Oral Gabapentinoids and Nerve Blocks for the Treatment of Chronic Ocular Pain

Molecular Mechanisms and Therapeutic Potential of Gabapentin with a Focus on Topical Formulations to Treat Ocular Surface Diseases

Gabapentin (GBP) was originally developed as a potential agonist for Gamma-Amino-Butyric-Acid (GABA) receptors, aiming to inhibit the activation of pain-signaling neurons. Contrary to initial expectations, it does not bind to GABA receptors. Instead, it exhibits several distinct pharmacological activities, including: (1) binding to the alpha-2-delta protein subunit of voltage-gated calcium channels in the central nervous system, thereby blocking the excitatory influx of calcium; (2) reducing the expression and phosphorylation of CaMKII via modulation of ERK1/2 phosphorylation; (3) inhibiting glutamate release and interfering with the activation of NMDA receptors; (4) enhancing GABA synthesis; (5) increasing cell-surface expression of δGABA_A receptors, contributing to its antinociceptive, anticonvulsant, and anxiolytic-like effects. Additionally, GBP displays (6) inhibition of NF-kB activation and subsequent production of inflammatory cytokines, and (7) stimulation of the purinergic adenosine A1 receptor, which supports its anti-inflammatory and wound-healing properties. Initially approved for treating seizures and postherpetic neuralgia, GBP is now broadly used for various conditions, including psychiatric disorders, acute and chronic neuropathic pain, and sleep disturbances. Recently, as an eye drop formulation, it has also been explored as a therapeutic option for ocular surface discomfort in conditions such as dry eye, neurotrophic keratitis, corneal ulcers, and neuropathic ocular pain. This review aims to summarize the evidence supporting the molecular effects of GBP, with a special emphasis on its applications in ocular surface diseases.

Genome Wide Association Study of Neuropathic Ocular Pain

Purpose

To conduct a genome-wide association study (GWAS) of individuals with neuropathic ocular pain (NOP) symptoms to identify genomic variants that may predispose to NOP development.

Design

Prospective study of individuals with NOP.

Participants

Three hundred twenty-nine patients recruited from the Miami Veterans Affairs eye clinic.

Methods

The Neuropathic Pain Symptom Inventory modified for the eye (NPSI-Eye) was completed to calculate a NPSI-Eye-Sub-Score (summed ratings of burning and wind sensitivity) as an indicator of NOP severity. A GWAS was performed for the NPSI-Eye-Sub-Score with a significance threshold of P < 5 × 10-8. A gene-based analysis was performed using the multimarker analysis of genomic annotation software (in the functional mapping and annotation of GWAS online platform). The 13 865 778 single nucleotide polymorphisms (SNPs) from our GWAS analysis were mapped to 10 834 protein coding genes, and significant genes were run through gene set enrichment analysis.

Main Outcome Measures

Identification of SNPs and protein products that may be associated with the development of NOP.

Results

One hundred seventy-one SNPs reached a threshold of P < 10-5, of which 10 SNPs reached the suggestive level of significance of P < 5 × 10-7 and 1 SNP met our genome-wide significance threshold of P < 5 × 10-8. This lead SNP, rs140293404 (P = 1.23 × 10-8), is an intronic variant found within gene ENSG00000287251 coding for transcript ENST00000662732.1. Rs140293404 is in linkage disequilibrium with exon variant rs7926353 (r2 > 0.8) within ENSG00000279046 coding for transcript ENST00000624288.1. The most significant genes from gene-based tests were matrix metalloproteinase-19 (MMP19) (P = 1.12 × 10-5), zinc finger RNA-binding motif and serine/arginine rich-1 (ZRSR1) (P = 1.48 × 10-4), CTC-487M23.8 (P = 1.79 × 10-4), receptor expression-enhancing protein-5 (REEP5) (P = 2.36 × 10-4), and signal recognition particle-19 (SRP19) (P = 2.56 × 10-4). From gene set enrichment analysis, the sensory perception (false discovery rate = 6.57 × 10-3) and olfactory signaling (false discovery rate = 1.63 × 10-2) pathways were enriched with the most significant genes.

Conclusions

Our GWAS revealed genes with protein products that may impact sensory perception, lending biological plausibility to a role for SNPs identified by our GWAS in the development of NOP. A better understanding of the biological relevance of these genes and pathways in the pathophysiology associated with NOP may facilitate future novel mechanism-based treatments.

