Chromatic Pupillometry to Assess the Melanopsin-Light Pathway in Progressive Supranuclear Palsy

Overview

The specific aim of this study is to investigate rod, cone and melanopsin driven pupillary light response in individuals with progressive supranuclear palsy (PSP), age-matched healthy controls and individuals with other neurodegenerative diseases using chromatic pupillometry, with special interest in assessing melanopsin-driven post-illumination pupil response (PIPR) as an identifier for PSP. The study addresses the following hypotheses: 1. Chromatic pupil responses, including rod/cone-driven rapid phase constriction and melanopsin-driven PIPR, are reduced in subjects with PSP compared to age-matched normal healthy control subjects, 2. Pupil parameters of the melanopsin-driven PIPR are abnormal in PSP subjects without supranuclear palsy, which is indicative of a subclinical physiological deficit of the OPN in the early stages of PSP. If these hypotheses are upheld, chromatic pupillometry to measure the PIPR promises to be a reliable in vivo, non-invasive, convenient and inexpensive technique to detect asymptomatic pupillomotor impairment in advance of diagnostic oculomotor signs and deterioration of cognitive function.

In 1963, Richardson, Steele and Olszewski published a landmark clinical report on 8 cases of supranuclear ophthalmoplegia, pseudobulbar palsy, nuchal dystonia and dementia and established the syndrome of heterogeneous system degeneration as a clinicopathological entity now known as PSP. The disease has a characteristic onset in the sixth decade (range 45 to 75 years) with some combination of impaired balance, abrupt falls, visual disturbances, slurred speech, dysphasia and vague changes in personality. Slowing of voluntary vertical saccades, either down, up or both are a diagnostic marker of PSP and later impairment of voluntary horizontal saccades are characteristic in more than half of the cases. However, a proportion of PSP patients do not demonstrate these eye signs for a year or more after the onset of the disease. This pilot study will use chromatic pupillometry to determine whether such a novel methodology may be used as an objective in vivo identifier of PSP. The rationale for the study is based in part on: 1. Clinicopathological correlation between the key clinical signs of a supranuclear gaze palsy with pathological verification that the degenerative process affects the pretectum and rostral midbrain, 2. The melanopsin-signaling pathway from ipRGCs (intrinsically photosensitive retinal ganglion cells) in the eye projects to the OPN (olivary pretectal nucleus) in the midbrain, 3. Chromatic pupillometry is a non-invasive technique suitable for elderly subjects with or without dementia.