Neuropathic pain in the eye – A/Prof Alex Hui

Common causes of neuropathic pain include dry eye, post-corneal and refractive surgery, and herpes infection.

The International Association for the Study of Pain recently updated its definition of pain for the first time since 1979 as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such lesions”.¹

It is important for clinicians to understand this definition because pain is not just limited to tissue damage or injury, but also to the experience associated with the stimulus.

Assistant Professor Alex Hui.

Indeed, analgesia, which is commonly used to refer to pain management, is more specifically defined as the reduction or absence of the sensation of pain in response to a typically painful stimulus.¹ Unfortunately, for a subset of patients, it is the system that signals pain through the nerves and gives the perception of pain that may be dysfunctional; this leads to neuropathic pain which can be very different from the pain associated with direct tissue damage.

Neuropathic pain is inherently a disease of the somatosensory system and as such is colloquially referred to as “nerve pain.”² There are many etiologies for neuropathic pain. In many cases, the link between a disorder of the somatosensory system and the presence of neuropathic pain is apparent, while in other cases it is a side effect of the disease that can only occur in some cases. For example, in patients who have suffered injuries resulting in paralysis or who have diseases of the nerves themselves, as in multiple sclerosis, damage to the somatosensory system is part of the primary pathophysiology of the disease and pain resulting is a manifestation of this evil.³

For other etiologies, nerve damage and thus pain signaling may not always be present. Examples of this include diabetic neuropathy as well as shingles, where the pathophysiology of the primary disease may lead to nerve dysfunction and pain sensation, but is not always a hallmark of these diseases. the pain caused by the stimulation of our pain receptors due to aggressions, can also be converted into neuropathic pain if it is not treated quickly and persists for long periods of time.

In the eye, common causes of neuropathic pain include dry eye, post-corneal and refractive surgery, and herpes infection. These perfectly represent some of the possible locations along the trigeminal nerve to the cornea that can be afflicted.2,4.5 Reactivation and replication of herpes viruses in the nasociliary branch of the trigeminal nerve not only lead to viral shedding and the clinical manifestations of ocular herpes, but can also damage the nerves themselves and disrupt nerve signaling, resulting in persistent pain.4

Anterior segment and refractive surgery damages nerves iatrogenically: corneal nerves can be cut or suffer trauma from the procedure.5 Dry eye also involves neurosensory abnormalities, as stated in the definition of the disease in the TFOS DEWS II report.6 Here, it is thought that there may be a link between chronic disease-induced inflammation that affects the health of ocular surface nerves.²

For the eye care practitioner, identifying patients at risk of developing neuropathic pain is crucial to ensure prompt and appropriate management. Clinicians should identify patients who experience pain disproportionate to their clinical signs (hyperalgesia), pain due to stimuli that are not normally painful (allodynia), or pain that persists after the causative insult has ceased. ‘exist.³ Use of a questionnaire, including the Eye Pain Assessment Survey (OPAS), is recommended to document pain.seven

The management of neurological pain focuses on controlling the primary condition as much as possible. Earlier involvement of the patient’s GP or referral to a pain specialist should be considered, particularly for conditions such as zoster ophthalmicus where postherpetic neuralgia can be severe and have a potential long-term impact on life quality.4 For the ocular surface, management of neuropathic pain often begins with the use of anti-inflammatory therapy or autologous serum, as decreasing inflammation has been shown to have a positive impact on the reduction of pain.seven

ABOUT THE AUTHOR:

Name: Deputy A/Prof Alex Hui Qualifications: OD, PhD, GradCertOcTher, FAAO
Organisation: School of Optometry and Vision Sciences, Faculty of Medicine and Health, UNSW Sydney; Center for Eye Research and Education, School of Optometry and Vision Sciences, University of Waterloo
Position: Adjunct Associate Professor (UNSW); Head, Biosciences (Waterloo)
Location: Toronto, Canada
Years in business: 12

References

1. International Association for the Study of Pain. Terminology. 2020 [cited 2022 19/07/2022]; Available at: https://www. iasp-pain.org/resources/terminology/.

2. Rosenthal, P. and D. Borsook, Ocular neuropathic pain. British Journal of Ophthalmology, 2016. 100(1): p. 128.

3. Jensen, TS and NB Finnerup, Allodynia and hyperalgesia in neuropathic pain: clinical manifestations and mechanisms. Lancet Neurol, 2014. 13(9): p. 924-35.

4. Sampathkumar, P., LA Drage and DP Martin, Shingles (shingles) and postherpetic neuralgia. Mayo Clin Proc, 2009. 84(3): p. 274-80.

5. Moshirfar, M., et al., Neuropathic corneal pain after LASIK surgery: a retrospective case series. Ophthalmology and Therapeutics, 2021. 10(3): p. 677-689.

6. Craig, JP, et al., TFOS DEWS II Definition and Classification Report. Ocul Surf, 2017. 15(3): p. 276-283.

7. Nortey, J., et al., Topical treatment options in corneal neuropathic pain. Frontiers in Pharmacology, 2022. 12.

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