Hypokalemic Periodic Paralysis (HypoKPP)

Definition & Classification

Genetics & Molecular Pathophysiology

Types

Mechanism

• Reduced activity of ATP-sensitive K⁺ channels (K_ATP) in muscle fibers leads to unopposed Na⁺/K⁺-ATPase activity and further K⁺ influx into cells
• Insulin (triggered by carbohydrate meals) inhibits residual K_ATP activity, causing a depolarizing shift toward the Cl⁻ equilibrium potential (~-50 mV), at which voltage-gated Na⁺ channels are largely inactivated - resulting in paralysis
• This insulin effect can trigger paralysis even without significant hypokalemia
• Adams & Victor's Principles of Neurology, p. 1465
• Brenner and Rector's The Kidney, p. 752-753

Clinical Features

Typical Attack

• Onset usually in the second decade of life; most severe in males
• Classic timing: patient awakens in the early morning hours (during the night or after waking) after a preceding day of strenuous exercise
• Precipitants: high-carbohydrate or high-sodium meals, rest after exercise, napping after a large meal, corticosteroids, epinephrine, norepinephrine
• Prodrome (variable): hunger, thirst, dry mouth, palpitations, sweating, fatigue
• Attack evolves over minutes to several hours; at its peak the patient may be unable to call for help
• Duration: hours (mild) to several days (severe)

Distribution of Weakness

• Proximal > distal, legs often before arms
• Muscles typically spared: eyes, face, tongue, pharynx, larynx, diaphragm, sphincters (respiratory involvement is rare but possible)
• Tendon and cutaneous reflexes are reduced or absent at peak
• Muscles may feel swollen and firm to palpation

Important Negative Feature

• No myotonia - clinical or EMG evidence of myotonia essentially excludes the diagnosis

Course

• Attacks occur every few weeks, tending to diminish with advancing age
• Permanent proximal weakness may develop over time, particularly in those with frequent severe attacks
• Vacuolation on muscle biopsy is a classic pathological finding; tubular aggregates are seen specifically in Type 2 (SCN4A mutations)
• Rarely fatal (respiratory paralysis, cardiac arrhythmia) - mostly a historical concern before modern ICU care
• Adams & Victor's Principles of Neurology, p. 1465-1466
• Bradley and Daroff's Neurology in Clinical Practice, p. 2825

Laboratory Findings

Provocative Test (between attacks)

• Oral glucose (50-100 g) or NaCl 2 g/hour x7 doses + vigorous exercise under ECG monitoring
• Attack terminated by 2-4 g oral KCl
• Long exercise NCS test: CMAP amplitude increment during 5-min exercise, then significant decrement 10-20 minutes post-exercise - not subtype-specific

Differential Diagnosis & Secondary Causes

• Thyrotoxic periodic paralysis (TPP) - most important secondary form
• Fanconi syndrome, Gitelman syndrome
• Hypokalemic distal renal tubular acidosis
• Diarrhea-associated hypokalemia (rare paralysis)

Thyrotoxic Periodic Paralysis (TPP)

• Clinically resembles familial HypoKPP but is not familial
• Predominantly affects Asian and Hispanic males in early adult life
• Attacks peak between 1-6 AM; precipitated by carbohydrate meals and rest after exercise
• Hypokalemia is often profound (1.1-3.4 mmol/L), frequently with hypophosphatemia and hypomagnesemia
• Signs of hyperthyroidism may be absent or subtle
• Mechanism: thyroid hormone directly upregulates Na⁺/K⁺-ATPase subunit expression in muscle, plus enhanced β-adrenergic sensitivity drives K⁺ into cells; reduced outward Kir2.1/2.2 current compounds the effect
• Susceptibility linked to variants in KCNJ18 (Kir2.6) and KCNJ2 (Kir2.1)

• Brenner and Rector's The Kidney, p. 753

Treatment

Acute Attack

1. Oral KCl is first-line: 0.2-0.4 mmol/kg every 30 minutes
   • Alternative dose from Adams & Victor: 0.25 mEq KCl/kg orally
2. IV potassium (0.05-0.1 mEq/kg/h) only if oral route unavailable (vomiting, dysphagia) - use non-dextrose IV fluids (glucose worsens attacks)
3. TPP special case: High-dose propranolol (3 mg/kg) rapidly reverses hypokalemia, hypophosphatemia, and paralysis without rebound hyperkalemia
   • Aggressive K⁺ replacement in TPP carries ~25% risk of rebound hyperkalemia, which can be fatal - use cautiously

Long-term Prevention (Familial HypoKPP)

Important note: Acetazolamide and dichlorphenamide can exacerbate HypoKPP Type 2 (SCN4A mutations) - genetic testing before prescribing is advisable.

• Adams & Victor's Principles of Neurology, p. 1466-1467
• Harrison's Principles of Internal Medicine 22E, p. 3689-3690
• Bradley and Daroff's Neurology in Clinical Practice, p. 2825-2826

Muscle Pathology

• Vacuolization of sarcoplasm (round/oval vacuoles of clear fluid + PAS-positive granules) - classic, especially with permanent weakness
• Vacuoles arise from proliferation and degeneration of membranous organelles within the sarcoplasmic reticulum and T-tubules (Engel, 1970)
• Tubular aggregates - specifically seen in HypoKPP Type 2 (SCN4A)
• Myopathic changes common; muscle fibers uniformly somewhat enlarged

Summary Points for High-Yield Review

• HypoKPP Type 1 = CACNA1S (Ca²⁺ channel); Type 2 = SCN4A (Na⁺ channel)
• Mechanism = gating pore current + insulin-mediated K_ATP inhibition → paradoxical depolarization
• Triggers: carbohydrate meals, rest after exercise, morning hours
• No myotonia (distinguishes from hyperKPP and paramyotonia)
• K⁺ shifts into muscle; urinary K⁺ not increased during attack
• ECG: prolonged intervals, T-wave flattening, U waves
• Treatment: oral KCl acutely; acetazolamide for prevention (avoid in Type 2)
• TPP: treat hyperthyroidism; use propranolol acutely (not aggressive K⁺ - rebound hyperkalemia risk)
• Permanent proximal weakness can develop with repeated attacks