Protocol for Evaluation of Myotonia & Periodic Paralysis

05 November, 2004

1. Routine motor & sensory nerve conduction studies in upper & lower limbs.

2. EMG: include distal, proximal, facial and paraspinal muscles.

3. Muscle cooling if suspicion of  paramyotonia congenita.

a. Wrap arm in plastic bag, immerse in ice water for 10-20 min till skin temperature is 20°C. Remove hand if weakness develops.

b. EMG hand muscle.

c. Allow muscle to rewarm to precooling temp & continue to record EMG (may take >1 hour)

4. Short exercise test if above test does not yield definitive result.

a. Immobilize hand. Record ADM CMAP (Make sure immobilize hand).

b. Record CMAP every minute for 5 minutes with muscle at rest to ensure stable baseline.

c Perform max voluntary contraction for 10 sc.

d. Record CMAP immediately. If a decrement in CMAP is seen, continue to record CMAP every 10 seconds till returns to baseline (usually 1-2 mins).

e. In cases where decrement is seen after exercise, repeat  same procedure several times to see if reduction in CMAP continues or habituates.

5. Prolonged exercise test if above do not provide diagnostic clues.

a. Immobilize hand. Record ADM CMAP (Make sure immobilize hand).

b. Record CMAP every minute for 5 minutes with muscle at rest to ensure stable baseline.

c. Perform max voluntary contraction for 5 minutes: resting every 15 sec for 3 seconds.

d. Ask patient to relax completely.

e. Record CMAP immediately, then every 2 minutes for 60 minutes afterwards or until there is no further decline (maybe >1 hour).

Decrement = (highest CMAP postexercise - smallest CMAP postexercise)/(highest CMAP postexercise) X 100.

Any decrement >40% is abnormal.

f. NB: Immediately after exercise, CMAP maybe larger before slow decline in amplitude.

Comments:

CMAP often increases with cooling in NORMAL individuals.

1. Myotonia congenita may have an initial but transient decrement after exercise that quickly resolves. Not worsened by cooling, and, as in normality, may increase with cooling.

2. Paramyotonia congenita have a prolonged decrement with combination of exercise and cooling that is still often apparent on rewarming.

Table of  EMG findings in Myotonia & Periodic Paralysis.

References:

McManis PG, Lambert EH, Daube JR.    The exercise test in periodic paralysis. Muscle Nerve 1986; 9 : 704.