Isaacs Disease

In this syndrome, neuromyotonia or hyperexcitability of the peripheral nerves results in spontaneous and continuous muscle electrical activity. Although in some cases the disease is inherited, in the majority it is acquired. The continuous discharges may originate anywhere along the length of the peripheral nerve. There is some association between Issac's syndrome and autoimmune disease, and it is thought that the acquired varieties are autoimmune in origin. In support of this idea, antibodies against potassium channels have been demonstrated. The hereditary disease is believed to be a genetic defect in the potassium channel, an assumption strengthened by the coexistence of myokymia in autosomal dominant episodic ataxia type I, which is a known genetic defect of the potassium channel. y The syndrome is rare, and assessment of its frequency and its definitive clinical features is hampered by the variable names given to the disorder in the past. Typically, patients have muscle stiffness, cramps, and myokymia. They may also have generalized weakness and sweating. Onset occurs at any time during life, and the condition tends to be chronic. Motor features are most prominent in the limbs and trunk. Respiratory muscle involvement can cause breathing difficulties, and laryngeal involvement can cause stridor. Physical examination may reveal mild weakness with reduced or absent tendon reflexes and evidence of myokymia, which results in occasional muscle hypertrophy.

Myokymia can arise from peripheral nerve damage of any origin. It is also a feature of autosomal dominant episodic ataxia type I. The features of cramps and stiffness should raise the differential of Brody's disease, stiff person syndrome, and the myotonias. The EMG can confirm the diagnosis by showing doublet, triplet, or multiplet spontaneous motor unit discharges, which may be present even in the absence of visible myokymia. Nerve and muscle biopsies show nonspecific features, and in a few cases, oligoclonal IgG has been found in the CSF. y

Anticonvulsant medications such as phenytoin and carbamazepine have been the first line of treatment. Plasma exchange may be appropriate when an autoimmune cause is thought to be likely. In some of these latter cases, immune suppression with corticosteroids and azathioprine has been tried. y


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