corneal structural features and tear molecular factors. All subjects underwent slit lamp examination, dry eye evaluation, and ocular surface disease index (OSDI) scoring. Subjects were stratified into group 1—without symptoms or clinical signs; group 2—without symptoms but with signs; group 3—with similar severity of symptoms and signs; and group 4—with symptom severity greater than that of the signs. Laser scanning in vivo confocal microscopy (IVCM) and tear fluid analysis for soluble factors by multiplex ELISA was performed for all subjects. Patients with a higher grade of symptoms and signs showed increased corneal dendritic cell (cDC) density (p < 0.05) which was more pronounced in subjects with discordance between the symptoms and signs (group 4). A significantly higher proportion of microneuroma-like structures and cDC were observed in group 4. IL-17A levels were significantly elevated in the tears of subjects with more discomfort. Our results demonstrate that corneal IVCM and the measurement of tear film factors can help clinicians improve diagnosis and treatment choice. Stratifying patients with ocular surface discomfort on the basis of discordance between symptoms and clinical signs may help identify patients who need additional adjunctive targeted therapy to resolve their condition.

Ocular Pain Symptoms in Individuals With and Without a History of Refractive Surgery: Results From a Cross-Sectional Survey

PURPOSE

This study characterized ocular pain symptoms in individuals with and without a history of refractive surgery (RS) using a cross-sectional survey of individuals with ocular pain.

METHODS

A link to an anonymous survey was posted on a corneal neuralgia Facebook group that included individuals with ocular pain from any etiology and sent to individuals seen in our clinic with ocular pain. The survey asked about medical history, ocular pain symptoms (using standardized questionnaires), and treatment responses. Respondents were split into 2 groups based on a history of RS.

RESULTS

One hundred one individuals responded to the survey. The mean age for all respondents was 41.6 ± 15.6 years, and 50% reported a history of RS. A total of 46% of individuals with a history of RS reported that their ocular pain started within 1 month of surgery, with median pain duration of 36 (interquartile range 22-84) months. The median Dry Eye Questionnaire-5 (range 0-22) scores were 16 and 15 for the RS and no-RS groups, respectively. Most individuals in both groups characterized their pain as burning (score ≥1: RS, 86%; no-RS, 80%) and reported evoked pain to wind, light, or temperature (score ≥1: RS, 97%; no-RS, 85%). Fifty-nine of 101 individuals responded to treatment questions. Individuals in both groups reported >30% improvement in pain symptoms with some topical and systemic approaches.

CONCLUSIONS

Individuals with a history of RS developed ocular pain soon after surgery, which persisted for years. Symptom profiles were similar between those with and without RS. Topical and systemic approaches can treat pain in both groups.

Pain mechanisms and management in corneal cross-linking: a review

Though corneal collagen cross-linking (CXL) is an increasingly available and effective treatment for keratoconus, few reports have considered its impact on pain-related physiology in depth. This comprehensive narrative review summarises mechanisms underlying pain in CXL and clinical care possibilities, with the goal of future improvement in management of CXL-related pain. Postoperative pain associated with CXL is largely due to primary afferent nerve injury and, to a smaller extent, inflammation. Chronification of pain after CXL has not been reported, even as long-term nerve damage without regeneration following standard CXL treatment is frequently observed. The lack of pain chronification may be due to the minimally invasive nature of the procedure, with its rapidly recovering superficial corneal wound, and to the positive anti-inflammatory changes of the tear film that have been described after CXL. Different CXL approaches have been developed, with the transepithelial epithelial-on technique (epi-on) associated with less postsurgical pain than the gold standard, epithelial-off technique (epi-off). After the first few days, however, the difference in pain scores and need for analgesics between epi-on and epi-off disappear. Patients experience relatively high-intensity pain the first few days post-CXL, and many strategies for acute pain control following CXL have been studied. Currently, no method of pain management is considered superior or universally accepted. Acute pain following CXL is a recognised and clinically significant side effect, but few CXL studies have systematically investigated postoperative pain and its management. This review aims to improve patient pain outcomes following this increasingly common procedure.

