Aromatic L-amino acid decarboxylase (AADC) deficiency is a rare disease caused by mutations in a gene called DDC. The protein that is encoded by this gene makes is an enzyme called AADC, which is responsible for making important cell signaling molecules in the brain (neurotransmitters) like dopamine and serotonin.

Mutations in DDC lead to insufficient amounts of the AADC enzyme and, in turn, too little dopamine and serotonin being produced in the brain. A number of disease symptoms result from this lack, including dystonia.

What is dystonia?

Dystonia is a type of movement disorder characterized by involuntary muscle tensing or contracting, resulting in twisting motions, unusual postures, or slow and repetitive movements. Tremors may also be evident. The tensing or cramping of muscles can be painful.

What causes dystonia?

The causes of dystonia are not well understood, although it is thought to be caused by faulty communication between different areas of the brain.

In AADC deficiency, the lack of neurotransmitters can lead to faulty communication between different areas of the brain. Moreover, the lack of neurotransmitters during important developmental stages means that some brain areas may be poorly developed or have structural problems that also affect communication.

How is dystonia treated?

There is currently no cure for dystonia, but medications can help to ease its symptoms and improve patients’ quality of life.

These treatments focus on reducing muscle cramps by limiting communication between nerves and muscles. There are several approaches to doing this; a physician may want to try various strategies or some combination of them, depending on the patient’s symptoms.

The transmission of nerve signals from the brain to muscles occurs at the neuromuscular junction (the point where a nerve cell connects to a muscle). When nerve signals are sent from the brain (faulty or otherwise), a neurotransmitter called acetylcholine is released from the nerve cell ending. Acetylcholine binds to receptors found on the muscle side of the neuromuscular junction, causing the muscle to contract. After the nerve signal has been sent, an enzyme called acetylcholinesterase breaks down the excess acetylcholine, removing it from the neuromuscular junction and resetting the junction for the next signal.

Acetylcholinergic agents are those that block acetylcholine from activating its receptors, thus preventing or reducing the nerve signals that produce dystonia. Trihexyphenidyl and benztropine are examples of acetylcholinergic agents that can be used to treat dystonia in people with AADC deficiency.

Pain caused by muscle cramps may be treated with over-the-counter medications like acetaminophen, or by prescription with opioid narcotics like hydrocodone.


Last updated: Oct. 1, 2019


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