A case of abdominal functional myoclonus analyzed by movement related cortical potentials

Diagnosis of functional (psychogenic) movement disorders is challenging, causing concern for many clinicians. Here, we describe a case in which MRCP (movement related cortical potentials) was useful for understanding pathophysiology.

The patient was a 16-year-old male with history of glycogenosis and Crohn's disease. He also had a history of possible autism spectrum disorder and psychogenic gait disturbance at age 12. On admission, persistent abdominal jerk had appeared. Abdominal jerks presented like hiccups, appearing once every few seconds and reducing in amplitude and frequency with distraction. Surface electromyogram with EEG excluded epileptic spasms. MRCP, with rectus abdominis muscle contractions set as a trigger, showed slowly rising negative waves (Bereitschaftspotentials, BP) from 0.6 s prior to the trigger. The presence of BP suggests that the abnormal movement emerges through a voluntary motor pathway. We comprehensively diagnosed his involuntary jerks as functional abdominal myoclonus.

In a voluntary movement, after the drive to move from frontal and limbic regions, MRCP shows slow negative waves originating from pre-Supplementary Motor Area (SMA) and SMA, as BP, before real muscle contraction. The sense of agency is also simultaneously generated in this process by the collaborative work of the frontal and parietal lobes. Even though this abnormal movement goes through the voluntary motor pathway, the patient may not be aware of voluntariness due to lack of sense of agency.

MRCP is still challenging, but useful for the diagnosis of functional (psychogenic) movement disorders.