X-linked intellectual disability, Siderius type

What it is, symptoms and effects

From: MedlinePlus Genetics, National Library of Medicine

X-linked intellectual disability, Siderius type is a condition characterized by mild to moderate intellectual disability that affects only males. Affected boys often have delayed development of motor skills such as walking, and their speech may be delayed.

Individuals with X-linked intellectual disability, Siderius type frequently also have an opening in the lip (cleft lip) with an opening in the roof of the mouth (cleft palate). A cleft can occur on one or both sides of the upper lip.

Some boys and men with this condition have distinctive facial features, including a long face, a sloping forehead, a broad nasal bridge, a prominent bone in the lower forehead (supraorbital ridge), and outside corners of the eyes that point upward (upslanting palpebral fissures). Affected individuals may also have low-set ears and large hands.

ORPHANET DEFINITION X-linked intellectual disability, Siderius type is characterised by mild to borderline intellectual deficit associated with cleft lip/palate. Preaxial polydactyly, large hands and cryptorchidism are sometimes present. The syndrome has been described in seven boys from two families. Transmission is X-linked and the syndrome is caused by mutations in the PHF8 gene, localised to the p11.21 region of the X chromosome.

Inheritance

From: MedlinePlus Genetics, National Library of Medicine

X-linked recessive

Frequency in the source

From: MedlinePlus Genetics, National Library of Medicine

Reported family(ies): 2.0; Worldwide. This is a published case count, not prevalence. Point prevalence: <1 / 1 000 000; Worldwide; Class only.

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.

Broad nasal tip · Very frequent (99-80%)

Increase in width of the nasal tip.

Cleft upper lip · Very frequent (99-80%)

A gap or groove in the upper lip. This is a congenital defect resulting from nonfusion of tissues of the lip during embryonal development.

Intellectual disability, mild · Very frequent (99-80%)

Mild intellectual disability (ID) is defined as a type of ID characterized by mildly sub-average adaptive functioning and intellectual functioning, with an intelligence quotient (IQ) the range of 50-69.

Long face · Very frequent (99-80%)

Facial height (length) is more than 2 standard deviations above the mean (objective); or, an apparent increase in the height (length) of the face (subjective).

Orofacial cleft · Very frequent (99-80%)

The presence of a cleft (gap, opening, or groove) in the oral cavity, including cleft of the upper lip and/or cleft of the palate. Cleft of the upper lip is visible as a groove or fissure in the lip, most frequently due to a congenital failure of the maxillary and median nasal processes to fuse. Cleft palate is characterized by a grooved depression or fissure in the roof of the mouth, most often resulting from a congenital failure of the palate to fuse properly. Clefts of the lip and palate can occur individually or together. It is preferable to code each defect separately.

Large hands · Very frequent (99-80%)

Cryptorchidism · Frequent (79-30%)

Testis in inguinal canal. That is, absence of one or both testes from the scrotum owing to failure of the testis or testes to descend through the inguinal canal to the scrotum.

Decreased testicular size · Frequent (79-30%)

Reduced volume of the testicle (the male gonad).

Low posterior hairline · Occasional (29-5%)

Hair on the neck extends more inferiorly than usual.

Preaxial hand polydactyly · Occasional (29-5%)

Supernumerary digits located at the radial side of the hand. Polydactyly (supernumerary digits) involving the thumb occurs in many distinct forms of high variability and severity. Ranging from fleshy nubbins over varying degrees of partial duplication/splitting to completely duplicated or even triplicated thumbs or preaxial (on the radial side of the hand) supernumerary digits.

Scoliosis · Occasional (29-5%)

The presence of an abnormal lateral curvature of the spine.

Synophrys · Occasional (29-5%)

Meeting of the medial eyebrows in the midline.

When it may begin

From: MedlinePlus Genetics, National Library of Medicine

Infancy; Neonatal

Inheritance in the source

From: MedlinePlus Genetics, National Library of Medicine

X-linked recessive

Frequency and the population described

From: MedlinePlus Genetics, National Library of Medicine

Reported family(ies): 2.0; Worldwide. This is a published case count, not prevalence. Point prevalence: <1 / 1 000 000; Worldwide; Class only.

Understanding the inheritance label

From: MedlinePlus Genetics

An X-linked recessive pattern involves a gene on the X chromosome. The number of X chromosomes and the particular genetic change influence how a condition is expressed. A genetic counsellor should interpret the result and the family history before discussing risks for relatives or future pregnancies.

Which doctor should you see?

The suggested department for discussing X-linked intellectual disability, Siderius type is Clinical Genetics, with a clinical geneticist as the relevant type of clinician. Paediatrician (children) or physician (adults), with 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.

• Does the exact genetic or chromosome finding explain the observed features?
• Would a genetic counsellor help the family understand the result?
• Which organ-specific assessments are appropriate for this particular diagnosis?

Treatment discussions and follow-up

The material gathered for this draft does not provide a complete condition-specific treatment pathway for X-linked intellectual disability, Siderius type. 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.