Diagnosis: Visual Evoked Response and Spinal Fluid Analysis

CEREBROSPINAL FLUID (CSF) ANALYSIS

A spinal tap is performed less often today than formerly but may be helpful when the diagnosis is in doubt or to satisfy diagnostic criteria. Total white cell count is elevated in about 25% of MS patients, but rarely above 25 mononuclear cells per mm³. The total protein concentration is slightly elevated in 40% of patients but is seldom greater than 75 mg/dL. In 40% to 60%, the γ-globulin (IgG) fraction is elevated above 15% of the total CSF protein, reflecting an increased production of IgG within the CNS. The IgG index provides a more precise estimate of IgG synthesis within the CNS. It is calculated as the ratio of IgG in CSF/IgG in serum divided by albumin in CSF/albumin in serum. A value greater than 0.7 is taken as abnormal and is found in 70% of MS patients. Unfortunately, the IgG index is not entirely specific for MS. Oligoclonal bands in the IgG sector of the protein isoelectric focusing pattern or immunofixation pattern, but not found in blood, are detected in more than 90% of cases and in fewer than 5% of controls once CNS infections are excluded. CSF pressure and glucose content are normal.

EVOKED POTENTIALS (EPS)

These neurophysiologic studies permit an objective analysis of the integrity of neuronal pathways in the CNS. Before the ready availability of MRI, EPs were widely used to identify subclinical CNS disease. Testing is easily performed and requires minimal patient cooperation, particularly when testing the visual pathways by means of visual evoked responses (VERs). In centers where MRI is at hand, brainstem and somatosensory EPs are seldom required today to confirm a tentative diagnosis of MS.

Today the primary value of EP testing occurs when a patient presents with an acute, seemingly monophasic myelopathy plus a normal brain MRI and one wishes to determine whether there might be dissemination of disease in space and time. Some patients may have had an earlier subclinical optic neuritis. Others may have either forgotten a prior episode of visual loss or offer an uninterpretable history of earlier visual disturbance now recovered. In such instances, VER testing may establish or exclude the presence of prior damage to visual pathways. The combination of an abnormal VER and a myelopathy is essentially confined to MS or neuromyelitis optica.

With VERs, a retinal stimulus, typically a reversing high-contrast checkerboard pattern, provides a means to study the integrity of the visual system. Response latencies provide objective data regarding the ability of the nervous system to transmit impulses efficiently from the optic nerve to the occipital cortex. If absence or delay of conduction is unilateral, one can conclude that there is slowed conduction between the retina and the optic chiasm typical for unilateral optic neuritis. VERs are most helpful when patients have fully regained their vision. Remyelination is never perfect so that a VER-documented slowing of conduction velocity provides an indelible marker of prior damage. When delayed latencies, attenuated potentials, or conduction block are bilateral, the lesion cannot be localized precisely because it could be situated anywhere along the visual pathway.

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