Corneal Nerve Abnormalities in Painful Dry Eye Disease Patients

Background: This study aimed to compare the corneal nerve structural abnormalities detected using in vivo confocal microscopy (IVCM) in patients with neuropathic corneal pain (NCP) secondary to primary meibomian gland dysfunction (MGD) or autoimmune dry eye (AIDE). Methods: A two-stage retrospective nested case-control study was conducted. First, data from patients with either MGD or AIDE were assessed, selecting only cases with no corneal pain (VAS = 0) or severe pain (VAS ≥ 8). Ocular signs and symptoms of the 238 selected patients were compared between painful and painless cases. Next, painful patients with no corneal damage (Oxford score ≤ 1) were selected within each study group, defining the cases with NCP (i.e., "pain without stain"). IVCM images from all groups were compared with prospectively-recruited healthy controls, focusing on dendritiform cell density and nerve abnormalities (density, tortuosity, microneuromas). Results: AIDE patients had more ocular signs/symptoms than MGD patients. Compared with healthy controls, AIDE-related NCP patients showed increased nerve tortuosity and number of neuromas, whereas MGD-related NCP patients had reduced nerve density and increased number, perimeter, and area of microneuromas. Microneuromas were also observed in healthy controls. Furthermore, a higher number of microneuromas was found in MGD-related NCP compared to AIDE-related NCP or painless MGD. Conclusions: MGD-related NCP was associated with significantly more corneal nerve abnormalities than AIDE-related NCP or healthy controls. Although IVCM can be useful to detect NCP-related corneal nerve changes in such patients, the diagnosis of dry eye disease-related NCP will require an association of several IVCM-based criteria without relying solely on the presence of microneuromas.

Dry eye disease flares: A rapid evidence assessment

PURPOSE

Characteristics of periodic flares of dry eye disease (DED) are not well understood. We conducted a rapid evidence assessment to identify evidence for and characteristics of DED flares.

METHODS

Literature searches were performed in Embase® via Ovid®, MEDLINE®, and PubMed®. Clinical trials and observational studies published 2009-2019 were included if they investigated patients aged ≥18 years with clinically diagnosed DED who experienced a flare, defined as a temporary or transient episode of increased ocular discomfort, typically lasting days to a few weeks. Triggers of flares, patient-reported outcomes (symptoms), clinician-measured outcomes (signs), and changes in tear molecules were captured.

RESULTS

Twenty-one publications that included 22 studies met inclusion criteria. Five observational studies described evidence of DED flares in daily life, 5 studies reported changes following cataract/refractive surgery in patients with preoperative DED, and 12 studies employed controlled environment (CE) models. Real-world triggers of DED flares included air conditioning, wind, reading, low humidity, watching television, and pollution. CE chambers (dry, moving air) and surgery also triggered DED flares. Exacerbations of symptoms and signs of DED, assessed through varied measures, were reported during flares. Across studies, matrix metalloproteinase-9 and interleukin-6 increased and epidermal growth factor decreased during DED flares.

CONCLUSIONS

Evidence from 22 studies identified triggers and characteristics of DED flares. Further research is needed to assist clinicians in early diagnosis and treatment of patients experiencing flares.

Comprehensive Review and Update of Burning Eye Syndrome

Keratoconjunctivitis sicca ("dry eye") is a common (14%-30% of adults over age 48) though difficult to treat condition that causes both discomfort and disability with associated dryness, pain, and visual disturbances. Etiology is not clearly understood but is likely varied, with a subset of patients suffering from chronic neuropathic pain referred to as "burning eye syndrome." This review of existing literature summarizes the clinical presentation, natural history, pathophysiology, and treatment modalities of burning eye syndrome. Chronicity of burning eye syndrome is likely secondary to increased nociception from the cornea, decrease in inhibitory signals, and nerve growth factor expression alterations. Treatment centers around symptomatic alleviation and reduction of inflammation. Conservative treatments focus on well-being and perception and include exercise, acupuncture, and cognitive behavioral therapy. Topical treatment consists of the anti-adhesion T-cell antagonist lifitegrast, corticosteroids, and cyclosporine; all have moderate efficacy and good safety. Autologous serum eye drops are a second-line topical that may promote corneal and neural healing on top of symptomatic relief. When these treatments fail, patients may trial neuromodulation with transcranial magnetic stimulation. Despite general treatment safety, more research is needed to develop novel approaches to this condition, possibly focusing more directly on the neurological component.

