How to Differentiate Between Organic and Psychogenic Spinal Myoclonus
Abstract & Commentary
By Panida Piboolnurak, MD, Assistant Professor, Department of Neurology and Neuroscience, Weill Medical College, Cornell University. Dr. Piboolnurak reports no financial relationship relevant to this field of study.
Synopsis: Detection of Bereitschaftspotential (BP) via EEG-EMG back averaging is helpful in differentiating psychogenic myoclonus from organic myoclonus. But, technical limitations of this technique and its availability have limited its utilization in clinical practice.
Sources: Esposito M., et al. Idiopathic Spinal Myoclonus: A Clinical and Neurophysiological Assessment of a Movement Disorder of Uncertain Origin. Mov Disord Nov. 11, 2009. (www.interscience.wiley.com). DOI: 10.1002/mds.22812.
Myoclonus is a movement disorder described as brief, shock-like movements caused by muscle contraction or inhibition of ongoing muscle activity. It may originate from the central or peripheral nervous system. Spinal myoclonus originates from the spinal cord and can be subdivided into spinal segmental myoclonus and propriospinal myoclonus. In spinal segmental myoclonus, myoclonus is limited to muscles innervated by one or two contiguous spinal segments. Propriospinal myoclonus is characterized by axial muscle contractions as a result of a spinal myoclonic generator recruiting axial muscles up and down the spinal cord via propriospinal pathways. Although spinal cord lesions can be seen in patients with spinal myoclonus, structural lesions may not be present in some patients. Distinguishing myoclonus of organic origin from psychogenic myoclonus can also be difficult. Detection of a Bereitschaftspotential (BP) by EEG-EMG back averaging analysis can be helpful in differentiating between these two etiologies. BP consisted of early BP and late BP. The early BP starts two seconds and late BP starts about 400 milliseconds before the onset of a voluntary jerk.
In this study, the authors looked clinically and electrophysiologically at a series of 20 patients with idiopathic spinal myoclonus. EMG showed a pattern consistent with spinal segmental myoclonus in four patients and with propriospinal myoclonus in 16 patients. Seven patients showed a consistent pattern of muscle propagation, but 13 patients did not show a consistent pattern. A definite BP was detected in six patients and possible BP was detected in nine patients. BP was absent in five patients. Clinically, organic myoclonus was considered in five patients and psychogenic myoclonus was considered in 10 patients. In the remaining five patients, two clinicians had different opinions on the diagnosis. In the clinically diagnosed organic myoclonus group (n = 5), definite BP was detected in four cases and possible BP was detected in one case. In the clinically diagnosed psychogenic myoclonus group (n = 10), there was one case with definite BP, six cases with possible BP, and three cases with absent BP. In five patients with unclear clinical diagnosis, one case had definite BP, two cases had possible BP, and two cases had no BP.
The authors concluded that although BP detection is a useful tool, it has limitations because it may not be possible to reach a definite conclusion as to whether a BP is present or not. Moreover, if the jerking movements are too frequent (> 1 every 5 seconds), a BP may not be able to be recorded even if the movements are performed voluntarily. It is also difficult to assess for BP if the movements are very infrequent. In addition, head movements can create movement artifacts in the EEG recording.
It can be difficult to distinguish between psychogenic and organic myoclonus based on neurological examination and EMG pattern (burst length and pattern of muscle activation). Since BP precedes a voluntary jerk, the presence of BP would suggest myoclonus of psychogenic origin. However, it has technical limitations and there is no standardization of techniques to record BP. Moreover, there are only a few institutions that can perform EEG-EMG back averaging analysis. For these reasons, diagnosis of organic or psychogenic myoclonus still relies on careful history and neurological examination.