Huntington’s disease (HD) is a genetic disorder caused by an expanded CAG repeat in the huntingtin gene, and although there are currently no disease-modifying treatments, there is much excitement about the prospect of treatments targeting huntingtin expression. In a phase I/2A trial of an antisense oligonucleotide (ASO) treatment (Tominersen), no serious adverse events were recorded, and there was a dose-dependent reduction in cerebrospinal fluid (CSF) huntingtin levels1. In an open-label extension (OLE) study, patients received monthly or bimonthly Tominersen, with preliminary data confirming the reduction in mutant huntingtin levels2. Here we report on a unique major adverse effect occurring during this OLE.