Stimulus-induced drop episodes in Coffin-Lowry syndrome.

The Coffin-Lowry syndrome (CLS) is a rare but well-defined X-linked semidominant syndrome characterized by psychomotor and growth retardation, and progressive skeletal changes. CLS is caused by loss of function mutations in the Rps6ka3 gene encoding the ribosomal S6 kinase 2 (RSK2) protein. A distinctive paroxysmal disorder has been described in some CLS patients, characterized by episodes of sudden falling, without apparent alteration of consciousness, usually induced by unexpected tactile or auditory stimuli. Duration of episodes is very short, usually lasting a few seconds. The appellation "Stimulus-induced drop episodes" (SIDEs) was proposed for these non-epileptic events in CLS patients. SIDEs are clinically heterogeneous; with some patients exhibiting cataplexy-like events characterized by sudden hypotonia and collapse, and others hyperekplexia-like episodes with a startle response. The pathophysiology of SIDEs is not well understood.