The pupillary light reflex in aging and Alzheimer's disease

The pupillary light reflex is reported to be reduced in amplitude in Alzheimer's disease (AD). The purpose of this study was to determine whether this effect is measurable under conditions typical of clinical rather than laboratory settings. A head-mounted infra-red videopupillometer was used to measure the amplitude of pupillary constriction in 12 patients with probable AD, 12 healthy age-matched older adults and 12 young adults. The constriction to the onset of bright light relative to the resting amplitude was significantly reduced in AD compared with both control groups. This result is consistent with an acetylcholine-related deficit in AD and supports the findings of Prettyman et al. and Fotiou et al. The impairment is likely to be caused by degeneration in relays in the midbrain but cholinergic deficits in the peripheral parasympathetic pathway cannot be excluded. The variation in pupillary response between individuals may preclude its use for diagnostic purposes. However, if the changes in pupillary response in AD are related to change in neurotransmitter status, then the value of such a technique may be in its use in providing an objective, non-invasive monitor of physiological abnormality with which to follow disease progression and treatment efficacy.