NEUROPATHIC PAIN (NP) is defined as pain caused by a lesion or disease of the somatosensory nervous system (International Association for the Study of Pain [IASP], 2024). NP affects upwards of 17% of the general population (Cavalli et al, 2019). Common conditions causing NP include diabetes (resulting in peripheral neuropathy, herpes zoster infection, and multiple sclerosis (Galor et al, 2016).

Those who have NP may experience allodynia (i.e., increased pain sensation from stimuli that do not normally provoke pain) and/or paresthesia, which causes tingling or numbness. Functional sensory, motor, and cognitive deficits may also be present with NP (Finnerup et al, 2021).

Due to rich innervation, the cornea can be affected by NP. This is known as neuropathic corneal pain (NCP) and may be perceived as ocular pain, discomfort, aching, photoallodynia, burning, irritation, dryness, and grittiness. Symptoms of NCP overlap with those of dry eye disease (DED) (Dieckmann et al, 2017).

Diagnosing NCP is challenging because patients present with minimal or absent signs. Nevertheless, NCP can be diagnosed based on careful history, symptoms, biomicroscopic exam, and evidence of nerve disease or lesion (Rosenthal et al, 2016).

Of note, NCP is commonly comorbid with behavioral conditions including anxiety, depression, and post-traumatic stress disorder (Crane et al, 2017). For example, a patient who has NCP may report prolonged dry eye treatment with no improvement; have a history of a causal event (e.g., eye infection or surgery); or relate non-ocular pain, neurological symptoms, or a psychiatric condition.

 Determining the locality of the pain is important for prescribing appropriate treatment. NCP may have a peripheral origin (e.g., corneal ocular surgery) or a systemic origin (e.g., fibromyalgia) (Dieckmann et al, 2017). Also, it may originate in both peripheral and centralized areas (Dieckmann et al, 2017).

A simple diagnostic test using topical 0.5% proparacaine hydrochloride may help to determine location origin (Goyal and Hamrah, 2016). Because topical proparacaine temporarily anesthetizes corneal innervation, it will block pain related to peripheral sensitization. However, it will not affect pain from central sensitization. This easy in-office test may, therefore, help to differentiate central from peripheral NCP (Goyal and Hamrah, 2016). 

Those who have peripheral NCP should be considered for scleral lens therapy. The large GP lens vaults the cornea, shielding it from the external environment. Immediate symptom relief is thought to be due to the lens protecting corneal nociceptors from stimuli such as shearing eyelid forces (Romero-Rangel et al, 2000). Additionally, wearing scleral lenses decreases light sensitivity and discomfort in 92% of patients who have ocular surface disease (Romero-Rangel et al, 2000). Figure 1 shows a patient who has mixed NCP associated with small fiber neuropathy and is being protected from pain by scleral lenses. 

NCP is a complex pain condition with symptoms that overlap with those of DED. Scleral lenses may provide immediate relief of NCP associated with peripheral sensitization. Nevertheless, this pain may also present with central sensitization and thus should also include treatment of the associated systemic disease.

References

1. IASP. Terminology /Neuropathic Pain. 2024. Available at iasp-pain.org/terminology?navItemNumber=576. Accessed 2024 May 21.

2. Cavalli E, Mammana S, Nicoletti F, Bramanti P, Mazzon E. The neuropathic pain: An overview of the current treatment and future therapeutic approaches. Int J Immunopathol Pharmacol. 2019 Jan-Dec;33:2058738419838383.

3. Galor A, Covington D, Levitt AE, et al. Neuropathic Ocular Pain due to Dry Eye is Associated with Multiple Comorbid Chronic Pain Syndromes. J Pain. 2016 Mar;17:310-318.

4. Finnerup NB, Kuner R, Jensen TS. Neuropathic Pain: From Mechanisms to Treatment. Physiol Rev. 2021 Jan 1;101:259-301.

5. Dieckmann G, Goyal S, Hamrah P. Neuropathic Corneal Pain: Approaches for Management. Ophthalmology. 2017 Nov;124:S34-S47.

6. Rosenthal P, Borsook D, Moulton EA. Oculofacial Pain: Corneal Nerve Damage Leading to Pain Beyond the Eye. Invest Ophthalmol Vis Sci. 2016 Oct;57:5285-5287.

7. Crane AM, Levitt RC, Felix ER, Sarantopoulos KD, McClellan AL, Galor A. Patients with more severe symptoms of neuropathic ocular pain report more frequent and severe chronic overlapping pain conditions and psychiatric disease. Br J Ophthalmol. 2017 Feb;101:227-231.

8. Goyal S, Hamrah P. Understanding Neuropathic Corneal Pain--Gaps and Current Therapeutic Approaches. Semin Ophthalmol. 2016;31(1-2):59-70.

9. Romero-Rangel T, Stavrou P, Cotter J, Rosenthal P, Baltatzis S, Foster CS. Gas-permeable scleral contact lens therapy in ocular surface disease. Am J Ophthalmol. 2000 Jul;130:25-32. 

MELANIE FROGOZO, OD, FAAO, is the owner of Alamo Eye Care in San Antonio, TX, where she directs the Contact Lens Institute. She has received remuneration from ABB Optical Group.