Cause:Familial periodic paralysis (FPP) is inherited; unknown in other two

Pathophys:"Channel-opathies". Hyperkalemic myopathy (HyperK) may be mediated through an aldosterone-like steroid intermittently secreted

FPP is more common in Asians and associated with hyperthyroidism

Sx:Paralysis, intermittent, periodic, often precipitated by exercise or, in HyperK, insulin

Si:Flaccid paralysis

Most occur throughout life

Death due to cardiac arrhythmias and respiratory impairments especially; PVCs and ventricular tachycardia (Nejm 1972;286:253)

If K⁺ elevated, r/o rare hypoaldosterone syndrome probably due to absent renin (Nejm 1972;287:573)

Lab: Path:Muscle biopsy shows clear vacuoles throughout muscle, intracellular "focal hydropic changes," often PAS-positive (glycogen)

Renal biopsy shows similar vacuoles at base of tubular cells

Rx:

of FPP, rx of hypo/hyper K⁺

of normo- or hyperkalemic myopathy, acetazolamide 125-250 mg po bid (Ann IM 1977;86:169) probably works by increasing K⁺ loss by blocking H⁺ secretion

of HypoK, K⁺ iv, or po acetazolamide 125-250 mg po bid which works by causing an acidosis and thereby driving K⁺ up; or more likely, inhibits an aldosterone-like steroid (Ann IM 1970;73:39; Nejm 1968;278:582)