Hypokalemic Periodic Paralysis

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.

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