ALLAN-HERNDON-DUDLEY SYNDROME

ALTERNATE NAMES

Allan-Herndon Syndrome; X-linked Intellectual Deficit with Hypotonia; Monocarboxylate Transporter 8 Deficiency; MCT8 Deficiency; MCT8 Specific Thyroid Hormone Cell Transporter Deficiency; MCT8 SLC16A2

DESCRIPTION

Allan-Herndon-Dudley Syndrome (AHDS) is a rare inherited disorder of brain development that causes moderate to severe intellectual disability and problems with movement. Some children with AHDS have difficulty with communication and speech. It is characterized by weak muscle tone (hypotonia) and underdevelopment of muscles (muscle hypoplasia). Mutations in the SLC16A2 gene (also known as MCT8) cause AHDS. The SLC16A2 gene provides instruction for making a protein that transports thyroid hormone triiodothyronine (T3) into nerve cells during development. Because of the mutation, normal brain development is disrupted and T3 accumulates in the blood, causing toxicity in some organs and exacerbations of the symptoms of AHDS. This condition occurs exclusively in males.

DIAGNOSTIC TESTING, PHYSICAL FINDINGS, AND ICD-9-CM CODING

Diagnostic testing: The diagnosis is established by a combination of clinical examination and molecular genetic testing for mutations in the SLC16A2 gene; elevated T3 and decreased T4 in the blood.

Physical findings: The physical findings of AHDS may include poor growth, elongation of the face, abnormal folding of the ears, developmental delay, intellectual disability, poor head control, abnormal muscle tone (initially hypotonia, later evolving into spasticity), joint contractures, and unclear or no speech.

ICD-9: 359

ONSET AND PROGRESSION

Children with AHDS usually appear normal at birth with signs of the disease occurring shortly after birth with poor muscle tone, inability to control head movements, and physical and developmental delays. Progressively worsening muscle weakness, stiffness, exaggerated reflexes, joint deformities, and involuntary movements of the limbs eventually leads to wheel chair dependency by early adulthood. Some children may have delays in language development.

TREATMENT

Presently, there is no standardized treatment for AHDS. Treatment is symptom specific and supportive.

SUGGESTED PROGRAMMATIC ASSESSMENT*

Suggested MER for evaluation:

• Clinical history and examination that describes the progression of neurological and cognitive decline from the claimant’s medical source(s).

• Genetic testing for mutations in the SLC16A2 (or MCT8) gene.

• Laboratory blood testing with elevations in free T3 and decreased free T4.

Suggested Listings for Evaluation:

DETERMINATION

LISTING

Meets Listing

111.17

Medical Equals

111.07B