Financial Disclosures

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Neurophysiology of corneal neuropathic pain and emerging pharmacotherapeutics

The altered activity generated by corneal neuronal injury can result in morphological and physiological changes in the architecture of synaptic connections in the nervous system. These changes can alter the sensitivity of neurons (both second-order and higher-order projection) projecting pain signals. A complex process involving different cell types, molecules, nerves, dendritic cells, neurokines, neuropeptides, and axon guidance molecules causes a high level of sensory rearrangement, which is germane to all the phases in the pathomechanism of corneal neuropathic pain. Immune cells migrating to the region of nerve injury assist in pain generation by secreting neurokines that ensure nerve depolarization. Furthermore, excitability in the central pain pathway is perpetuated by local activation of microglia in the trigeminal ganglion and alterations of the descending inhibitory modulation for corneal pain arriving from central nervous system. Corneal neuropathic pain may be facilitated by dysfunctional structures in the central somatosensory nervous system due to a lesion, altered synaptogenesis, or genetic abnormality. Understanding these important pathways will provide novel therapeutic insight.

Ocular surface itch and pain: key differences and similarities between the two sensations

PURPOSE OF REVIEW

To review the pathophysiology and treatment of ocular itch and pain, encompassing nociceptive and neuropathic categories.

RECENT FINDINGS

Ocular itch and pain are sensations that arise from activation of ocular surface polymodal nerves. Nociceptive itch, commonly comorbid with ocular pain complaints, is mainly driven by a histamine-mediated type 1 hypersensitivity reaction. Beyond topical therapy, novel drug delivery systems are being explored to improve ocular residence time of nonsteroidal anti-inflammatory drugs (NSAIDs) and antihistamines. Nociceptive ocular pain can be driven by a variety of factors. Treatment focuses on addressing the causative sources of pain. Neuropathic ocular itch and pain are driven by nerve damage and dysfunction and as such, topical and oral neuromodulation have been explored as treatments. Oral neuromodulators include alpha 2 delta ligands, tricyclic antidepressants (TCAs), and low dose naltrexone. Novel therapies are being evaluated for both modalities such as difelikefalin (κ-opioid receptor agonist) for neuropathic itch and libvatrep (transient receptor potential vanilloid 1 antagonist) for neuropathic pain.

SUMMARY

Both ocular itch and pain can be driven by nociceptive and/or neuropathic mechanisms. Identifying contributors to abnormal ocular sensations is vital for precise medical care. Novel therapeutics for these conditions aim to improve patient outcomes and quality of life.

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.

Novel Treatments for Chronic Ocular Surface Pain

ABSTRACT

Several etiologies can contribute to ocular surface pain including nociceptive, peripheral neuropathic, and central neuropathic mechanisms. Clinical clues can help identify contributors to ocular surface pain in a patient. In individuals whose pain persists despite targeting nociceptive contributors, neuropathic mechanisms should be considered and addressed using oral, topical, and/or adjuvant agents.

Ocular pain after refractive surgery: interim analysis of frequency and risk factors

PURPOSE

To examine the frequency and risk factors for ocular pain after laser assisted in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK).

METHODS

Prospective study of individuals undergoing refractive surgery at two different centers. Participants rated their ocular pain on a 0 - 10 Numerical Rating Scale (NRS) pre-surgery and 1 day, 3 months, and 6 months post-surgery. A clinical examination focused on ocular surface health was performed 3- and 6-months post-surgery. Persistent ocular pain was defined as an NRS score ≥ 3 at both 3- and 6-months post-surgery (cases) and this group was compared to individuals with NRS scores <3 at both time points (controls).

RESULTS

109 individuals underwent refractive surgery (87% LASIK; 13% PRK) and were followed for 6 months after surgery. Mean age was 34±8 years (range 23 to 57); 62% self-identified as female, 81% as White, 33% as Hispanic. Eight (7%) individuals reported ocular pain (NRS ≥ 3) prior to surgery, with the frequency of ocular pain increasing after surgery to 23% (n=25) at 3 months and 24% (n=26) at 6 months. Twelve individuals (11%) reported an NRS ≥ 3 at both time points and comprised the persistent pain group. Factors that predicted persistent pain after surgery in a multivariable analysis were: (a) ocular pain pre-surgery (b) symptom report of depression prior to surgery (Patient Health Questionnaire-9: PHQ9: odds ratio (OR) 1.3, 95% confidence interval (CI) 1.1-1.6, p=0.01); (c) use of an oral anti-allergy medication pre-surgery (OR 13.6, 95% CI 2.1-89.3, p=0.007). (d) pain intensity day 1 after surgery (OR: 1.6, 95% CI 1.2-2.2, p=0.005); Ocular surface signs of tear dysfunction were not related to pain complaints. Most individuals (>90%) were completely or somewhat satisfied with their vision at 3 and 6 months.

