Ataxia, tremor, intellectual disability: a case of STXBP1 encephalopathy with a new mutation

Abstract

STXBP1 gene mutations are among the most common mutations in earlyonset epileptic encephalopathies. The clinical spectrum of STXBP1 mutationsis not limited to epileptic phenotypes and also includes atypical Rett syndromeand non-syndromic sporadic severe intellectual disability. Tremor, dystonia,choreiform movements, stereotypical head movements and ataxia may also beseen. However, the phenotypical spectrum is not as well-known as the othercommon SCN1A or CDKL5 gene mutations, making the clinical diagnosisdifficult and usually requiring gene panel studies or whole exome sequencingfor the diagnosis. We present a 17-year-old male patient whose seizures startedat the age of 12 years. The patient could only make limited eye contact, wouldcontinuously scream, and also had severe intellectual disability, marked ataxicwalking and a very significant coarse tremor. The patient was clinically thoughtto have STXBP1 encephalopathy due to the presence of severe intellectualdisability together with tremor, and ataxia. STXBP1 gene analysis revealed anew c.9_13delCATTG (pIle4Profs*12) (p.I4Pfs*12) (heterozygous) frameshiftmutation. In conclusion, STXBP1 encephalopathy should be considered ifsevere intellectual disability is accompanied by severe tremor and ataxia ina patient with epileptic and developmental encephalopathy. A normal headcircumference supports the diagnosis in such patients.