Reproducibility and correlation with clinical severity of the McManis long-exercise test in patients with hypokalemic periodic paralysis

Hypokalemic periodic paralysis (HypoPP) is a rare, monogenetic muscle disorder
caused by a mutation in the skeletal muscle calcium (CACNA1S) or sodium
ionchannel gene (SCN4A). Clinically, these disorders are characterized by
attacks of flaccid muscle weakness of the extremities with an accompanying
shift in serum potassium from the serum compartment into the muscle. The
long-exercise test (LET) is a well validated diagnostic test for HypoPP that is
performed in between attacks. During the LET, a partial paralytic attack is
provoked in a small muscle (m. abductor digiti minimi) of the little finger by
five minutes of voluntary maximal exercise. After this exercise, the compound
muscle action potential (CMAP) amplitude is measured every two minutes with
evoked potential (EMG). The CMAP amplitude provides information on changes in
the number of active fibres and their ability to depolarize and repolarize.
Most studies in the field have only focused on the diagnostic quality of the
test; no previous study has investigated the reproducibility of this test, nor
its correlation with the clinical severity experienced by patients. More
knowledge on these aspects of the test will tell us more about the reliability
of the test as a diagnostic test as well as an objective assessment of
treatment status.

- To assess the reproducibility of the LET.
- To define the relation between the LET and clinical severity in HypoPP
patients.

Explorative, diagnostic study

Patients will be subjected to the LET four times, two times on study day 1 and
two times on study day 2. A single test takes about one hour to perform. During
the test, a transient paralytic attack in the musculus abductor digiti minimi
of the little finger of the patient will be provoked by 5 minutes maximal use
of this muscle. The paralytic attack only effects this muscle and therefore the
patient will not experience much inconvience. During the following 50 minutes,
a single electrical pulse to the ulnar nerve will be used to assess the level
of paralyses every 1-2 minutes. These pulses are not painful to the patient.
Next to this, patients will be asked to keep a diary of the paralytic attacks
they experience during one month, noting both length and severity of each
attack.