Determining the cause of a patient’s myoclonus can involve a lengthy elimination process. Doctors usually begin by taking a full history including a description of the myoclonus, other neurological problems, any history of seizures, current or past drug or toxin exposure, other medical problems and family history.
Next, doctors will typically perform a physical examination noting where the myoclonus occurs, if it’s rhythmic or irregular, continuous or sporadic, if it occurs at rest or with voluntary movement, and whether it responds to stimuli such as touch, light, sound, or stretching a muscle.
To rule out potential causes, doctors may run tests to check for:
Electrolyte, glucose, kidney, liver or thyroid malfunction
Presence of paraneoplastic antibodies
Presence of drugs or other toxins
Irregularities in the brain or spine
To determine where in the nervous system the myoclonus originates, doctors will perform a series of neurophysiological tests, including specialized EEG (electrocephalography), EMG (electromyography) and/or combined EEG-EMG with evoked potential testing.
Types of myoclonus according to location include:
Cortical — In the cortex or surface of the brain
Cortical-subcortical — Co-activation between the cortex and subscortex (deeper in the brain)
Subcortical-supraspinal — A focal or spreading activation from the subcortex to the top of the spine
Spinal — In the spinal cord
Peripheral — In the nerves outside of the brain or spine
