Aromatic l-amino acid decarboxylase deficiency

What it is, symptoms and effects

From: MedlinePlus Genetics, National Library of Medicine

Aromatic l-amino acid decarboxylase (AADC) deficiency is an inherited disorder that affects the way nerve cells (neurons) transmit information to other cells.

Signs and symptoms of AADC deficiency typically appear in the first six months of life. Affected infants may have feeding problems, weak muscle tone (hypotonia), and sleep disturbances. Most children with AADC deficiency experience episodes called oculogyric crises that involve involuntary upward-rolling movements of the eyes. Additional signs and symptoms typically include developmental delays and intellectual disabilities.

Movement disorders are common in people with AADC deficiency. Involuntary movements that can occur in people with AADC deficiency include muscle contractions that may cause unusual body positions (dystonia) and writhing movements of the limbs (athetosis). Dystonia tends to become worse when the individual is tired, but it usually improves after sleep. Affected individuals may also experience slow or diminished movements (hypokinesia).

AADC deficiency may also affect the autonomic nervous system, which controls involuntary body processes such as the regulation of blood pressure and body temperature. This can lead to signs and symptoms such as droopy eyelids (ptosis), constriction of the pupils of the eyes (miosis), inappropriate or impaired sweating, nasal congestion, drooling, poor control of body temperature, low blood pressure (hypotension), low blood glucose (hypoglycemia), and abnormal heart rhythms.

People with AADC deficiency have an increased risk of infection, which can lead to life-threatening complications.

Causes and biological mechanisms

From: MedlinePlus Genetics, National Library of Medicine

Variants (also called mutations) in the DDC gene cause AADC deficiency. The DDC gene provides instructions for making the AADC enzyme, which plays an important role in the nervous system. Within neurons, the AADC enzyme helps produce dopamine and serotonin, which are two types of neurotransmitters. Neurotransmitters are chemical messengers that transmit important signals from neurons to other cells in the body. Dopamine can be used to create additional neurotransmitters called epinephrine and norepinephrine.

Variants in the DDC gene can reduce the activity of the AADC enzyme. This causes neurons to produce less dopamine and serotonin than normal, which in turn reduces the production of epinephrine and norepinephrine. Changes in the levels of these four neurotransmitters contribute to the developmental delays, intellectual disabilities, abnormal movements, and autonomic nervous system dysfunction seen in people with AADC deficiency.

Inheritance and family implications

From: MedlinePlus Genetics, National Library of Medicine

This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell must have a variant to cause the disorder. The parents of an individual with an autosomal recessive condition each carry one copy of the altered gene, but they typically do not show signs and symptoms of the condition.

How common is it?

From: MedlinePlus Genetics, National Library of Medicine

AADC deficiency is a rare disorder. Approximately 350 people with this condition have been reported in the literature. The condition is more prevalent in certain Asian populations, particularly among people from Taiwan, Japan, and China.

Reported clinical features and what the terms mean

The following findings are associated with this condition in Orphanet. They are not a checklist for diagnosing yourself, and they do not all occur in every affected person. Some are examination, imaging or laboratory findings that cannot be recognised at home.

The frequency labels describe how often a finding was reported among people with the condition in the source. They do not give the chance that a person with that symptom has the condition. Definitions below reproduce HPO terminology; they explain the term, not the likely severity in an individual.

Decreased CSF 5-hydroxyindolacetic acid concentration · Very frequent (99-80%)

5-HIAA (5-hydroxyindolacetic acid) concentration in the cerebrospinal fluid (CSF) is below the lower limit of normal.

Decreased CSF homovanillic acid concentration · Very frequent (99-80%)

Decreased concentration of homovanillic acid (HVA) in the cerebrospinal fluid. HVA is a metabolite of dopamine.

Global developmental delay · Very frequent (99-80%)

A delay in the achievement of motor or mental milestones in the domains of development of a child, including motor skills, speech and language, cognitive skills, and social and emotional skills. This term should only be used to describe children younger than five years of age.

