OCULAR PHOTORECEPTORS

The contribution of the third class ocular photoreceptors (non rod or cone) to a range of light detection tasks, as well as its role in circadian rhythms, should now be factored into all assessments of clinical blindness, the XXXV United Kingdom & Ireland Society of Cataract & Refractive Surgeons (UKISCRS) Congress heard.

One of the highlights of the event was the Choyce Medal Lecture given by Russell Foster BSc, PhD, FRS, professor of circadian neuroscience, and chair of the Nuffield Laboratory of Ophthalmology, University of Oxford, whose research over 10 years led to the discovery of a third class of ocular photoreceptor in 1999.

Prof Foster discussed the link between eyes, light and the regulation of body clocks, and the importance of circadian rhythms in the healthy functioning of humans. “The body clock is constantly adjusting physiology and behaviour across the 24-hour day, anticipating and fine-tuning physiology to the varying demands of activity and sleep. Blood pressure, temperature and metabolic rate all rise before we wake, and fall in anticipation of sleep. Even our pain sensitivity varies across the day. Furthermore, there is a 49 per cent higher chance of having a stroke between 6am and midday, while overall there is a 29 per cent greater chance of dying during this period. We are very different creatures at midnight compared to midday. For example, even accounting for fatigue, the human ability to process information at 4am or 5am is worse than the drop in brain performance we experience when we have consumed sufficient alcohol to make us legally drunk,†Prof Foster commented.

He stressed the impact of sleep and body clock disruption on health. Disrupted sleep can have effects that range across activation of the stress axis, immune suppression, metabolic abnormalities and greater risk of heart disease. Prof Foster said that because the eyes are so vital in regulating the 24-hour body clock, ophthalmologists have an important role to play in advising patients about the relationship between light and the body clock.

Role of the eyes

The eye and the retina play a vital role in regulating the body clock system. As eye loss places patients into a world that lacks both vision and a proper sense of time, clinical guidelines should incorporate this information, he contended. “If you have no eyes your ability to coordinate internal time is lost completely.â€

The discovery of a third photoreceptor system within the eye based on melanopsin photosensitive retinal ganglion cells (pRGCs) unlocked the key to understanding the regulation of the body clock, and many other responses to light including pupil constriction, alertness, hormone release, and even our subconscious awareness of light. “We found a mouse could be visually blind but not blind to these non-visual responses to light,†Prof Foster noted.

Thus, he recommended that a ‘blind’ individual who exhibits a bright-light-dependent pupil constriction should be encouraged where possible to expose their eyes to sufficient daytime light or even use light boxes to maintain normal circadian entrainment and sleep/wake timing.

Patients with diseases of the inner retina that result in RGC death such as glaucoma are at particular risk of body clock issues and sleep disruption and should receive targeted treatment, including potentially melatonin, to address this, he suggested.

Meanwhile, Prof Foster said that because so much of our physiology and behaviour is being affected by these new receptors, the clinical diagnosis of complete blindness should assess the state of both the rod/cone and pRGC photoreceptive systems.