INTRODUCTION: Monocarboxylate Transporter 8 (MCT8) is a thyroid hormone transporter expressed in various human organs, including brain tissue. Loss-of-function mutations in the MCT8 gene, located on the X chromosome, manifest as mild-to-moderate intellectual disability and moderate-to-severe psychomotor impairment in boys with pleasant dispositions and elevated serum T3 levels.  While affected boys present with marked axial hypotonia and quadriparesis, they have surprisingly little spasticity early in the disease course.  Rather, their hypotonia tends to be accompanied by subtle involuntary movements, specifically dystonia.  The lack of spasticity represents a challenge for their rehabilitation and explains why spasticity-directed therapies have produced suboptimal responses.  Our goal was to better define the spectrum of motor disabilities in MCT8 deficiency in order to improve the rehabilitation results of patients with this condition.

METHODS: To accomplish our goal, we evaluated a multi-national cohort of patients with MCT8 mutations.  Direct clinical evaluations, along with clinical video recordings (n=7), were reviewed by our pediatric neurogeneticist and pediatric movement disorder specialist.

RESULTS: Patient evaluations and videos reviewed on multiple MCT8 deficiency patients revealed a common pattern of hypotonic quadriparesis with superimposed dystonia.

CONCLUSION/DISCUSSION: This multi-national pilot study has better characterized the motor impairments associated with MCT8 deficiency.  It is anticipated that this information will allow physicians and therapists to focus their treatment efforts on the hyperkinetic elements of MCT8 deficiency, rather than the conventionally suspected spasticity.