Walsh & Hoyt: Pathophysiology

Whatever the pathogenesis of MS, the neurologic deficits that occur in patients with this disease are caused primarily by the effects of inflammation, demyelination, and axonal loss. Demyelinated nerve fibers have an impaired ability to conduct trains of impulses at physiologic frequencies. Such fibers may experience a complete block of conduction, an intermittent conduction block, or simply slowed conduction, depending in part on changes in the concentrations of physiologically active ions (e.g., Na+, K+, Ca2+). This inability of demyelinated fibers to transmit impulses reliably and without delay impairs functions that depend on the delivery of a precisely timed burst of impulses (e.g., appreciation of vibration, visual function). In addition, demyelinated fibers are thermolabile. Changes of as little as 0.5 degree centigrade within the physiologic range may convert normal conduction to intermittent or complete block. This phenomenon may account for the temporary production or exacerbation of neurologic symptoms, visual symptoms, or both in patients with MS during a hot bath, shower, or exerciseUhthoffs symptom. Conversely, conduction may be improved when the temperature of the environment is lowered. Visual or neurologic function improves in some patients if they take a cold bath or shower, drink cold water, or hold an ice cube in their mouth. Indeed, many patients with MS report that they cannot function during hot weather unless the temperature is substantially lowered by air conditioning. A randomized, controlled study by researchers at the National Aeronautics and Space Administration (NASA) demonstrated that cooling therapy, using a liquid cooling garment, was associated with measurable yet modest improvements in visual and motor function (low-contrast Sloan letter charts, MS Functional Composite) in patients with MS.