Effects of Topical Gabapentin on Ocular Pain and Tear Secretion

Neuropathic ocular pain is a frequent occurrence in medium to severe dry eye disease (DED). Only palliative treatments, such as lubricants and anti-inflammatory drugs, are available to alleviate patients’ discomfort. Anesthetic drugs are not indicated, because they may interfere with the neural feedback between the cornea and the lacrimal gland, impairing tear production and lacrimation. Gabapentin (GBT) is a structural analog of gamma-amino butyric acid that has been used by systemic administration to provide pain relief in glaucomatous patients. We have already shown in a rabbit model system that its topic administration as eye drops has anti-inflammatory properties. We now present data on rabbits’ eyes showing that indeed GBT given topically as eye drops has analgesic but not anesthetic effects. Therefore, opposite to an anesthetic drug such as oxybuprocaine, GBT does not decrease lacrimation, but–unexpectedly–even stimulates it, apparently through the upregulation of acetylcholine and norepinephrine, and by induction of aquaporin 5 (AQP5) expression in the lacrimal gland. Moreover, data obtained in vitro on a primary human corneal epithelial cell line also show direct induction of AQP5 by GBT. This suggests that corneal cells might also contribute to the lacrimal stimulation promoted by GBT and participate with lacrimal glands in the restoration of the tear film, thus reducing friction on the ocular surface, which is a known trigger of ocular pain. In conclusion, GBT is endowed with analgesic, anti-inflammatory and secretagogue properties, all useful to treat neuropathic pain of the ocular surface, especially in case of DED.

Neuropathic corneal pain and dry eye: a continuum of nociception

Throughout the body, damage to peripheral nerves normally involved in nociception may produce a constellation of symptoms-including irritation, itchiness and pain. The neurobiological processes involved in corneal symptoms of dry eye (DE) and neuropathic corneal pain (NCP) have not been clearly considered in terms of nociceptive processing. The conventional underlying presumption is that a labelled line principle is responsible; that these distinct perceptions are hard coded by primary afferent inputs to the central nervous system. This presumption oversimplifies the neurobiological mechanisms underlying somatosensory perception. The labelled line perspective that DE represents a chronic pain condition does not make intuitive sense: how can an eye condition that is not painful in most cases be considered a pain condition? Does not chronic pain by definition require pain to be present? On the other hand, NCP, a term that clearly denotes a painful condition, has historically seemed to resonate with clinical significance. Both DE and NCP can share similar features, yet their differentiation is not always clear. As is often the case, clinical terms arise from different disciplines, with DE evolving from ophthalmological findings and NCP inspired by pain neurophysiology. This review evaluates the current definition of these terms, the rationale for their overlap and how the neurophysiology of itch impacts our understanding of these conditions as a continuum of the same disease. Despite the complexity of nociceptive physiology, an understanding of these mechanisms will allow us a more precise therapeutic approach.

Corneal Nerve Abnormalities in Ocular and Systemic Diseases

The trigeminal nerve gives rise to the corneal subbasal nerve system, which plays a crucial role in sensations of touch, pain, and temperature and in ocular healing processes. Technological advancements in instruments, in particular in vivo confocal microscopy and aethesiometry, have allowed for the structural and functional evaluation of corneal nerves in health and disease. Through application of these technologies in humans and animal models, structural and functional abnormalities have been detected in several ocular and systemic disorders, including dry eye disease (DED), glaucoma, migraine, and fibromyalgia. However, studies across a number of conditions have found that structural abnormalities do not always relate to functional abnormalities. This review will discuss instruments used to evaluate corneal nerves and summarize data on nerve abnormalities in a number of ocular and systemic conditions. Furthermore, it will discuss potential treatments that can alleviate the main manifestations of nerve dysfunction, namely ocular surface pain and persistent epithelial defects.

Ocular Neuropathic Pain: An Overview Focusing on Ocular Surface Pains

Objective

This paper reviews ocular pain with the main focus on ocular surface discomfort and dry eye pain. Anatomy, physiology, epidemiology, assessment, and treatment are discussed in this paper.

Methods

A PubMed search was conducted for studies published from 2000 to 2019 on the anatomy, pathophysiology, epidemiology, assessment, and treatment of ocular pain. Reviews, meta-analyses, and randomized clinical trials were included. Inclusion criteria focused on ocular surface discomfort, dry eye pain and neuropathic pain.

Results

A total of 112 articles were found through searches, 45 of which were selected and studied in this review.

