Genetics:
Autosomal dominant, reduced penetrance in females
Ca++ CACN1A3 muscle DPH sensitive Ca++ channel. CACNL1A3 (CACNA1S) gene chr. 1q32 protein= calcium channel L type 1S subunit,
Na+ SCNA4A 17q23-q25.3. Or sodium channel alfa subunit SCN4A gene chr. 17q25
Or potassium channel KCNE3 gene chr. 11q13
Clinical features:
Commonest form.
Recurrent attacks of flaccid weakness. Attacks are longer 12-24 hrs
Weakness:
- Precipitated by rest or high carbohydrates or low serum K+.
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
Acute: Potassium P.O.
Prophylactic: Acetazolamide P.O.