Aromatic L-Amino Acid Decarboxylase Deficiency
What else is it called?
- AADC deficiency
- DDC deficiency
- Deficiency of aromatic-L-amino-acid decarboxylase
- Dopa decarboxylase deficiency
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What causes it?
This disorder is caused by a problem with the DDC gene. This gene provides instructions for making the AADC enzyme which helps produce neurotransmitters from other molecules. Specifically, it helps produce serotonin and dopamine that transmit signals between nerve cells in the brain, spinal cord and other parts of the body.
If you have a problem with the DDC gene, you will not produce enough AADC enzymes. Therefore, you will be unable to produce enough serotonin and dopamine, which will cause disruptions in the nervous system functions.
How common is it?
We do not know how many people have this disorder. It is very rare with only a small number of cases that are known. It can affect people from any race or background and affects males and females equally. Affected individuals from Taiwan have made up around 20% of all people living with this condition worldwide.
What are the signs and symptoms?
Signs and symptoms usually develop in the first years of life. Some symptoms may be mild, some may be severe. They may include:
- Severe developmental delay
- Weak muscle tone (hypotonia)
- Muscle stiffness
- Difficulty moving
- Involuntary writhing movements of the limbs (athetosis, dystonia, tremors, chorea)
- Lack of energy (lethargic)
- Poor feeding
- Startle easily
- Sleep disturbances
- Episodes called oculogyric crises that involve abnormal rotation of the eyeballs, extreme irritability and agitation, muscle spasms, uncontrolled movements, especially of the head and neck
If affected the autonomic nervous system, you may have:
- Droopy eyelids (ptosis)
- Constriction of the pupils of the eyes (miosis)
- Inappropriate or impaired sweating
- Nasal congestion
- Reduced ability to control body temperature Low blood pressure (hypotension)
- Backflow of acidic stomach contents into the esophagus (gastroesophageal reflux)
- Low blood sugar (hypoglycemia)
- Fainting (syncope)
- Cardiac arrest
The biorhythm affects the course of signs and symptoms throughout the day. They tend to be milder in the morning or after sleep and get worse later in the day or with tiredness. In most cases, symptoms do not get worse as you become older. However, the prognosis depends on the severity of each case. Therefore, the deficiency may be life-threatening or fatal in some cases.
How is it diagnosed?
You will be diagnosed after a clinical examination by your specialist or doctor. However, as the signs and symptoms of this disorder are similar to other conditions, you will need further tests. You will need to undertake an examination called the lumbar puncture. Doctors may also run specialised urine, blood and plasma tests. Diagnosis is confirmed by a DNA test. In some countries, this deficiency can be detected by newborn screening. Unfortunately, this is not yet available in the UK.
Can it be treated?
Unfortunately, there is no cure for AADC deficiency. The treatment is focused on managing individual symptoms and providing comfort and relief in the form of supporting the individual and any family members. A multi-disciplinary team of specialists and support networks will be involved in the management of the condition and to offer practical and emotional support.
Your doctor or specialist will most likely prescribe you dopamine agonists, MAO (monoamine oxidase) inhibitors, pyridoxine/pyridoxal phosphate (PLP), anticholinergic and antiepileptic drugs, and melatonin. To prevent secondary complications and promote development, physiotherapy, speech therapy, occupational therapy, feeding and nutritional assessments, and (neuro)psychological treatment and support will be required. Gene therapy has shown some clinical improvements in its clinical trials.
Do my family need to be tested?
Aromatic l-amino acid decarboxylase deficiency can only be passed on to a child if both parents have a copy of the faulty gene. This is called autosomal recessive inheritance. A person who has a copy of the faulty gene is known as a carrier.
If both parents are carriers, their child has a one in four (25%) chance of inheriting the disorder, and a one in two chance (50%) of being a carrier. This is the same for each child the parents have.
If the child only inherits one copy of the faulty gene, they will be a carrier but will not have the disorder. In some rare cases, carriers have had mild symptoms of the disorder for which they carry the faulty gene.
Once you are diagnosed, you can speak to a genetic counsellor. They can explain how you may have inherited Aromatic l-amino acid decarboxylase deficiency. They can also tell you about genetic testing for the rest of your family. They can provide advice and support if you go on to have children of your own.