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Q1: What is the diagnosis?
The diagnosis is thyrotoxic hypokalaemic periodic paralysis (THPP). This rare condition is seen in young Latin American and East Asian men with thyrotoxicosis. The clinical features of thyrotoxicosis are usually not apparent. The human leucocyte antigen types A2 Bw22 and Aw19 B17 increase the relative risk. The age of onset is usually before 30 years, and familial occurrence is unusual. The frequency of attacks varies from daily to yearly, and each attack lasts two to 12 hours. The attacks are precipitated by a high carbohydrate diet, sodium intake, emotional stress, and strenuous exertion followed by rest/sleep. It never occurs during vigorous physical activity. The higher mental functions and sensations are not affected. It is characterised by episodic, sudden onset, flaccid quadriparesis, with absent or hypoactive reflexes. The proximal limb weakness is more than the distal, and lower limbs are more prominently involved. The ocular, bulbar, and respiratory muscles are rarely involved. It responds to potassium administration. The frequency and severity of THPP episodes is reduced by β-adrenergic blockers, antithyroid drugs, and by avoidance of strenuous physical activity and a high carbohydrate diet. Rendering the patient euthyroid abolishes this condition. In the absence of effective treatment chronic progressive interattack weakness may develop. The muscle biopsy specimen often shows vacuolar myopathy.1
Q2: What is the basic pathophysiology of this disorder?
The basic pathophysiology in THPP is the development of hypokalaemia due to an intracellular shift of potassium. The total body potassium is normal. The cause of this shift is uncertain. Molecular defects in skeletal muscle calcium channels, leading to a decrease in activity of the calcium pump have been identified.1Calcium transport disturbances may adversely change muscle excitation-contraction coupling and account for acute paresis in THPP. The contractile apparatus is normal. Effects of insulin on potassium uptake in muscle suggest that an abnormality of muscle membrane may be involved. Increased activity and numbers of Na+, K+-ATPase channels in skeletal muscle2 and platelets3 have been identified in hyperthyroid patients with or without periodic paralysis. Hyperthyroidism leads to increased β-adrenergic receptor responsiveness and this also contributes to increased activity of Na+, K+-ATPase channels.4 This may explain the mechanism of β-blockers in prevention and of the hyperadrenergic state of exercise in provocation of THPP attacks. Hyperinsulinaemia is present in many patients with THPP and increases the Na+, K+-ATPase induced transport of potassium.5 This is important in the precipitation of paralytic attacks and may underlie the provocation of paralysis with high carbohydrate intake.
It is essential that THPP be excluded in acute pure motor flaccid hyporeflexic quadriparesis with clear sensorium, particularly in young men.
Q3: How will you prevent further attacks?
Establishment of the euthyroid state will abolish the attacks. β-Adrenergic blocking agents are useful for reducing the frequency and severity of attacks while measures to control thyrotoxicosis are instituted. Strenuous exertion and high carbohydrate diet avoidance also helps. Acetazolamide is not useful in attack prevention.1
This patient was being managed as a hysterical conversion reaction by primary care physicians. Once the diagnosis of THPP was established the patient was managed with intravenous potassium supplements under electrocardiographic monitoring. Six hours later his paralysis resolved and serum potassium concentrations rose to 4.1 mmol/l. Subsequently he was put on propranolol 80 mg/day and carbimazole 30 mg/day. Six weeks later his thyroid function tests were normal. There has been no recurrence of paralytic attacks over a follow up of nine months. The patient refused radioiodine therapy, and is on carbimazole and propranolol.
This patient emphasises the lesson that though THPP is a well documented condition, the diagnosis may be delayed or missed. This is due to the rarity of the condition and absence of typical signs and symptoms of thyrotoxicosis.6
Thyrotoxic hypokalaemic periodic paralysis.