Discussion

Pain in general can be acute or chronic. Acute pain is usually a physiologic response to a serious damage to the tissues and alleviates with pain relief treatments. Chronic pain is defined as the persistence of pain for more than three months. From another point of view, pain has been classified into either nociceptive or neuropathic. Nociceptive pain is a physiologic response to a noxious stimulus. Both central and peripheral nervous systems can be involved in the development of a neuropathic pain, which is characterized by positive or negative sensory signs, a pain perceived disproportionate to a noxious stimulus, and/or not responsive to analgesics. Chronic pain usually has a neuropathic component. Ocular surface pain is a well-known complaint after any corneal surgery. This is mainly due to abnormal regeneration of damaged corneal nerve endings and abnormal connections with adjacent nerve endings which produce spontaneous activity. Tear hyperosmolarity and the resultant ocular surface inflammation can also trigger voluntary activity of corneal nerve endings. Referral pain to the first and second division of the trigeminal nerve has been reported. Interference with vision and even sleep, which is out of proportion to the examination are among patients' complaints. All of these elements proposed the new concept of ocular neuropathic pain syndrome. The first step in conventional evaluation of ocular discomfort is search for tear insufficiency. Pathologies of lid and blinking as well as conjunctival irregularities should be addressed. Anti-inflammatory agents and, in resistant cases, systemic neuromodulators are shown to be helpful. Education on behavioral changes and reassurance are essential steps. Considering the neuropathic origin for the ocular pain, treatment modalities used for such pain in other parts of the body can be considered for this syndrome.

Ocular Surface Pain: A Narrative Review

Ocular surface pain is a frequent cause of visits to an eye care provider and has a substantial impact on healthcare cost, yet a complete understanding of its causative factors and tools for diagnostic workup are notably missing in many eye clinics. The cornea has the densest sensory innervation in the human body and has the potential to be a powerful producer of pain. Pain can manifest as a result of a noxious stimulus or disruption in the ocular surface anatomy (nociceptive pain), or it can result from abnormalities in the ocular surface neurosensory apparatus itself (neuropathic pain). Novel advances in neurobiology have sought to differentiate the two entities, particularly to identify when chronic dry eye symptomatology is driven by neuropathic ocular pain. In this review, we seek to provide an overview of the prevalence, physiologic factors, and management of ocular surface pain complaints.

Comparison of clinical characteristics of post-refractive surgery-related and post-herpetic neuropathic corneal pain

Purpose:

To compare the clinical characteristics and in vivo confocal microscopy (IVCM) findings of patients with neuropathic corneal pain (NCP) due to refractive surgery (RS-NCP) and herpetic eye disease (H-NCP) to controls.

Methods:

Sixteen patients with RS-NCP and 7 patients with H-NCP, and 37 healthy reference age- and sex-matched healthy controls were included to the study. The medical records were reviewed for demographic features, detailed disease history, ocular surface disease index (OSDI), ocular pain assessment survey (OPAS) scores. IVCM images of patients were analyzed and compared to reference controls by two masked observers.

Results:

The mean pain intensity score for the last 24 h (5.1 ± 2.4 vs. 3.9 ± 1.2; p = 0.27), last 2 weeks (6.1 ± 2.5 vs. 4.8 ± 2.3; p = 0.13) for RS-NCP vs. H-NCP respectively, and quality of life scores (p = 0.23) were similar in both groups. Quality of life, especially mood (p = 0.06) and enjoying life/relations to others (p = 0.10) were affected in both groups, but were not statistically significant between groups. The mean total nerve density was lower in RS-NCP (5,702.4 ± 4,599.0 μm/mm²) compared to their respective controls (26,422.8 ± 4,491.0; p < 0.001) and in the H-NCP group (2,149.5 ± 2,985.9) compared to their respective controls (22,948.8 ± 3,169.0; p < 0.001). Alterations in DC density were similar between all groups (38.3 ± 48.0 cells/mm² in RS-NCP, 61.0 ± 76.9 in H-NCP, p = 0.95).

Conclusion:

Neuropathic corneal pain patients due to refractive surgery show similar clinical characteristics, pain levels, quality of life impact, and IVCM findings as patients with NCP due to herpetic eye disease.

Impact of Attrition, Intercellular Shear in Dry Eye Disease: When Cells are Challenged and Neurons are Triggered

The mechanical component in the pathophysiology of dry eye disease (DED) deserves attention as an important factor. The lubrication deficit induced impaired mechano-transduction of lid pressure to the ocular surfaces may lead to the dysregulation of homeostasis in the epithelium, with sensations of pain and secondary inflammation. Ocular pain is possibly the first sign of attrition and may occur in the absence of visible epithelial damage. Attrition is a process which involves the constant or repeated challenge of ocular surface tissues by mechanical shear forces; it is enhanced by the thinning of corneal epithelium in severe DED. As a highly dynamic process leading to pain and neurogenic inflammation, the identification of the impact of attrition and its potential pathogenic role could add a new perspective to the current more tear film-oriented models of ocular surface disease. Treatment of DED addressing lubrication deficiencies and inflammation should also consider the decrease of attrition in order to stimulate epithelial recovery and neural regeneration. The importance of hyaluronic acid, its molecular characteristics, the extracellular matrix and autoregulative mechanisms in this process is outlined. The identification of the attrition and recognition of its impact in dry eye pathophysiology could contribute to a better understanding of the disease and optimized treatment regimens.

