Idiopathic spinal myoclonus: a clinical and neurophysiological assessment of a movement disorder of uncertain origin.

Spinal Myoclonus (SM) is characterized by brief and sudden movements caused by the activation of muscles belonging to adjacent spinal myotomes. Recent reports have indicated that "typical" clinical and electrophysiological features of SM can be mimicked voluntarily. A useful tool that can distinguish between organic and psychogenic jerks is the detection of a Bereitschaftspotential (BP). In this study, we looked for evidence of a BP in a cohort of patients with idiopathic SM. A clinical and neurophysiological assessment of 20 patients affected by idiopathic SM was performed. A video EEG-EMG multichannel recording was performed in each patient to detect BP. An expert neurophysiologist (PB) reviewed the BP recordings and divided them into those showing a definite, possible, and no BP. A clinical assessment of the videoed movements was performed by two neurologists expert in movement disorders (KB and MJE) who indicated if the movements were compatible with organic or psychogenic myoclonus. A definite or possible BP was recorded in 15 out of 20 patients. Clinical raters agreed in their clinical opinion on 15 patients (75%). All patients where both raters agreed the movements appeared to be organic had definite or possible BP. BP are commonly seen in patients with idiopathic SM. There is discordance between clinicians in their clinical rating of SM as organic or psychogenic, but even in those patients where movements appear clinically to be organic, a BP is commonly detected, indicating that the aetiology is psychogenic. This suggests that BP recordings are a useful adjunct to clinical assessment in the accurate diagnosis of patients with idiopathic SM.