Atypical behavior · Frequent (79-30%)

Atypical behavior is an abnormality in a person's actions that can be controlled or modulated by the will of the individual. While abnormal behaviors can be difficult to control, they are distinct from other abnormal actions that cannot be affected by the individual's will.

Dystonia · Frequent (79-30%)

An abnormally increased muscular tone that causes fixed abnormal postures. There is a slow, intermittent twisting motion that leads to exaggerated turning and posture of the extremities and trunk.

Failure to thrive · Frequent (79-30%)

Failure to thrive (FTT) refers to a child whose physical growth is substantially below the norm.

Feeding difficulties · Frequent (79-30%)

Impaired ability to eat related to problems gathering food and getting ready to suck, chew, or swallow it.

Gastroesophageal reflux · Frequent (79-30%)

A condition in which the stomach contents leak backwards from the stomach into the esophagus through the lower esophageal sphincter.

Other findings in the same source

From: Orphanet

Additional reported features include Hyperhidrosis (Frequent (79-30%)); Hypotonia (Frequent (79-30%)); Intellectual disability (Frequent (79-30%)); Irritability (Frequent (79-30%)); Motor delay (Frequent (79-30%)); Oculogyric crisis (Frequent (79-30%)); Poor head control (Frequent (79-30%)); Seizure (Frequent (79-30%)); Sleep abnormality (Frequent (79-30%)); Autistic behavior (Occasional (29-5%)). This is a selected summary, not a complete description of the condition.

Which doctor should you see?

The suggested department for discussing Aromatic l-amino acid decarboxylase deficiency is Metabolic Medicine, with a metabolic specialist / clinical geneticist as the relevant type of clinician. Paediatrician (children) or physician (adults), with metabolic specialist / clinical geneticist referral.

This is an editorial referral starting point. The appropriate clinic depends on the person’s age, symptoms, previous diagnosis and local services. The first clinician can decide whether another specialty or a team is needed; a department label does not confirm the diagnosis.

How to prepare for an assessment

Bring a short timeline of the main symptoms: when they first appeared, whether they are constant or episodic, what seems to change them, and how they affect daily activities. Include previous reports, discharge summaries, current medicines and supplements, allergies, and any relevant family history. A dated record is more useful than trying to match every feature in an online article.

Ask the clinician what is already established and what remains uncertain. If a test is suggested, ask what question it answers, what its limitations are and how the result would change the next step. The information here is not an instruction to arrange every possible test. In children, bring growth, developmental and school information if it is relevant to the concern.

• Is a biochemical or molecular result needed to clarify the diagnosis?
• Does this condition require an individual plan for illness or reduced food intake?
• Should nutrition advice come from a specialist metabolic dietitian?

Treatment discussions and follow-up

The material gathered for this draft does not provide a complete condition-specific treatment pathway for Aromatic l-amino acid decarboxylase deficiency. That gap does not mean that treatment is unavailable. A clinician needs to establish the diagnosis and review current guidance before recommending medicines, procedures, rehabilitation or other support.

Before leaving the appointment, clarify the next review date, who will communicate results, and whom to contact if the situation changes. Discuss difficulties with sleep, work, school, mobility, eating or emotional wellbeing when these are relevant. Practical support may require coordination between the treating clinician and other services.

The collected references do not establish a complete prevention or long-term outlook section for this entry. Missing information should not be interpreted as proof that prevention is impossible or that a particular outcome is inevitable. Ask what is known for the exact subtype, stage and personal circumstances, and which uncertainties remain.

When to seek emergency help

Severe breathing difficulty, collapse, new stroke-like symptoms, a seizure that is prolonged or repeated without recovery, uncontrolled major bleeding, or an immediate risk of self-harm require emergency help. In India, call 112 or reach the nearest emergency department. This is a general, non-exhaustive warning list; it is not a condition-specific triage tool.