Identification of Differentially Expressed Genes and Key Pathways in the Dorsal Root Ganglion After Chronic Compression

Neuropathic pain (NP) is caused by primary or secondary impairment of the peripheral or central nervous systems. Its etiology is complex and involves abnormal patterns of gene expression and pathway activation. Using bioinformatics analysis, we aimed to identify NP-associated changes in genes and pathways in L4 and L5 dorsal root ganglia (DRG) in a rat model of NP induced by chronic compression of the DRG (CCD). Genome-wide transcriptional analyses were used to elucidate the molecular mechanisms underlying NP. We screened differentially expressed genes (DEGs) 7 days after CCD in comparison with sham-operated controls. Quantitative real-time polymerase chain reaction (RT-qPCR) and western blotting were used to confirm the presence of key DEGs. Kyoto Encyclopedia of Genes and Genomes (KEGG)-pathway analysis of DEGs and global signal transduction network analysis of DEGs were also conducted. The CCD group developed clear mechanical and thermal allodynia in the ipsilateral hind paw compared with the sham group. This comparison identified 1,887 DEGs, with 1156 upregulated and 731 downregulated DEGs, and 123 DEG-enriched pathways. We identified the key candidate genes that might play a role in the development of NP, namely syndecan 1 (Sdc1), phosphatidylinositol-4,5-bisphosphate 3-kinase, catalytic subunit gamma (Pi3k), Janus kinase 2 (Jak2), jun proto-oncogene, AP-1 transcription factor subunit (Jun), and interleukin 6 (IL-6) by analyzing the global signal transduction network. RT-qPCR and western blot analysis confirmed the microarray results. The DEGs Sdc1, Pi3k, Jak2, Jun, and IL-6, and the cytokine signaling pathway, the neuroactive ligand-receptor interaction, the toll-like receptor signaling pathway, and the PI3K-Akt signaling pathway may have decisive modulatory roles in both nerve regeneration and NP. These results provide deeper insight into the mechanism underlying NP and promising therapeutic targets for its treatment.

Allosteric Cannabinoid Receptor 1 (CB1) Ligands Reduce Ocular Pain and Inflammation

Cannabinoid receptor 1 (CB1) activation has been reported to reduce transient receptor potential cation channel subfamily V member 1 (TRPV1)-induced inflammatory responses and is anti-nociceptive and anti-inflammatory in corneal injury. We examined whether allosteric ligands, can modulate CB1 signaling to reduce pain and inflammation in corneal hyperalgesia. Corneal hyperalgesia was generated by chemical cauterization of cornea in wildtype and CB2 knockout (CB2-/-) mice. The novel racemic CB1 allosteric ligand GAT211 and its enantiomers GAT228 and GAT229 were examined alone or in combination with the orthosteric CB1 agonist Δ8-tetrahydrocannabinol (Δ8-THC). Pain responses were assessed following capsaicin (1 µM) stimulation of injured corneas at 6 h post-cauterization. Corneal neutrophil infiltration was also analyzed. GAT228, but not GAT229 or GAT211, reduced pain scores in response to capsaicin stimulation. Combination treatments of 0.5% GAT229 or 1% GAT211 with subthreshold Δ8-THC (0.4%) significantly reduced pain scores following capsaicin stimulation. The anti-nociceptive effects of both GAT229 and GAT228 were blocked with CB1 antagonist AM251, but remained unaffected in CB2-/- mice. Two percent GAT228, or the combination of 0.2% Δ8-THC with 0.5% GAT229 also significantly reduced corneal inflammation. CB1 allosteric ligands could offer a novel approach for treating corneal pain and inflammation.

