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Thyrotoxic periodic paralysis
Thyrotoxic periodic paralysis (TPP) is a rare neurological disease characterized by recurrent episodes of paralysis and hypokalemia during a thyrotoxic state.
It is most common in Asian males (with male-to-female ratios ranging from 17:1 to 70:1) during the third decade of life: the annual incidence in Chinese and Japanese thyrotoxicosis patients is estimated at around 1/50, whereas it is estimated at 1-2/1,000 among non-Asian thyrotoxicosis patients.
TPP manifests as recurrent episodes of acute muscular weakness of the four extremities that vary in severity from paresis to complete paralysis. Recovery occurs within 2-72 hours. Attacks typically occur at night and may be preceded by muscle cramps, aches and stiffness. Ocular, bulbar and respiratory involvement has also been reported but is rare. Attacks are triggered by ingestion of a high carbohydrate load or strenuous physical activity followed by a period of rest. Episodes only occur when patients are thyrotoxic but symptoms and classical signs of hyperthyroidism are often absent at the time of the first attack. TPP can occur in association with any cause of hyperthyroidism, but is most commonly associated with Graves' disease.
The pathogenesis remains unclear. Genetic predisposition is thought to play a role in the pathogenesis of TPP and single nucleotide polymorphisms (SNPs) of the CACNA1S (1q32) and GABRA3 (Xq28) genes have been associated with TPP susceptibility in some Asian populations. Hypokalemia is the consequence of an extra- to intracellular potassium shift due to an increase in Na/K-ATPase pump activity, either as a direct response to thyroid hormone or indirectly via adrenergic stimulation, insulin or exercise.
Biochemical studies are essential for diagnosis, with mild-to-severe thyrotoxicosis and hypokalemia during attacks being the major findings. Other anomalies may include hypophosphatemia and hypomagnesemia, without changes in acid-base balance. EMG reveals myopathic changes during attacks and muscle excitability anomalies after a prolonged exercise test. ECG anomalies are also noted. Thyrotoxicosis is the key finding for distinguishing TPP from other causes of hypokalemic periodic paralysis (HOP), such as familial HOP (FHOP; see this term), and from other hypokalemic disorders with a transcellular potassium shift, and excessive renal potassium wasting or gastrointestinal losses.
The differential diagnosis may also include Guillain-Barré syndrome, transverse myelitis (see these terms), spinal cord compression and hysteria, as well as muscle weakness and fatigue associated with hyperthyroidism in patients with thyrotoxic myopathy or myasthenia gravis (see this term).
TPP usually occurs sporadically
Management and treatment
Management of TPP includes definitive control of hyperthyroidism, prevention of attacks with propranolol and avoidance of precipitating factors. Potassium administration during attacks may prevent cardiac arrhythmias and hasten the recovery from paralysis.
The prognosis for patients is good and TPP resolves when euthyroid status is achieved.