Disorders of Myelin

Abstract

The chapter begins with a discussion of common major clinical features and mechanisms of damage to myelin. Myelin is produced in the peripheral nervous system (PNS) by Schwann cells and in the central nervous system (CNS) by oligodendrocytes. Key functions of myelin are to house axons and to provide for the axons hollow tubular channels of extracellular matrix, provide physical strength to the axon, insulate the axon from environmental toxins, and allow saltatory conduction increasing nerve conduction velocity as much as 100-fold. The four major mechanisms of demyelination are death of oligodendrocyte or Schwann cell, interference with myelin synthesis, interference with myelin turnover, and immune-mediated destruction of myelin. In the CNS, myelin tracts commonly damaged are the corticospinal tract (weakness, spasticity), spinothalamic tract (sensory loss), visual pathway (visual disturbance), and spinocerebellar pathways (ataxia). In the PNS, motor (flaccid weakness) and sensory (position sense loss) are often involved. The chapter then discusses multiple sclerosis, neuromyelitis optica, acute disseminated encephalomyelitis, and Guillain–Barré syndrome, with attention to their pathophysiology, major clinical features, major laboratory findings, and principles of management and prognosis.

A 27 year-old female administrative assistant developed acute monocular decreased vision in her left eye three years ago that spontaneously cleared over 6 weeks. Last week she developed the onset of left arm and leg clumsiness, moderate weakness and mild numbness over one day. Her physician found she had had normal mental status and a left relative afferent pupillary defect (Marcus Gunn pupil). The patient’s pupils constricted less (therefore appeared to dilate) when a bright light was swung from the unaffected eye to the affected eye. A left hemiparesis was present along with very brisk left-sided deep tendon reflexes and a left Babinski sign. A T2-weighted MRI demonstrated 1 cm hyperintense oval lesion perpendicular to the right lateral ventricle, which enhanced with gadolinium, as well as two 1 cm hypodense lesions in the left parietal and frontal lobe white matter on T-1 weighted images. Her CSF demonstrated 3 WBC/mm ³ , normal glucose, protein of 50 mg/dL, and 4 oligoclonal bands not seen in the serum. A diagnosis of relapsing remitting multiple sclerosis was made.