MZF1 in the Dorsal Root Ganglia Contributes to the Development and Maintenance of Neuropathic Pain via Regulation of TRPV1

Previous studies have demonstrated that myeloid zinc finger 1 (MZF1) in the dorsal root ganglion (DRG) participates in neuropathic pain induced by chronic-constriction injury (CCI) via regulation of voltage-gated K⁺ channels (Kv). Emerging evidence indicates that transient receptor potential vanilloid 1 (TRPV1) is involved in the development and maintenance of neuropathic pain. Although it is known that the transcription of TRPV1 is regulated by Kruppel-like zinc-finger transcription factor 7 (Klf7)—and that the structure of TRPV1 is similar to that of Kv—few studies have systematically investigated the relationship between MZF1 and TRPV1 in neuropathic pain. In the present study, we demonstrated that CCI induced an increase in MZF1 and TRPV1 in lumbar-level 4/5 (L4/5) DRGs at 3 days post-CCI and that this increase was persistent until at least 14 days post-CCI. DRG microinjection of rAAV5-MZF1 into the DRGs of naïve rats resulted in a decrease in paw-withdrawal threshold (PWT) and paw-withdrawal latency (PWL) compared with that of the rAAV5-EGFP group, which started at four weeks and lasted until at least eight weeks after microinjection. Additionally, prior microinjection of MZF1 siRNA clearly ameliorated CCI-induced reduction in PWT and PWL at 3 days post-CCI and lasted until at least 7 days post-CCI. Correspondingly, microinjection of MZF1 siRNA subsequent to CCI alleviated the established mechanical allodynia and thermal hyperalgesia induced by CCI, which occurred at 3 days postinjection and lasted until at least 10 days postinjection. Microinjection of rAAV5-MZF1 increased the expression of TRPV1 in DRGs. Microinjection of MZF1 siRNA diminished the CCI-induced increase of TRPV1, but not P2X7R, in DRGs. These findings suggest that MZF1 may contribute to neuropathic pain via regulation of TRPV1 expression in DRGs.

Latent Sensitization in a Mouse Model of Ocular Neuropathic Pain

Purpose

Chronic ocular pain is poorly understood and difficult to manage. We developed a murine model of corneal surface injury (CSI)-induced chronic ocular neuropathic pain. The study focuses on changes in corneal nerve morphology and associated short- and long-term pain-like behavior after CSI.

Methods

CSI was induced in mice by local application of an alkali solution (0.75 N NaOH). Corneal nerve architecture, morphology, density, and length were studied. Eye-wiping was evaluated before and after CSI in response to hypertonic saline (2 M NaCl). Naltrexone (NTX) or Naloxone-methiodide (NLX-me), opioid receptor antagonists, were given subcutaneously (s.c., 3 mg/kg) or topically (eye drop, 100 μM), and then an eye-wiping test was performed.

Results

CSI caused partial corneal deinnervation followed by gradual reinnervation. Regenerated nerves displayed increased tortuosity, beading, and branching. CSI enhanced hypertonic saline-induced eye-wiping behavior compared to baseline or sham-injury (P < 0.01). This hypersensitivity peaked at 10 days and subsided 14 days after CSI. Administration of NTX, or NLX-me, a selective peripheral opioid antagonist, reinstated eye-wiping behavior in the injury group, but not in the sham groups (P < 0.05).

Conclusions

This study introduces a model of chronic ocular pain and corneal neuropathy following CSI. CSI induces central and peripheral opioid receptor-dependent latent sensitization (LS) that is unmasked by systemic or topical administration of opioid antagonists.

Translational Relevance

This model of chronic ocular pain establishes LS as a new inhibitory mechanism in the oculotrigeminal system and may be used for potential diagnostic and therapeutic interventions for ocular neuropathy.

Tivanisiran, a novel siRNA for the treatment of dry eye disease

INTRODUCTION

Dry eye disease (DED) is characterized by an alteration of the tear film with ocular inflammation and neurosensory abnormalities. The main clinical signs of this condition are tear instability and ocular damage. Although DED has gained significant attention in the past few years, limited prescription treatment options are available for patients. Areas covered: The current manuscript summarizes the pre-clinical and clinical development of tivanisiran, a novel small interfering oligonucleotide of RNA (siRNA) used for the treatment of DED. Tivanisiran was designed to silence Transient Receptor Potential Vanilloid 1 (TRPV1); herein the chemistry and mechanism of action of this new compound is also described. Expert opinion: Drugs currently on the market mostly target the inflammatory component of the disease and show only partial efficacy. New compounds addressing other aspects of the disease would provide significant advantages and contribute to a more personalized treatment of the disease. Tivanisiran has been designed to reduce ocular discomfort and pain, and was shown to improve ocular hyperemia and tear quality in human and animal models. Consequently, if the results of the ongoing and future clinical trials meet their study endpoints, tivanisiran could be submitted to obtain approval for the treatment of DED.