MRI Sheds Light on Chronic Ocular Surface Pain

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While ocular surface pain has long been attributed to tear abnormali­ties, recent research indicates that nerve dysfunction also plays a role. Now researchers report that in patients with ocular surface pain and photophobia, the brain systems related to sensations of physical pain can be provoked by viewing light.¹

Their preliminary investigation, the first to examine neural mechanisms in individuals with chronic ocular surface pain and photophobia, suggests that the trigeminal pain pathway may contrib­ute to photophobia. The study’s results also demonstrated a partial benefit of topical anesthetic for the pain.

“The findings may explain why light can result in pain for these patients,” said Eric A. Moulton, OD, PhD, at Boston Children’s Hospital. Moreover, he said, the results “suggest that when a patient presents with photophobia and chronic ocular surface pain, clinicians should consider that neural changes are also likely occurring in pain process­ing areas within the central nervous system.”

BENEATH THE SURFACE. Light-induced activation of the somatosensory cortices and supplementary motor area in a patient with chronic ocular surface pain.

A role for fMRI. For this study, the researchers recruited 16 patients from the Miami Veterans Affairs eye clinic. Half reported experiencing chronic ocular surface pain for six months or longer; the remaining eight served as controls. Those with chronic ocular surface pain also reported experiencing light sensitivity at least most of the time over one week.

Using functional magnetic reso­nance imaging (fMRI), the research­ers measured brain activity triggered by light. In a single session, subjects viewed light stimuli during two fMRI scans—one before and one after application of a single drop of .5% proparacaine topical anesthetic in each eye. Each scan presented two screen conditions: 1) a resting black screen, which featured a white fixation cross on a black background, and 2) a light stim­ulus white screen, which had a black fixation cross on white background. Following each scan, patients rated their level of pain to the stimulus.

Light-induced outcomes. In all participants, light-induced activity occurred in brain areas related to visual processing. However, those with chronic pain and photophobia reported evoked pain, while those serving as controls did not.

Before receiving proparacaine, the case patients reported that viewing the white light stimulus screen evoked pain. This decreased following treat­ment. Furthermore, significantly de­creased light-evoked fMRI activity was detected in pain-related areas following proparacaine application.

Potential pain relief? The proparacaine findings indicate that topical treatments may be able to mitigate symptoms of light sensitivity in a subset of patients, the researchers wrote.

For refractory cases of ocular surface pain, Dr. Moulton advised considering use of centrally acting pain medica­tions as well as referral to chronic pain specialists. And while sunglasses and specialized tints may offer some relief, he noted that the underlying pathol­ogy remains. Finally, he said, doctors should be aware of comorbidities that often occur with other chronic pain conditions, such as mood disorders.

—Miriam Karmel

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1 Choudhury A et al. Am J Ophthalmol. 2023;246:20-30.

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Relevant financial disclosures: Dr. Moulton—NEI: S; U.S. Department of Veterans Affairs: S.

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