Partially Characterized Syndromes

Other AChR deficiencies

Congenital myasthenic syndrome resembling LEMS Congenital myasthenic syndrome with facial malformations Familial limb girdle myasthenia

LEMS, Lambert-Eaton myasthenic syndrome; ACh, acetylcholine; AChR, acetyl-choline receptor; AChE, acetylcholinesterase.

upset and at times with no obvious reason. The physical examination is characterized by fatigable weakness. Fixed weakness is usually generalized, but very selective weakness is described. Delayed pupillary responses occur in some cases. Reflexes are generally but not invariably preserved.

Differential Diagnosis. The same differential diagnosis applies to these conditions as to the congenital myopathies. Clinical distinction from mitochondrial myopathy may be difficult because this latter disorder can also present with fatigable weakness. Neonatal botulism must be excluded. Once the diagnosis of a myasthenic syndrome has been made, a differentiation of congenital from acquired autoimmune myasthenia may be required, particularly in patients with late-onset disease who lack a family history. Neonatal myasthenia due to passive placental transfer of AChR antibodies usually occurs in offspring of obviously affected AChR antibody-positive mothers.

Evaluation. A positive AChR antibody test excludes a congenital myasthenia, but because cases of AChR antibody-negative myasthenia gravis exist, a negative test is less helpful. A positive edrophonium (Tensilon) test confirms the presence of a myasthenic syndrome but does not differentiate congenital myasthenia from myasthenia gravis. It may be negative in some patients with congenital myasthenia in whom there is a deficiency of acetylcholinesterase. EMG studies may show a decrement in the compound muscle action potential to repetitive stimulation but at 10 Hz rather than the typical 3 Hz. Single-fiber EMG may show excess jitter and blocking. Morphological, immunostaining, and immunochemical techniques and in vitro neurophysiological tests on intercostal muscle containing motor end plates are research methods for confirming and characterizing the nature of a congenital myasthenia. y , y Genetic analysis may show, for example, mutations of genes encoding for subunits of AChR.y

Management. Respiratory and bulbar complications require appropriate supportive measures. Depending on the precise nature of the congenital myasthenia, some patients may respond to anticholinesterase treatment. Those patients who do not respond to anticholinesterases may benefit from treatment with 3,4,-diaminopyridine.y

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