[MIM 300 523]

(deficiency in MCT8, deficiency in monocarboxylate carrier 8, X-linked intellectual deficiency-hypotonia)

Very rare, but could be the cause of 1 to 2 % of the X-linked intellectual deficiencies of unknown origin. Mutations of the SLC16A2 gene (Xq13.2) coding for the MCT8 (monocarboxylate carrier 8) protein, one of the T3 thyroid hormone specific carriers in neurons. 

Biology: very high free T3, normal or low free T4 , and normal TSH.

Clinical presentation: only boys are suffering from that syndrome:

-         congenital hypotonia progressing to spasticity in adolescence (contractures, hyperreflexia, clonus)

-         muscular hypoplasia with a delay in motor development (difficulty to hold the head), delay of speech

-         severe intellectual deficiency

-         typical face: elongated, poorly expressive, open mouth (drooling), upper lip in a gendarme's shape, abnormal ears, thickened soft tissues in the nose and ears 

-         long, fine and everted feet

-         absence of speech or severe dysarthria

-         dystonic movements or athetosis

-         sometimes: convulsions (25%), pectus excavatum, scoliosis

Symptomatic treatment: anticholinergics, L-Dopa, glycopyrrolate to decrease oral secretions, antiepileptics, antispasticity. Thyroid opotherapy has no effect.

Anesthetic implications: 

hypotonia, contractures, mental retardation, epilepsy.

References:

-         Schwartz CE, May MM, Carpenter NJ, Rogers RC et al. 
Allan-Herndon-Dudley syndrome and the monocarboxylate carrier 8 (MCT8) gene. 
Am J Hum Genet 2005; 77: 41-53. 

Updated: December 2019