CONCLUSIONS

Eleven percent of individuals reported persistent ocular pain after refractive surgery, with several pre- and peri-operative factors predicting pain after surgery.

Efficacy and safety of pregabalin in eye pain: A systematic review

BACKGROUND

The pregabalin is approved for the management of persistent pain. The aim of this study is to assess the advantages and disadvantages of the use of pregabalin in eye pain management.

METHODS

The PubMed, Cochrane Library, Embase, and Web of Science databases were searched until January 2022 for randomized controlled trials. Randomized, double-blinded trials comparing pregabalin with placebo in eye pain management were included. The primary outcome was visual analog scale or numerical rating scale at acute (24 hours) and chronic (≥7 days after surgery) timepoints. The secondary outcomes were analgesic medication requirements and pregabalin-related complications (nausea, vomiting, dizziness, and headache). We also compared the effect of pregabalin on dry-eye syndrome.

MAIN RESULTS

Six relevant articles were identified that studied the use of pregabalin as pain relief for photorefractive keratectomy (n = 2), laser epithelial keratomileusis (n = 1), laser-assisted in situ keratomileusis (n = 1), eyelid surgery (n = 1), and dacryocystorhinostomy (n = 1). Pregabalin was associated with a significant reduction in pain scores (95% confidence interval = -0.41 [-0.76--0.06]) 24 hours after surgical procedures. The data were insufficient to draw conclusions regarding dry eye symptoms. Because of the high heterogeneity of outcomes regarding adverse effects, there is no conclusion regarding the safety of pregabalin in eye pain.

CONCLUSIONS

Pregabalin reduced acute eye pain but had no significant effect on long-term analgesia after ophthalmological surgery in adults. It had no effect on dry-eye symptoms after ocular surgery. Further studies on the safety of pregabalin in eye pain management are required to draw solid conclusions.

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.

Neuropathic ocular surface pain: Emerging drug targets and therapeutic implications

INTRODUCTION

Dysfunction at various levels of the somatosensory system can lead to ocular surface pain with a neuropathic component. Compared to nociceptive pain (due to noxious stimuli at the ocular surface), neuropathic pain tends to be chronic and refractory to therapies, making it an important source of morbidity in the population. An understanding of the options available for neuropathic ocular surface pain, including new and emerging therapies, is thus an important topic.

AREAS COVERED

This review will examine studies focusing on ocular surface pain, emphasizing those examining patients with a neuropathic component. Attention will be placed toward recent (after 2017) studies that have examined new and emerging therapies for neuropathic ocular surface pain.

EXPERT OPINION

Several therapies have been studied thus far, and continued research is needed to identify which individuals would benefit from specific therapies. Gaps in our understanding exist, especially with availability of in-clinic diagnostics for neuropathic pain. A focus on improving diagnostic capabilities and researching gene-modulating therapies could help us to provide more specific mechanism-based therapies for patients. In the meantime, continuing to uncover new modalities and examining which are likely to work depending on pain phenotype remains an important short-term goal.

Dry Eye Symptoms and Signs in US Veterans With Gulf War Illness

PURPOSE

To examine dry eye (DE) symptoms and signs in individuals with vs without Gulf War illness (GWI).

DESIGN

Prospective cross-sectional study.

METHODS

We performed a prospective, cross-sectional study of South Florida veterans who were active duty during the Gulf War era (GWE; 1990-1991) and seen at an eye clinic between October 1, 2020, and March 13, 2021. Veterans were split into 2 groups: those who met Kansas criteria for GWI (cases, n = 30) and those who did not (controls, n = 41). DE symptoms were assessed via standardized questionnaires whereas DE signs were assessed using a series of ocular surface parameters. Differences between groups were assessed via Mann-Whitney U test. Linear regression analyses were used to examine which GWI symptoms most closely aligned with DE symptoms.

