Investigations in channelopathies
Neurophysiology
Neurophysiology is a key part of the investigation of muscle channelopathies.
EMG & Nerve conduction studies
In patients with non-dystrophic myotonia, EMG should demonstrate myotonia. Rarely muscles may need to be cooled to demonstrate the myotonia. If no myotonia is documented on EMG then it is very unlikely that a patient has non-dystrophic myotonia.
Short Exercise Tests
The short exercise test involves briefly exercising the adductor digitii minimi muscle and recording the change in compound muscle action potential (CMAP) over time and with repeated contractions. This is usually done both at room temperature and after cooling. This can be very helpful in distinguishing which type of non-dystrophic myotonia a patient has and can help direct genetic testing.
Long Exercise Test
This test involves exercising the adductor digitii minimi muscle for 5 minutes and then recording the change in CMAP over 50 minutes. The test is positive if there is a >40% decrement in CMAP during the 50 minute period of monitoring. This can be very useful in diagnosing periodic paralysis. Patients with frequent attacks of weakness and a normal long exercise test and normal genetic testing are very unlikely to have periodic paralysis.
Genetic Testing
Genetic testing is the gold standard for diagnosing muscle channelopathies. Patients with a suspected muscle channelopathy can have the skeletal muscle channeopathy panel run on their DNA sample. This panel currently covers 11 genes, although it is frequently being updated as new genes are discovered. It covers the main channel genes CLCN1, SCN4A, CACNA1S, and KCNJ2 as well as a number of rarer genes. It also includes testing for deletions and duplications in the CLCN1 and KCNJ2 genes. If this panel is negative then a diagnosis of a muscle channelopathy is less likely and patients will often require clinical reassessment to look for other possible diagnoses.
MRI
MRI of the lower limbs can be useful in identifying whether a patient has active disease or any muscle damage. In patients with periodic paralysis, MRI can give an indication of whether treatment is effective in reducing water accumulation and active disease.
