Since the 1960s, science has struggled to find answers for the treatment of Parkinson’s disease. We believe we have now found an important part of the answer.

The eye may be involved in the cause and progression of Parkinson’s disease

Accumulating evidence shows that the eyes are involved in the development of Parkinson's disease. Parkinson’s patients have damaged retinal dopamine neurons. Recent discoveries show a link between these neurons and the substantia nigra, and that damage to one area causes corresponding damage to the other.

Lesioning the substantia nigra causes corresponding retinal damage

Confocal images of rat retina – damaged DA
layer (shown left) corresponds to SN lesioning
undertaken only in contralateral (opposite)
hemisphere; other retina remains unaffected.

In a recent study, the substantia nigra in one hemisphere of animals was lesioned. Within three weeks, the majority of the dopamine neurons in the contralateral retina were destroyed. Retinal dopamine production significantly decreased while melatonin significantly increased. The ipsilateral retina was not affected, showing a unilateral connection between the SNc and retina.

Parkinson’s symptoms may also be caused by damage to the eye

In a separate study, toxins that cause parkinsonism (6-OHDA, MPTP, paraquat, and rotenone) were placed in animals’ eyes in amounts too small to diffuse into the brain. In each case, the animals developed parkinsonian symptoms similar to those obtained by directly injecting these toxins into the SNc. This evidence suggests that the eyes may be a disease pathway for Parkinson’s disease.

Parkinson’s symptoms may be treated through the eye

If the eyes are involved as a disease pathway, it follows that Parkinson’s could also be treated via that pathway. Retinal injections of dopamine and melatonin antagonists, in amounts too small to diffuse into the brain, resulted in recovery from Parkinson’s symptoms. In PD retinal dopamine neuronal damage results in an increase in retinal melatonin which antagonizes dopamine.

Light may be an effective treatment option

Light activates retinal dopamine and suppresses melatonin. Exposing the animals to experimental light conditions produced a two-fold recovery compared to dopamine or melatonin antagonists alone.

Light therapy studies in humans have shown similar improvement

To date, five published studies in humans have shown improvement of PD symptoms with light therapy. However most of these studies were either short term or not well controlled. PhotoPharmics is sponsoring a pivotal multi-center, double-blind study that is currently underway at Amsterdam, Boston and Salt Lake City, to verify the findings of these earlier investigations.