RESULTS

Veterans with GWI had higher DE symptoms scores compared to controls (Ocular Surface Disease Index [OSDI] scores: mean 41.20±22.92 vs 27.99±24.03, P = .01). In addition, veterans with GWI had higher eye pain scores compared with controls (average eye pain over past week: 2.63±2.72 vs 1.22±1.50, P = .03), including on neuropathic ocular pain questionnaires (Neuropathic Pain Symptom Inventory modified for the Eye [NPSI-E]: 17.33±17.20 vs 9.63±12.64, P = .03). DE signs were mostly similar between the groups. GWI symptoms "nausea or upset stomach" (β=14.58, SE = 3.02, P < .001) and "headache" (β=7.90, SE = 2.91, P = .011) correlated with higher OSDI scores.

CONCLUSION

Individuals with GWI have more severe DE symptoms and ocular pain scores but similar tear and ocular surface parameters compared to controls without GWI. This finding suggests that mechanisms beyond tear dysfunction drive eye symptoms in GWI.

Self-Report of Severity of Ocular Pain Due to Light as a Predictor of Altered Central Nociceptive System Processing in Individuals With Symptoms of Dry Eye Disease

Dry eye disease (DED) is a diagnosis given to individuals with a heterogeneous combination of symptoms and/or signs, including spontaneous and evoked ocular pain. Our current study evaluated whether and which ocular pain assessments could serve as screening tools for central sensitization in individuals with DED. A cohort of individuals with DED symptoms (n = 235) were evaluated for ocular pain, DED signs (tear production, evaporation), evoked sensitivity to mechanical stimulation at the cornea, and evidence of central sensitization. Central sensitization was defined for this study as the presence of pain 30 seconds after termination of a thermal noxious temporal summation protocol (ie, aftersensations) presented at a site remote from the eye (ventral forearm). We found that combining ratings of average intensity of ocular pain, ratings of average intensity of pain due to light, response to topical anesthetic eye drops, and corneal mechanical pain thresholds produced the best predictive model for central sensitization (area under the curve of .73). When examining ratings of intensity of ocular pain due to light alone (0-10 numerical rating), a cutoff score of 2 maximized sensitivity (85%) and specificity (48%) for the presence of painful aftersensations at the forearm. Self-reported rating of pain sensitivity to light may serve as a quick screening tool indicating the involvement of central nociceptive system dysfunction in individuals with DED. PERSPECTIVE: This study reveals that clinically-relevant variables, including a simple 0 to 10 rating of ocular pain due to light, can be used to predict the contribution of central sensitization mechanisms in a subgroup of individuals with DED symptoms. These findings can potentially improve patient stratification and management for this complex and painful disease.

How Should Corneal Nerves be Incorporated Into the Diagnosis and Management of Dry Eye?

Purpose

a)Confocal microscopy and aethesiometry have allowed clinicians to assess the structural and functional integrity of corneal nerves in health and disease. This review summarizes literature on nerves in dry eye disease (DED) and discusses how this data can be applied to DED diagnosis and treatment.

Recent findings

b)Subjects with DED have a heterogenous symptom and sign profile along with variability in nerve structure and function. Most studies have reported lower nerve density and sensitivity in aqueous tear deficiency, while findings are more inconsistent for other DED subtypes. Examining nerve status, along with profiling symptoms and signs of disease, can help categorize subjects into disease phenotypes (structural and functional patterns) that exist under the umbrella of DED. This, in turn, can guide therapeutic decision-making.

Summary

c)Due to the heterogeneity in symptoms and signs of DED, corneal nerve evaluations can be valuable for categorizing individuals into disease sub-types and for guiding clinical decision making.

Differential Effects of Treatment Strategies in Individuals With Chronic Ocular Surface Pain With a Neuropathic Component

