The brain is connected to each eye by an optic nerve, so any degeneration of the brain caused by such diseases can also damage cells along the nerve and in the retina, says Helen Danesh-Meyer, an eye surgeon and neuro-ophthalmologist at the University of Auckland Medical School in New Zealand. Indeed, a loss of visual function is one of the first symptoms in many people with a neurodegenerative condition.
Although evidence of a link between degeneration of the optic nerve and diseases such as Alzheimer's has been around since the late 1980s, without instruments capable of measuring the retinal changes accurately it is only recently that this knowledge could be put to use, says Danesh-Meyer.
The accuracy of ophthalmological tools has greatly improved in the last few years. Developments include a type of laser-camera technique called Heidelberg retina tomography (HRT), and a laser device called GDx, both of which can be used to scan the shape and thickness of optical nerve fibres at the back of the eye.
Both tools are now widely used to manage glaucoma, but in 2006 Danesh-Meyer became one of the first researchers to use them to study neurodegenerative diseases by looking at the region of the retina where ganglion cells meet to form the optic nerve - a region known as the optic nerve disc (OND). In a trial involving 40 Alzheimer's patients and 50 healthy volunteers, she was able to show that people with Alzheimer's had a distinctive enlargement to a cup-shaped part of their OND and a progressive thinning of the retinal nerve fibres within the disc.
People with Alzheimer's have a distinctive shape to the disc of their optic nerve
Following this discovery, researchers have been using even more accurate instruments to track degenerative changes in the OND to monitor the progression of diseases like Alzheimer's, Parkinson's and MS. But it has been the emergence of optical coherence tomography (OCT) that appears most promising: it became commercially available in 2006 and is fast becoming a standard tool for the management of glaucoma and diabetic retinopathy. When applied to the OND, it produces highly detailed two and three-dimensional images of the subsurface retinal tissue, says Denise Valenti at Boston University, who has been using OCT to study Parkinson's and Alzheimer's.
The technique works very much like ultrasound, but bounces light off the tissue instead of sound waves. One beam of light is fired at the tissue and another at a reference mirror. When the reflected beams have travelled an identical distance, interference will make their combined beam brighter than if the distances are different. So by reflecting one beam off of different layers of tissue, and moving the reference mirror until the combined reflected beam is brightest, the technique can measure the depths of each section of tissue and build up a detailed image of its structure. It has proved particularly useful in ophthalmology because the semi-transparent nature of retinal tissue makes it possible for OCT to penetrate to greater depths - up to several millimetres. When applied to the OND it can give information about both the shape and thickness of retinal nerve fibres, allowing even subtle changes to be tracked.
Such changes can be used to monitor the progression of diseases non-invasively and relatively cheaply. Unlike MRI, which is expensive and can require patients to remain still for an hour or more, OCT is increasingly available in clinics and can be carried out in a few minutes. "It's extremely inexpensive compared to other tests," says Valenti.
One possibility is to use OCT to monitor the effectiveness of treatments for neurodegenerative diseases, says Danesh-Meyer: "These drugs can have a lot of side effects, so if they are not having a benefit then you won't want to continue with them."
Laura Balcer, a neurologist at the University of Pennsylvania School of Medicine in Philadelphia, has been using OCT on patients taking part in MS drug trials to try to establish if the system can accurately gauge drug efficacy. Such an objective tool would allow symptoms to be picked up that might otherwise go unreported, she says. For example, OCT has already shown that even in people with MS whose eye function is normal, there are marked differences in OND shape and fibre thickness compared with healthy people. "MS researchers are very excited about OCT," she says.
The technology is also proving its value as a tool for monitoring brain tumours, which can affect vision by pressing on the optic nerve. Such pressure will cause damage to different parts of the OND, depending on where in the brain the tumour is located, says Danesh-Meyer. What's more, the extent of the thinning of the nerve-disc fibre can also reveal whether vision will be restored upon removal of the tumour.Continue reading
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