Allan-Herndon-Dudley syndrome is a condition in which the brain does not develop properly. This manifests as a varying range of intellectual disability and difficulty with normal movement. This disorder occurs wholly in males and affects development from when the foetus is in the womb. Even though affected males have poor speech development and limited communication skills, they seem to like interaction with other people.
What are the other names for Allan-Herndon-Dudley syndrome ?
- Allan-Herndon syndrome
- MCT8 (SLC16A2)-specific thyroid hormone cell transporter deficiency
- Mental retardation, X-linked, with hypotonia
What is the pathology behind Allan-Herndon-Dudley syndrome ?
Mutations in the specific SLC16A2 gene causes Allan-Herndon-Dudley syndrome. This SLC16A2 gene, otherwise known as MCT8, gives instructions for production of a protein that plays a vital role in the development of the body’s nervous system.
The Monocarboxylate Transporter 8 (MCT8) is a significant and widely expressed transporter of several thyroid hormones. This protein has the job of transporting triiodothyronine or T3, a thyroid hormone into the nerve cells of developing brain. Triiodothyronine or T3 is secreted by a very important, butterfly-shaped gland in the lower neck called the thyroid. T3 is significant, as it contributes to the regular formation and growth of both nerve cells as well as the junctions between nerve cells (called synapses) from which cell-to-cell communication occurs. T3 and other thyroid hormones regulate the development of other organs as well and control the body’s metabolism.
Hence, gene mutations will also alter the SLC16A2 protein. Consequently, this defective protein will be unable to move T3 into nerve cells successfully. It is an absence of this critical hormone in particular parts of the brain that disrupts normal brain and neural development, resulting in both intellectual and motion disability. And another effect of this is that as T3 is not taken up by nerve cells, surplus amounts of this hormone circulate in the bloodstream. These amplified T3 levels in the blood may be deadly to some organs and bring about the signs and symptoms of Allan-Herndon-Dudley syndrome.
How is the Allan-Herndon-Dudley syndrome passed on ?
It is an X-linked recessive disease. What this means is that in males (who have XY genetic make-up with only one X chromosome), a single altered copy of the gene in each cell is enough to cause the condition. However, in females (who have two X chromosome copies), a mutation must be present in both to cause the disorder. Hence, males are affected by X-linked recessive disorders far more frequently than females. A distinguishing feature of X-linked inheritance is that fathers cannot pass the X-linked traits to their sons.
What are the Symptoms of Allan-Herndon-Dudley syndrome ?
Patients with the Allan–Herndon–Dudley syndrome present in infancy with poor muscle tone (hypotonia), weakness, and a general failure to gain weight. They have overall developmental delays in childhood, and they display both spasticity (continuous contraction of muscles) and hyperreflexia (overreaction of your body’s involuntary nervous system to stimuli) as adults. Their thyroid-function tests show a characteristic pattern—high T3, low-normal T4, and high-normal TSH, which manifest as hyperthyroidism.
What you need to do if you think you have family history of Allan-Herndon-Dudley syndrome ?
Parents with family history of Allan-Herndon-Dudley syndrome need to undergo genetic counseling.
Thyroid analogs have to be administered prenatally and these are important for protecting cells from premature neuronal maturation.
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Disclaimer: The content provided here is meant for general informational purposes only and hence SHOULD NOT be relied upon as a substitute for sound professional medical advice, care or evaluation by a qualified doctor/physician or other relevantly qualified healthcare provider.
