Ocular Itch Potentiates Reactivity of Pain Processing Brain Regions in Chronic Ocular Pain

Abstract

Purpose : Ocular pain and itch are both multidimensional sensations with overlapping features in terms of quantitative, qualitative, and motor aspects. In patients with chronic neuropathic ocular pain, light can be perceived as painful (photophobia) and can activate neural circuits related to somatic pain processing. We used functional magnetic resonance imaging (fMRI) to explore whether the presence of itch impacts brain networks associated with pain and photophobia.

Methods : 30 subjects with ocular pain (OP+, worst pain rating 1-week recall>1) were split into two groups based on ocular itch (OP+Itch+: “itching eye pain sensation”≥ mild, n=23; OP+Itch-: “itching eye pain sensation”=none, n=7). Using fMRI, blood oxygen level-dependent (BOLD) responses to light-induced pain were measured and analyzed to identify brain regions related to pain processing routinely found in pain literature. Subjects reported light-evoked unpleasantness (Verbal Rating Score: 0-100) after each scan.

Results : Mean age of the study sample was 55.6±10.9 years old, 60% identified as male, 66.7% as White, and 43.3% as Hispanic. Unpleasantness ratings to the light stimuli were comparable in both groups (OP+Itch+: 39.61±34.42 vs. OP+Itch-: 32.86±35.81, p=0.66). Separate group analyses of OP+Itch+ and OP+Itch- subjects revealed light-evoked BOLD responses in similar brain regions, including the left primary somatosensory (S1), precuneus, anterior cingulate, paracingulate, bilateral insular, and frontal pole cortices as well as bilateral cerebellar hemispheric lobule VI, crus I and II, and vermis. The OP+Itch+ group had additional active regions: the spinal trigeminal nucleus, right temporal pole, and bilateral secondary somatosensory (S2) cortices, whereas the right S1 cortex was only present in the OP+Itch- group. In a direct group contrast analysis with OP+Itch- patients, OP+Itch+ individuals had greater BOLD activity in the left S1, bilateral S2, and left insular cortices.

Conclusions : In our cohorts, individuals with ocular pain and itch had greater BOLD activation in areas involved in sensory (S1) and emotional (S2 and insula) pain processing than individuals who had ocular pain alone. Brain processing related to ocular pain appears augmented by the presence of itch.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.