Synonyms:
formerly Gamstorps disease, adynamia episodica hereditaria
Genetics:
SCN4A 17q23, M1592V. Cold induced weakness I693T
Autosomal dominant
Mutations in sodium channel alfa subunit SCN4A
Clinical features:
Recurrent attacks of weakness.
All limbs, spares, respiratory and ocular
Weakness after exercise. Myotonia may occur e.g. cold induced
Precipitated by K+. Alleviated by Ca++.
Improves with age
Findings on investigations:
Electrophysiology NCS/EMG:
- CMAP: decrement in amplitude after exercise
Long exercise test a.k.a. McManis Protocol:
- Good specificity, better for hyperkalemic periodic paralysis than hypokalemic periodic paralysis
- CMAPs on abductor digiti minimi ADM, Intermittent strong voluntary contraction: during first 2-5 minutes increase in CMAP. During following 20 minutes decrease in CMAP to below pre-exercise level
Pathology, Muscle biopsy:
H and E stain: Usually normal. Vacuolation in some. GMT: red subsarcolemmal aggregates.
Ultrastructure/Electron microscopy: vacuoles/aggregates are expansion of T tubules and sarcoplasmic reticulum.
Treatment:
Lifestyle management (reduce K+)
Reduce K+ (thiazide diuretics, beta agonists)
Acute and prophylactic: Acetazolamide P.O.
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