Background: Dysfunction at the ocular system via nociceptive or neuropathic mechanisms can lead to chronic ocular pain. While many studies have reported on responses to treatment for nociceptive pain, fewer have focused on neuropathic ocular pain. This retrospective study assessed clinical responses to pain treatment modalities in individuals with neuropathic component ocular surface pain. Methods: 101 individuals seen at the University of Miami Oculofacial Pain Clinic from January 2015 to August 2021 with ≥3 months of clinically diagnosed neuropathic pain were included. Patients were subcategorized (postsurgical, post-traumatic, migraine-like, and laterality) and self-reported treatment outcomes were assessed (no change, mild, moderate, or marked improvement). One-way ANOVA (analysis of variance) was used to examine relationships between follow up time and number of treatments attempted with pain improvement, and multivariable logistic regression was used to assess which modalities led to pain improvement. Results: The mean age was 55 years, and most patients were female (64.4%) and non-Hispanic (68.3%). Migraine-like pain (40.6%) was most common, followed by postsurgical (26.7%), post-traumatic (16.8%) and unilateral pain (15.8%). The most common oral therapies were α2δ ligands (48.5%), the m common topical therapies were autologous serum tears (20.8%) and topical corticosteroids (19.8%), and the most common adjuvant was periocular nerve block (24.8%). Oral therapies reduced pain in post-traumatic (81.2%), migraine-like (73%), and unilateral (72.7%) patients, but only in a minority of postsurgical (38.5%) patients. Similarly, topicals improved pain in post-traumatic (66.7%), migraine-like (78.6%), and unilateral (70%) compared to postsurgical (43.7%) patients. Non-oral/topical adjuvants reduced pain in postsurgical (54.5%), post-traumatic (71.4%), and migraine-like patients (73.3%) only. Multivariable analyses indicated migraine-like pain improved with concomitant oral α2δ ligands and adjuvant therapies, while postsurgical pain improved with topical anti-inflammatories. Those with no improvement in pain had a shorter mean follow-up (266.25 ± 262.56 days) than those with mild (396.65 ± 283.44), moderate (652 ± 413.92), or marked improvement (837.93 ± 709.35) (p < 0.005). Identical patterns were noted for number of attempted medications. Conclusion: Patients with migraine-like pain frequently experienced pain improvement, while postsurgical patients had the lowest response rates. Patients with a longer follow-up and who tried more therapies experienced more significant relief, suggesting multiple trials were necessary for pain reduction.

Alternative therapies for dry eye disease

PURPOSE OF REVIEW

Dry eye disease (DED) is a multifactorial disease affecting approximately 5-50% of individuals in various populations. Contributors to DED include, but are not limited to, lacrimal gland hypofunction, meibomian gland dysfunction (MGD), ocular surface inflammation, and corneal nerve dysfunction. Current DED treatments target some facets of the disease, such as ocular surface inflammation, but not all individuals experience adequate symptom relief. As such, this review focuses on alternative and adjunct approaches that are being explored to target underlying contributors to DED.

RECENT FINDINGS

Neuromodulation, stem cell treatments, and oral royal jelly have all been studied in individuals with DED and lacrimal gland hypofunction, with promising results. In individuals with MGD, devices that provide eyelid warming or intense pulsed light therapy may reduce DED symptoms and signs, as may topical Manuka honey. For those with ocular surface inflammation, naturally derived anti-inflammatory agents may be helpful, with the compound trehalose being farthest along in the process of investigation. Nerve growth factor, blood-derived products, corneal neurotization, and to a lesser degree, fatty acids have been studied in individuals with DED and neurotrophic keratitis (i.e. corneal nerve hyposensitivity). Various adjuvant therapies have been investigated in individuals with DED with neuropathic pain (i.e. corneal nerve hypersensitivity) including nerve blocks, neurostimulation, botulinum toxin, and acupuncture, although study numbers and design are generally weaker than for the other DED sub-types.

SUMMARY

Several alternatives and adjunct DED therapies are being investigated that target various aspects of disease. For many, more robust studies are required to assess their sustainability and applicability.

Photophobia: shared pathophysiology underlying dry eye disease, migraine and traumatic brain injury leading to central neuroplasticity of the trigeminothalamic pathway

BACKGROUND

Photophobia is a potentially debilitating symptom often found in dry eye disease (DE), migraine and traumatic brain injury (TBI).

METHODS

We conducted a review of the literature via a PubMed search of English language articles with a focus on how photophobia may relate to a shared pathophysiology across DE, migraine and TBI.

RESULTS

DE, migraine and TBI are common conditions in the general population, are often comorbid, and share photophobia as a symptom. Across the three conditions, neural dysregulation of peripheral and central nervous system components is implicated in photophobia in various animal models and in humans. Enhanced activity of the neuropeptide calcitonin gene-related peptide (CGRP) is closely linked to photophobia. Current therapies for photophobia include glasses which shield the eyes from specific wavelengths, botulinum toxin, and inhibition of CGRP and its receptor. Many individuals have persistent photophobia despite the use of these therapies, and thus, development of new therapies is needed.

