Eye scan allows fast diagnosis of MS

A simple eye scan may provide an accurate diagnosis of multiple sclerosis, according to a study published last week by Hopkins researchers in the journal Neurology. Less expensive and more easily administered than magnetic resonance imaging (MRI) - the currently favored diagnostic tool - the authors report that the test is an effective identifier of neuronal deterioration, the disease's most damaging and distinctive aspect.

The neuronal degeneration associated with MS is of a specific type. Current scientific consensus holds that, in MS, an individual's immune cells do not "recognize" his or her nervous system and subsequently attack it as if it were a foreign object.

A particular target of many of these misplaced attacks is myelin, a fatty sheath that surrounds the axon, the part of a neuron that passes signals on to its neighbors. Normally, axon myelination underlies a neuron's high transmission speed (up to 200 miles per hour). Without it, neurons become slow and inefficient.

In patients with MS, the myelin sheath is slowly eaten away, producing lesions - literally holes - in the brain and spinal cord. Typically, this results in characteristic motor and speech problems, as well as less well-understood cognitive and behavioral deficits.

Diagnosing and tracking MS can be an expensive and time-consuming proposition. A series of MRI tests has to be administered, the purpose of which is to determine how much the whole brain has atrophied.

Usually, this is quantified by measuring the amount of cerebrospinal fluid (CSF) - a watery liquid found throughout the central nervous system - in the brain. CSF fills in almost any space not occupied by neurons, so more deterioration means more CSF.

While MRI is relatively accurate and widely accepted, researchers and clinicians have been looking for a faster, more easily administered alternative that could also serve as supplementary evidence to MRI-based diagnoses.

The Hopkins group, led by Paul Calabresi of the School of Medicine's Department of Neurology, chose to examine the feasibility of one technique in particular. Optical coherence tomography (OCT), a relatively new imaging technique, allows scientists to visualize any given tissue to the micron (one one-millionth of a meter).

This precision makes possible the imaging of the most delicate pieces of the body. Indeed, its most common application is in studying the retina, the thin layer of light-sensitive cells that lines the back of the eyeball.

Although it consists of 10 distinct layers, the retina in its entirety is only about half a millimeter thick. Each layer is, consequently, even more miniscule.

The Hopkins team focused its efforts on one layer in particular: the retinal nerve fiber layer (RNFL).

The RNFL consists of fibers destined to form the optic nerve, which transmits to the brain an electrical signal indicating the presence of light. This means that RNFL fibers are subject to the same kind of degeneration as neurons in higher brain areas. However, because of their more accessible location, the status of RNFL fibers is more easily quantified than that of neurons deep within the brain.

Calabresi and his colleagues used OCT to assess the thickness of the RNFL in MS patients and controls. This was then compared to MRI data from each subject.

The results showed a significant correlation between decreased RNFL thickness and increased CSF volume in MS patients; no correlation was seen in controls.

In other words, RNFL thickness appears to be an accurate measure of the progression of MS and may provide a way to bypass the expense and time associated with MRI.

Nonetheless only 40 patients were enrolled in this study; a larger project with more subjects who are followed over a period of years will be required before OCT is fully accepted as an alternative to MRI.