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

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.

Rethinking Sjögren Beyond Inflammation: Considering the Role of Nerves in Driving Disease Manifestations

ABSTRACT

Sjögren syndrome (SS) is a chronic inflammatory autoimmune disease characterized by destruction of mucosal glands resulting in dry eye and dry mouth. Ocular presentations can be heterogenous in SS with corneal nerves abnormalities that are structural, functional, or both. Some individuals present with corneal hyposensitivity, with a phenotype of decreased tear production and epithelial disruption. Others present with corneal hypersensitivity, with a phenotype of neuropathic pain including light sensitivity and pain out of proportion to signs of tear dysfunction. A similar correlate can be found outside the eye, with dry mouth predominating in some individuals while pain conditions predominate in others. Understanding how nerve status affects SS phenotype is an important first step to improving disease management by targeting nerve abnormalities, as well as inflammation.

Coexistence of neuropathic corneal pain, corneal nerve abnormalities, depression, and low quality of life

PURPOSE

To evaluate the quality of life (QoL), mental health conditions and corneal morphology in neuropathic corneal pain (NCP) subjects without a significant ocular surface disease.

METHODS

A composite questionnaire was administered to 228 consecutive subjects, assessing the pain intensity, duration, and quality using a modified version of the Self-Administered Leeds Assessment of Neuropathic Symptoms and Signs (S-LANSS) and Pain Detect (PD) questionnaires. Subjects diagnosed with possible central NCP and two sub-groups of patients diagnosed with peripheral ocular pain completed an additional battery of mental health questionnaires and were examined by In Vivo Confocal Microscopy (IVCM).

RESULTS

Of the 76 subjects that reported chronic ocular pain (duration >1 month), 53 were classified with probable NCP. Nine subjects without signs that justify the pain and non-responding to topical anaesthesia, were considered affected by central NCP. In these patients, a significant negative correlation was found between the presence pain and the mental component of the QoL (R2 = 0.733), and a positive correlation between the severity of pain the presence post-traumatic stress disorder (R2 = 0.83) and depression (R2 = 0.93). Although neuromas and sprouting had higher frequency in the central NCP group compared the control groups, these differences was not statistically different.

CONCLUSIONS

The assessment of ocular pain characteristics using multiple questionnaires and IVCM may help to recognize differences between nociceptive and neuropathic pain. An association between pain intensity and mental health condition may guide the therapeutical choices.

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.

Painful-blind eye: A forgotten palliative care

Painful-blind eye (PBE) is a challenging and debilitating condition that greatly affects the quality of life of patients. Although PBE can result from a variety of etiologies, currently there is no guideline or consensus on how to approach therapeutically these patients, and most treatments are experience-based. We summarized the evidence from available studies to investigate the current state of PBE treatment strategies. This review revealed that the information available about therapeutic approaches in patients with PBE is insufficient and outdated, therefore, new experimental and larger studies are needed to reach an agreement about this condition.

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.

Neuropathic pain and itch: mechanisms in allergic conjunctivitis

PURPOSE OF REVIEW

Allergic conjunctivitis is highly prevalent and affects up to one third of the general population. The current understanding of the pathophysiology and therapeutic strategies center around the type 2 inflammatory pathway. However, there is an increasing body of evidence that suggests neurogenic mechanisms also play a role in allergic inflammation, with a substantial proportion of allergic conjunctivitis patients experiencing both ocular itch and pain.

RECENT FINDINGS

Unmyelinated C fibres on the ocular surface transmit histaminergic itch and can be directly activated by mast cell mediators. The conjunctival mucosa also contains TRPV1+ (histamine-dependent) and TRPA1+ (histamine-independent) neurons that enhance ocular pain and itch in allergic conjunctivitis. Allergen-complexed IgE also binds directly to FcεRI expressed on peripheral neurons. Environmental aeroallergens can also directly stimulate neuronal nociceptors to release inflammatory substances. Allergic inflammation thus stimulates nerve terminals to release vasoactive and inflammatory neuropeptides, leading to a cyclical neuronal dysregulation that augments mast cell activity. These repetitive cycles lead to both peripheral and central sensitization and neuronal plasticity, resulting in decreased itch/pain thresholds and a heightened itch/pain response.

SUMMARY

Neurogenic mechanisms including peripheral and central sensitization may drive chronic ocular itch and pain secondary to allergic inflammation. Research into these pathways may help to identify therapeutic targets in allergic conjunctivitis patients with refractory symptoms.

Pathogenic Mechanism of Dry Eye-Induced Chronic Ocular Pain and a Mechanism-Based Therapeutic Approach

Purpose

Dry eye–induced chronic ocular pain is also called ocular neuropathic pain. However, details of the pathogenic mechanism remain unknown. The purpose of this study was to elucidate the pathogenic mechanism of dry eye–induced chronic pain in the anterior eye area and develop a pathophysiology-based therapeutic strategy.

Methods

We used a rat dry eye model with lacrimal gland excision (LGE) to elucidate the pathogenic mechanism of ocular neuropathic pain. Corneal epithelial damage, hypersensitivity, and hyperalgesia were evaluated on the LGE side and compared with the sham surgery side. We analyzed neuronal activity, microglial and astrocytic activity, α₂δ–1 subunit expression, and inhibitory interneurons in the trigeminal nucleus. We also evaluated the therapeutic effects of ophthalmic treatment and chronic pregabalin administration on dry eye–induced ocular neuropathic pain.

Results

Dry eye caused hypersensitivity and hyperalgesia on the LGE side. In the trigeminal nucleus of the LGE side, neuronal hyperactivation, transient activation of microglia, persistent activation of astrocytes, α₂δ–1 subunit upregulation, and reduced numbers of inhibitory interneurons were observed. Ophthalmic treatment alone did not improve hyperalgesia. In contrast, continuous treatment with pregabalin effectively ameliorated hypersensitivity and hyperalgesia and normalized neural activity, α₂δ–1 subunit upregulation, and astrocyte activation.

Conclusions

These results suggest that dry eye–induced hypersensitivity and hyperalgesia are caused by central sensitization in the trigeminal nucleus with upregulation of the α₂δ–1 subunit. Here, we showed that pregabalin is effective for treating dry eye–induced ocular neuropathic pain even after chronic pain has been established.

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.

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.

Topical TRPM8 Agonist for Relieving Neuropathic Ocular Pain in Patients with Dry Eye: A Pilot Study

Background: Activation of TRPM8, a cold-sensing receptor located on the cornea and eyelid, has the potential to relieve the neuropathic ocular pain (NOP) in dry eye (DE) by inhibiting other aberrant nociceptive inputs. We aimed to investigate the effect of a topical TRPM8 agonist, cryosim-3 (C3), on relieving DE-associated NOP. Methods: We conducted a prospective pilot study of 15 patients with DE-associated NOP. These patients applied topical C3 to their eyelid, 4 times/day for 1 month. The patients underwent clinical examinations. They also completed the Ocular Pain Assessment Survey (OPAS), which is a validated questionnaire for NOP, at baseline, 1 week, and 1 month after treatment. Result: At 1 week, the OPAS scores of eye pain intensity, quality of life (driving/watching TV, general activity, sleep, and enjoying life/relations with other people), and associated factors (burning sensation, light sensitivity, and tearing) improved. The total OPAS scores of eye pain intensity, quality of life, and associated factors remained improved at 1 month. The Schirmer test scores also improved at 1 month. Conclusion: TRPM8 agonist (C3) could be a novel agent for treating patients with DE-associated NOP who are unresponsive to conventional treatments.