Congenital Myasthenias

The onset of myasthenic symptoms at birth or in patients with a family history of this disease has always made congenital forms of myasthenia a diagnostic possibility. With the advent of the acetylcholine receptor (AChR) and the calcium channel antibody test as markers for myasthenia gravis and Lambert Eaton myasthenia, it has become apparent that congenital myasthenia, like the congenital myopathies, may present later in life and, in some cases, without a family history.

Pathogenesis and Pathophysiology. Congenital defects of the neuromuscular junction can be divided into presynaptic, synaptic, and postsynaptic defects ( .Table.. 37-..3 ). Presynaptic disorders include defects in the synthesis of acetylcholine or in its packaging into the presynaptic vesicles, which reduces the amount of acetylcholine available for release. Synaptic defects involve a deficiency of acetylcholinesterase. Postsynaptic defects consist of abnormal function or numbers of acetylcholine receptors. Not all the congenital myasthenic syndromes have a well-defined cause. Some appear to be sporadic, but among the inherited cases, autosomal recessive inheritance is most common. The slow channel syndrome is inherited as an autosomal dominant trait.

Epidemiology and Risk Factors. Precise incidence figures are not available, but the congenital myasthenias represent a minority of patients with myasthenia and are rare diseases. Some patients with congenital myasthenia undoubtedly remain undiagnosed and later in life are presumed to have serologically negative autoimmune myasthenia, particularly if they respond well to anticholinesterase treatment.

Clinical Features and Associated Disorders. Only the broad clinical features of the congenital myasthenias are described; the individual syndromes are listed in Table.3Z-3 .[29] , '31' Presentation of the congenital myasthenias can be similar to that of congenital myopathies, with presentation as a floppy baby. These patients may have difficulty with feeding, delayed motor milestones, and persistent or sometimes progressive limb weakness. Skeletal abnormalities can result from this weakness. Ocular, bulbar, and respiratory weakness may occur, but some of the syndromes are notable for the lack of ocular involvement. Other syndromes have a characteristically later time of onset, typically in the second or third decade. The distinguishing feature is fatigability or fluctuating weakness, usually becoming worse with activities such as feeding, crying, or moving. In patients with some of the myasthenic syndromes, episodic exacerbations occur with fever or emotional

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