Histidinemia

What it is, symptoms and effects

From: MedlinePlus Genetics, National Library of Medicine

Histidinemia is an inherited condition characterized by elevated levels of histidine in blood, urine, and the fluid that surrounds the brain and spinal cord (cerebrospinal fluid). Histidine is an amino acid that acts as a building block for many different proteins.

In most cases, histidinemia does not cause health problems. Most people with elevated histidine levels are unaware that they have this condition. Rarely, people with histidinemia have intellectual disabilities, learning disabilities, or behavioral problems. Having a medical complication during or soon after birth might increase the risk of developmental problems in people with histidinemia.

Causes and biological mechanisms

From: MedlinePlus Genetics, National Library of Medicine

Histidinemia is caused by variants (also called mutations) in the HAL gene, which provides instructions for making an enzyme called histidase. Histidase breaks down histidine that the body gets from food. Histidase is active (expressed) primarily in the liver and the skin.

HAL gene variants impair the enzyme's ability to break down histidine, which causes histidine to build up in the blood, urine, and cerebrospinal fluid.

Inheritance and family implications

From: MedlinePlus Genetics, National Library of Medicine

This condition is typically 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.

How common is it?

From: MedlinePlus Genetics, National Library of Medicine

Since many people who have histidinemia do not have signs and symptoms, it is hard to know exactly how many people have the condition. The condition appears to be more common among people of Japanese descent and among the French Canadian population in Quebec, Canada.

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.

Histidinuria · Obligate (100%)

An increased concentration of histidine in the urine.

Hyperhistidinemia · Obligate (100%)

The concentration of histidine in the blood circulation is above the upper limit of normal.

Abnormality of speech or vocalization · Very rare (<4-1%)

An abnormality in the sound (volume) or cadence (rate) of speech.

Atypical behavior · Very rare (<4-1%)

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.

Hyperactivity · Very rare (<4-1%)

Hyperactivity is a condition characterized by constant and unusually high levels of activity, even in situations where it is deemed inappropriate.

Moderate global developmental delay · Very rare (<4-1%)

A moderate delay in the achievement of motor or mental milestones in the domains of development of a child.

Specific learning disability · Very rare (<4-1%)

Impairment of certain skills such as reading or writing, coordination, self-control, or attention that interfere with the ability to learn. The impairment is not related to a global deficiency of intelligence.

Which doctor should you see?

The suggested department for discussing Histidinemia 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 Histidinemia. 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.