SADDAN

What it is, symptoms and effects

From: MedlinePlus Genetics, National Library of Medicine

SADDAN (severe achondroplasia with developmental delay and acanthosis nigricans) is a rare disorder of bone growth characterized by skeletal, brain, and skin abnormalities.

All people with this condition have extremely short stature with particularly short arms and legs. Other features include unusual bowing of the leg bones; a small chest with short ribs and curved collar bones; short, broad fingers; and folds of extra skin on the arms and legs. Structural abnormalities of the brain cause seizures, profound developmental delay, and intellectual disability. Several affected individuals also have had episodes in which their breathing slows or stops for short periods (apnea). Acanthosis nigricans, a progressive skin disorder characterized by thick, dark, velvety skin, is another characteristic feature of SADDAN that develops in infancy or early childhood.

Causes and biological mechanisms

From: MedlinePlus Genetics, National Library of Medicine

Mutations in the FGFR3 gene cause SADDAN. The FGFR3 gene provides instructions for making a protein that is involved in the development and maintenance of bone and brain tissue. A mutation in this gene may cause the FGFR3 protein to be overly active, which leads to the disturbances in bone growth that are characteristic of this disorder. Researchers have not determined how the mutation disrupts brain development or causes acanthosis nigricans.

Inheritance and family implications

From: MedlinePlus Genetics, National Library of Medicine

SADDAN is considered an autosomal dominant disorder because one mutated copy of the FGFR3 gene in each cell is sufficient to cause the condition. The few described cases of SADDAN have been caused by new mutations in the FGFR3 gene and occurred in people with no history of the disorder in their family. No individuals with this disorder are known to have had children; therefore, the disorder has not been passed to the next generation.

How common is it?

From: MedlinePlus Genetics, National Library of Medicine

This disorder is very rare; it has been described in only a small number of individuals worldwide.

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.

Acanthosis nigricans · Very frequent (99-80%)

A dermatosis characterized by thickened, hyperpigmented plaques, typically on the intertriginous surfaces and neck.

Aplasia/Hypoplasia of the mandible · Very frequent (99-80%)

Absence or underdevelopment of the mandible.

Brain atrophy · Very frequent (99-80%)

Partial or complete wasting (loss) of brain tissue that was once present.

Enlarged cerebellum · Very frequent (99-80%)

An abnormally increased size of the cerebellum compared to other brain structures.

Generalized-onset seizure · Very frequent (99-80%)

A generalized-onset seizure is a type of seizure originating at some point within, and rapidly engaging, bilaterally distributed networks. The networks may include cortical and subcortical structures but not necessarily the entire cortex.

Hypoplasia of the corpus callosum · Very frequent (99-80%)

Underdevelopment of the corpus callosum.

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

Severe intellectual disability (ID) is defined as a type of ID characterized by severely sub-average adaptive functioning and intellectual functioning, with an intelligence quotient (IQ) the range of 20-34.

Metaphyseal chondrodysplasia · Very frequent (99-80%)

An abnormality of skeletal development characterized by a disturbance of the metaphysis and its histological structure with relatively normal epiphyses and vertebrae.

Severe global developmental delay · Very frequent (99-80%)

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

Abnormality of the clavicle · Frequent (79-30%)

Any abnormality of the clavicles (collar bones).

Femoral bowing · Frequent (79-30%)

Bowing (abnormal curvature) of the femur.

Fibular bowing · Frequent (79-30%)

A bending or abnormal curvature of the fibula.

Microcephaly · Frequent (79-30%)

Head circumference below 2 standard deviations below the mean for age and gender.

Tibial bowing · Frequent (79-30%)

A bending or abnormal curvature of the tibia.

Which doctor should you see?

The suggested department for discussing SADDAN is Orthopaedics, with a orthopaedic specialist as the relevant type of clinician. General physician / Family Medicine; paediatrician for children. Referral depends on symptoms.

Additional services that may be relevant, depending on the findings, include: Clinical Genetics.

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.

• What explains the change in pain, movement or function?
• Which activities need adjustment while the diagnosis is being clarified?
• What are the roles of rehabilitation, observation and surgery in this situation?

Treatment discussions and follow-up

The material gathered for this draft does not provide a complete condition-specific treatment pathway for SADDAN. 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.