Diagnosis

Differential Diagnosis

When patients are being evaluated for a possible diagnosis of myoclonus, it is important that the clinician rules out other movement disorders, including tremor, tics, chorea, and dystonia, although some patients may have a combination of myoclonus and these other movement disorders. The movements of myoclonus are most often confused with those of tremor, although the rapid speed and brief duration of myoclonus are unquestionably associated with myoclonus. Brief, explosive tics may appear to be myoclonic movements; however, the patients' ability to temporarily suppress their tics, as well as the waxing and waning of these tics, are not indicative of myoclonus.

• Tremor
• Tics
• Chorea
• Exaggerated startle response (hyperexplexia)
• Dystonia

Temporal Onset of Symptoms

It may be necessary for patients to keep a diary of when these symptoms occur during the course of their day (temporal onset). Changes in diets, drugs, or other alternative or homeopathic remedies, including recent or long-term exposure to toxins, should be noted.

The course of treatment is based upon the type of myoclonus; therefore, an accurate diagnosis—including identification of temporal onset of symptoms, triggering factors, and the source of the movements (e.g., cortical, subcortical, spinal, or peripheral)—are important.

During the diagnostic evaluation, the physician may ask some of the following questions.

• Is the myoclonus spontaneous?
  Answering this question requires observation, which may sometimes be prolonged. Attempts by the physician to bring out these movements may be necessary. For example, for some patients, sitting with their arms outstretched may cause the abnormal movements to occur. Evaluators may also clap their hands and note the response or they may flick the patients' distal finger joints and observe any responsive movement.
• What is the distribution of the movements?
  Evaluators typically note whether the movements are limited to one area of the body or muscle group or if many areas are involved
• Are there any associated systemic or neurologic abnormalities such as tremor, dystonia, epilepsy, or chorea?
• Are the movements positive (contracted) or negative (loss of muscle tone followed by contraction)?

Clinical Assessment

When evaluating a patient for possible myoclonus, the physician usually conducts a thorough physical examination and records a history of both the patient and the patient's family members. Sometimes, when myoclonus is mild, family members may have a "symptomless" (subclinical) form of the disorder, and the family history may seem to be negative when it is really positive for the presence of myoclonus. Testing may include a variety of laboratory, imaging, and neurophysiologic studies, as well as genetic testing. Some physicians use a rating scale known as the Unified Myoclonus Rating Scale to assist in evaluating myoclonic symptoms, as well as response to treatment.

Laboratory and Imaging Studies

Laboratory studies that may be helpful in evaluating the patient with myoclonus include measurements of circulating electrolytes and glucose in the blood, assessment of kidney and liver functions, and screening studies for drugs, toxins, and antibodies.

The purpose of conducting imaging studies is to help identify the possible underlying disease process causing the myoclonus, including cortical myoclonus in the brain and reticular myoclonus in the brainstem. Magnetic resonance imaging (MRI) of the brainstem may be ordered for those patients who exhibit symptoms associated with palatal myoclonus.

Neurophysiologic Studies

Neurophysiologic studies used in diagnosing and evaluating myoclonus include electroencephalography (EEG), electromyography (EMG), and other specialized studies as such EEG-EMG back-averaging, and evoked potentials. EEG is used to identify myoclonus of cortical origin—including epileptic syndromes and epilepsia partialis continua—and to identify symptomatic syndromes such as Creutzfeldt-Jakob disease. Children with sudden severe myoclonus or changes warrant study with an EEG. EEG-EMG back-averaging is used to localize the source of the movements; however, this technique is not routinely available in most healthcare settings. Motor and somatosensory evoked potentials (SEPs) may be used to assess the characteristics of myoclonus.

Genetic Testing

Genetic testing may be undertaken to identify associated conditions such as myoclonus dystonia, Huntington disease, any of the SCAs, myoclonic epilepsy, and dentatorubralpallidoluysian atrophy.

	Treatment