CONCLUSIONS

The presence of photophobia in DE, migraine and TBI suggests shared trigeminothalamic pathophysiologic mechanisms, as explained by central neuroplasticity and hypersensitivity mediated by neuropeptide CGRP. Treatment strategies which target neural pathways (ie, oral neuromodulators, transcutaneous nerve stimulation) should be considered in patients with persistent photophobia, specifically in individuals with DE whose symptoms are not controlled with traditional therapies.

Long-Term Trigeminal Nerve Stimulation as a Treatment for Ocular Pain

OBJECTIVES

Ocular pain symptoms (e.g., hypersensitivity to light and wind, "burning" sensations) can be debilitating and difficult to treat. Neuromodulatory therapies targeting sensory trigeminal and central pain pathways may help treat chronic ocular pain refractory to traditional therapies. The current study evaluates the long-term effects of a trigeminal neurostimulator (TNS) on ocular pain.

MATERIALS AND METHODS

Retrospective review of 18 individuals at the Miami Veterans Affairs Eye Clinic with chronic, severe ocular pain who were prescribed and used TNS at home for ≥3 months. The primary outcome measures were 1) ocular symptom intensity over a 24-hour recall period (dryness, pain, light sensitivity, wind sensitivity, burning; rated on 0-10 scales) captured pre-TNS and at monthly follow-up intervals and 2) side effects. The frequency and duration of TNS was a secondary outcome measure.

RESULTS

The mean age of the population (n = 18) was 57.5 years (range, 34-85 years) with a male majority (67%). Two individuals discontinued use due to lack of efficacy and one due to confounding health issues. Initial mean weekly frequency of TNS use was 3.7 ± 1.9 sessions of 25.8 min at month 1 and 2.7 ± 2.3 sessions of 28.0 min at month 6. At six months, pain intensity (↓ 31.4%), light sensitivity (↓ 36.3%), wind sensitivity (↓ 32.6%), and burning sensation (↓ 53.9%) were all decreased compared to baseline (p < 0.01 for all); greater decreases in ocular pain were noted in individuals with migraine (n = 10) than those without migraine (n = 8). No significant change was noted in mean dryness scores. Fifteen subjects experienced sedation with TNS use, persisting throughout the follow-up visits. No other adverse effects were communicated by any subjects.

CONCLUSION

Our study suggests TNS is a safe, adjunctive treatment option in individuals with severe, chronic ocular pain. Individuals demonstrated gradual, continual improvement in pain symptoms over time within a multimodal approach.

Dry eye: why artificial tears are not always the answer

Dry eye disease (DED) is a multifactorial disease that manifests in patients with a variety of symptoms and signs such as ocular pain, visual issues, rapid tear evaporation and/or decreased tear production. It is a global health problem and is the leading cause of optometry and ophthalmology clinic visits. The mainstay therapy for DED is artificial tears (ATs), which mimics tears and improves tear stability and properties. ATs have been found to improve symptoms and signs of disease in all DED subtypes, including aqueous deficient DED and evaporative DED. However, given the heterogeneity of DED, it is not surprising that ATs are not effective in all patients. When AT fails to relieve symptoms and/or signs of DED, it is critical to identify the underlying contributors to disease and escalate therapy appropriately. This includes underlying systemic diseases, meibomian gland dysfunction, anatomical abnormalities and neuropathic dysfunction. Thus, this review will discuss the benefits and limitations of ATs and review conditions when escalation of therapy should be considered in DED.

Exploring the Link Between Dry Eye and Migraine: From Eye to Brain

Dry eye and migraine are common diseases with large societal and economic burdens that have recently been associated in the literature. This review outlines the link between dry eye and migraine, which may have implications for reducing their respective burdens. We highlight possible shared pathophysiology, including peripheral and central sensitization, as the potential link between dry eye and migraine. Finally, therapies targeting similar pathophysiological mechanisms between dry eye and migraine are discussed.

Low-dose naltrexone is effective and well-tolerated for modulating symptoms in patients with neuropathic corneal pain

PURPOSE

Neuropathic corneal pain (NCP) is caused by damage or disease of the somatosensory nervous system that innervates the cornea and presents with symptoms of pain or persistent unpleasant sensations, such as burning, dryness, or light sensitivity. This retrospective study aims to assess the efficacy and tolerability of low-dose naltrexone (LDN) in refractory NCP patients.

