Another typical feature of MS is the temporary induction, by heat or exercise, of symptoms such as unilateral visual blurring (the Uhthoff phenomenon) or tingling and weakness of a limb (the basis of the hot-tub test used in previous years). This has been shown experimentally to represent an extreme sensitivity of conduction in demyelinated nerve fibers to an elevation in temperature. A rise of only 0.5°C (0.9°F) can block electrical transmission in thinly myelinated or demyelinated fibers. Likewise, hyperventilation slows conduction of the visual evoked response, an effect that is rarely perceived by the patient. The remarkable sensitivity of demyelinated and remyelinated regions to subtle metabolic and environmental changes provides an explanation for the rapid onset of symptoms in some patients and the apparent fluctuations of MS that show no laboratory evidence of active inflammatory changes in the CNS. Smoking, fatigue, hyperventilation, and a rise in environmental temperature are all capable of briefly worsening neurologic functioning and are easily confused with relapses of disease.
Weakness or numbness, sometimes both, in one or more limbs is the initial symptom in about one-half of patients. Symptoms of tingling of the extremities and tight band-like sensations around the trunk or limbs are commonly associated and are probably the result of involvement of the posterior columns of the spinal cord. The symptoms generally appear over hours or days, at times being so trifling that they are ignored, and other times coming on so acutely and prominently as to bring the patient urgently to the doctor. The resulting clinical syndromes vary from a mere dragging or poor control of one or both legs to a spastic or ataxic paraparesis. The tendon reflexes are retained and later become hyperactive with extensor plantar reflexes; varying degrees of deep and superficial sensory loss may be associated. It is a useful adage that the patient with MS often presents with symptoms in one leg but with signs in both; the patient will complain of weakness, incoordination, or numbness and tingling in one lower limb and prove to have bilateral Babinski signs and other evidence of bilateral corticospinal and posterior column disease.
There are, in addition, several syndromes that are typical of multiple sclerosis and may be the initial manifestations. These common modes of onset are (1) optic neuritis, (2) transverse myelitis, (3) cerebellar ataxia, and (4) brainstem syndromes (vertigo, facial pain or numbness, dysarthria, diplopia). When these are unaccompanied by other features of MS, they are termed “clinically isolated syndrome” (CIS) but they are often components of the established disease as well. In the initial phases of the illness, they may pose diagnostic questions, as they also certainly occur with numerous diseases other than MS.
Flexion of the neck may induce a tingling, electric-like feeling down the shoulders and back and, less commonly, down the anterior thighs. This phenomenon is known as the Lhermitte sign, although it is more a symptom than a sign and was originally described by Babinski in a case of cervical cord trauma. Lhermitte’s contribution was to draw attention to the frequent occurrence of this phenomenon in MS. It is probably attributable to an increased sensitivity of demyelinated axons to the stretch or pressure on the spinal cord induced by neck flexion, but it occurs in other conditions such as cervical spondylosis.