Differential Diagnosis of Periodic Paralysis

Periodic paralysis must be distinguished from other conditions.

Differential diagnosis may include:

(1) seizure disorder

(2) metabolic myopathy such as McArdle's disease

(3) vascular disorder

(4) syncope

When to consider another diagnosis:

(1) fluctuation of fixed weakness rather than attacks of weakness

(2) absence of myotonia on cooling OR weakness with exercise

(3) cramps with myoglobinuria

(4) altered consciousness

(5) rhythmic jerking

(6) cranial nerve symptoms

(7) pre-syncopal symptoms

(8) pain or numbness

Finding	Type of Periodic Paralysis (PP)
low serum TSH (hyperthyroidism)	thyrotoxic hypokalemic PP
cardac arrhythmia or long QT on ECG	Andersen-Tawil Syndrome
dysmorphic features	Andersen-Tawil Syndrome
onset with cooling	paramyotonia congenital (PC)
myotonia clinically or on EMG	presence suggests paramyotonia congenital or hyperkalemic PP; absence suggests hypokalemic PP

If a periodic paralysis is suspected, then consider the following in making the diagnosis:

(1) age at onset

(2) serum potassium

(3) frequency

(4) severity and duration of attacks

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