METHODS

Fifty-nine NCP patients with a centralized component treated with oral LDN 4.5  mg at bedtime for at least four weeks were identified. Thirty out of 59 patients who had a baseline pain score ≥4 on the visual analogue scale had completed the ocular pain assessment survey (OPAS) and presented persistent pain, despite instillation of topical anesthetic drops, were included. Changes in pain scores, comorbidities, side effects, among others, were analyzed. Change in ocular pain scores (scale 0-10) and quality of life (QoL) scores (scale 0-100%) were the main endpoints.

RESULTS

Mean age (years ± SD) was 45.60 ± 19.30 with a white (80.00%) female (73.33%) predominance. Duration of LDN use was 14.87 ± 11.25 months, and the duration of NCP before treatment was 17.53 ± 17.29 months. Eight patients used LDN as a monotherapy, whereas the remaining used it as an adjunct therapy. LDN resulted in a 49.22% decrease in mean pain score from 6.13 ± 1.93 to 3.23 ± 2.60 (p < 0.001). Mean QoL scores by the OPAS were 5.84 ± 2.57 at the first visit and improved to 3.77 ± 2.91 at the last visit (p = 0.023). Common side effects were vivid dreams, headaches, and stomachache.

CONCLUSION

LDN was effective and well-tolerated for NCP treatment.

Treatment Response to Gabapentin in Neuropathic Ocular Pain Associated with Dry Eye

PURPOSE

To investigate the response to gabapentin treatment in patients with dry eye (DE) accompanied by features of neuropathic ocular pain (NOP), and to analyze the differences between clinical manifestations of the groups according to treatment response.

METHODS

We retrospectively reviewed the records of 35 patients with DE accompanied by NOP features and obtained information on their medical history and previous ocular history. The patients underwent clinical examinations of the tear film, ocular surface, and meibomian gland and completed the Ocular Pain Assessment Survey (OPAS). One month after treatment with topical eye drops, add-on of gabapentin treatment was determined according to the Wong-Baker FACES Pain Rating Scale (WBFPS). A reduction of 2 points or more on the WBFPS was considered a positive treatment response. Enrolled patients were divided into three groups according to the treatment response: topical treatment response group (group 1, n = 11); gabapentin response group (group 2, n = 13); and gabapentin non-response group (group 3, n = 11). The medical history, clinical parameters, and OPAS scores were compared between groups.

RESULTS

The incidence of systemic comorbidities was higher in group 2 than in other groups. The corneal staining scores were lower in groups 2 and 3 than in group 1. Among the treatment response groups, group 2 showed improvements in OPAS scores of ocular pain severity, pain other than eyes, and quality of life, while group 1 showed improved OPAS scores of ocular pain severity and ocular associated factors. Group 2 exhibited lower scores of pains aggravated by mechanical and chemical stimuli than group 3.

CONCLUSIONS

Gabapentin could be effective in patients who have systemic comorbidity and less pain evoked by mechanical and chemical stimuli for the treatment of DE patients with NOP, which is refractory to topical treatment.

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.

Periorbital botulinum toxin A improves photophobia and sensations of dryness in patients without migraine: Case series of four patients

Purpose

Individuals receiving botulinum toxin A (BoNT-A) injections in the head and neck for migraine treatment have reported decreases in photophobia and sensations of dryness, independent of ocular surface parameters. We hypothesized that patients without migraine but with similar ocular neuropathic-like symptoms would also experience symptomatic improvement with periocular BoNT-A injections, independent of ocular surface changes.

Observations

We identified four individuals without a history of migraine but with neuropathic ocular pain (symptoms of dryness, burning, and photophobia that were out of proportion to ocular surface findings and unresponsive to ongoing dry eye (DE) therapies). Individuals underwent 1 session of periocular BoNT-A injections. Validated questionnaires (Visual Light Sensitivity Questionnaire-8, Dry Eye Questionnaire-5) assessed photophobia and DE symptoms pre- and 1-month post-injections. All four reported improvements in frequency and severity of photophobia and eye discomfort following BoNT-A injections. Tear film parameters (phenol red thread test, tear break-up time, corneal staining, and Schirmer test) and eyelid (palpebral fissure height and levator palpebrae superioris function) and eyebrow (position) anatomy were also evaluated before and after injections. Despite a unanimous improvement in symptoms, there were no consistent changes in ocular surface parameters with BoNT-A injections across individuals.

Conclusions

Periocular BoNT-A shows promise in reducing photophobia and sensations of dryness in individuals with neuropathic-like DE symptoms without a history of migraine, independent of tear film, eyelid, or eyebrow parameters.