A Cause of Severe Hypotonia in Infancy: Allan-Herndon-Dudley Syndrome

Background: We present a male case with the diagnosis of Allan-Herndon-Dudley syndrome (AHDS) which is an X-linked disease caused by mutations in the solute carrier family 16 member 2 (SLC16A2) gene. The phenotypic features of the disease are severely impaired intellectual and motor development, dysarthria, athetoid movements and spastic paraplegia. Also, as SLC16A2 encodes the monocarboxylate transporter 8 (MCT8), a thyroid hormone transporter, patients with Allan-Herndon-Dudley syndrome present a specific altered thyroid hormone profile. Patient and Methods: A 1-year-old boy was referred to our clinic due to mild dysmorphic signs such as large ears, elongated face,global developmental delay,and severe neonatal and infantil hypotonicity. A severe axial and peripheral hypotonicity, increased patellar deep tendon reflexes and clonus, and positive Babinski sign were determined in his neurological examination. Dystonic posturing was realized in his hand. He had a drop neck and he could not able tosit independently. Intermittent ataxia was observed in his upper extremities. Despite the normal serum levels of thyroid-stimulating hormone, free T3 had increased.The cranial MRI was reported as normal that was planned around 10 months old. Results: The SLC16A2gene hemizygous deletion was determined at c.467_469 in WES analysis. The detected variant was reported as pathogenic due to ACMG 2015 guideline. The family study is in progress. Conclusion: We emphasize that diagnosis of ADHS, is a very rare condition and can not be distinguished by phenotypic features; but should be remembered in patients, especially if there is an abnormal thyroid test, severe neonatal-infantil hypotonicity, and motor and cognitive delay.
Keywords: genetic, mutation, hypotonia