Larsen syndrome

What it is, symptoms and effects

From: MedlinePlus Genetics, National Library of Medicine

Larsen syndrome is a disorder that affects the development of bones throughout the body. The signs and symptoms of Larsen syndrome vary widely even within the same family. Affected individuals are usually born with dislocations of the hips, knees, or elbows. Foot abnormalities, such as inward- and upward-turning feet (clubfeet), are also common. Affected individuals generally have small extra bones in their wrists and ankles that are visible on x-ray images. The tips of their fingers, especially the thumbs, are typically blunt and square-shaped (spatulate).

Characteristic facial features in people with Larsen syndrome include a prominent forehead (frontal bossing), flattening of the bridge of the nose and middle of the face (midface hypoplasia), and wide-set eyes (ocular hypertelorism). Many people with Larsen syndrome have an opening in the roof of the mouth (a cleft palate). Affected individuals may also have hearing loss caused by malformations in tiny bones in the ears (ossicles).

Short stature is a common feature of Larsen syndrome. In addition, people with the condition may have an unusually large range of joint movement (hypermobility) or joint deformities (contractures) that restrict movement. People with Larsen syndrome can also have abnormal curvature of the spine (kyphosis or scoliosis) that can impair breathing or compress the spinal cord and lead to weakness of the limbs. Some affected individuals experience respiratory problems, such as partial closing of the airways, short pauses in breathing (apnea), and frequent respiratory infections. Heart and kidney problems can also occur in people with Larsen syndrome. People with this condition can survive into adulthood. Their intellectual function is usually unaffected.

Causes and biological mechanisms

From: MedlinePlus Genetics, National Library of Medicine

Mutations in the FLNB gene cause Larsen syndrome. The FLNB gene provides instructions for making a protein called filamin B. This protein helps build the network of protein filaments (cytoskeleton) that gives structure to cells and allows them to change shape and move. Filamin B attaches (binds) to another protein called actin and helps the actin form the branching network of filaments that makes up the cytoskeleton. It also links actin to many other proteins to perform various functions within the cell, including the cell signaling that helps determine how the cytoskeleton will change as tissues grow and take shape during development.

Filamin B is especially important in the development of the skeleton before birth. It is active (expressed) in the cell membranes of cartilage-forming cells (chondrocytes). Cartilage is a tough, flexible tissue that makes up much of the skeleton during early development. Most cartilage is later converted to bone (a process called ossification), except for the cartilage that continues to cover and protect the ends of bones and is present in the nose, airways (trachea and bronchi), and external ears. Filamin B appears to be important for normal cell growth and division (proliferation), for maturation (differentiation) of chondrocytes, and for the ossification of cartilage.

FLNB gene mutations that cause Larsen syndrome change single protein building blocks (amino acids) in the filamin B protein or delete a small section of the protein sequence, resulting in an abnormal protein. This abnormal protein appears to have a new, atypical function that interferes with the proliferation or differentiation of chondrocytes, impairing ossification and leading to the signs and symptoms of Larsen syndrome.

Inheritance and family implications

From: MedlinePlus Genetics, National Library of Medicine

This condition is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. In some cases, an affected person inherits the mutation from one affected parent. Other cases result from new mutations in the gene and occur in people with no history of the disorder in their family.

Autosomal recessive inheritance of Larsen syndrome has been reported in a small number of families. Autosomal recessive means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition. In some of these cases, the appearance of autosomal recessive inheritance may actually result from multiple siblings in a family each inheriting a single altered gene from an unaffected parent who has an FLNB mutation only in some or all of their sperm or egg cells. When a mutation is present only in reproductive cells, it is known as germline mosaicism.

A few rarer conditions with overlapping signs and symptoms and autosomal recessive inheritance have sometimes been diagnosed as Larsen syndrome, but they are now generally considered to be different disorders because they are typically more severe and are not caused by FLNB gene mutations.

How common is it?

From: MedlinePlus Genetics, National Library of Medicine

Larsen syndrome occurs in approximately 1 in 100,000 newborns. However, some doctors think the condition is more common and is misdiagnosed as other conditions with similar features.

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.

Brachydactyly · Very frequent (99-80%)

Digits that appear disproportionately short compared to the hand/foot. The word brachydactyly is used here to describe a series distinct patterns of shortened digits (brachydactyly types A-E). This is the sense used here.

Broad distal phalanx of finger · Very frequent (99-80%)

Abnormally wide (broad) distal phalanx of finger.

Broad thumb · Very frequent (99-80%)

Increased thumb width without increased dorso-ventral dimension.

Depressed nasal bridge · Very frequent (99-80%)

Posterior positioning of the nasal root in relation to the overall facial profile for age.

Flat face · Very frequent (99-80%)

Absence of concavity or convexity of the face when viewed in profile.

Hypertelorism · Very frequent (99-80%)

Interpupillary distance more than 2 SD above the mean (alternatively, the appearance of an increased interpupillary distance or widely spaced eyes).

Joint hypermobility · Very frequent (99-80%)

The capability that a joint (or a group of joints) has to move, passively and/or actively, beyond normal limits along physiological axes.

Malar flattening · Very frequent (99-80%)

Underdevelopment of the malar prominence of the jugal bone (zygomatic bone in mammals), appreciated in profile, frontal view, and/or by palpation.

Other findings in the same source

From: Orphanet

Additional reported features include Prominent forehead (Very frequent (99-80%)); Short distal phalanx of finger (Very frequent (99-80%)); Short nail (Very frequent (99-80%)); Large joint dislocations (Very frequent (99-80%)); Accessory carpal bones (Frequent (79-30%)); Abnormality of epiphysis morphology (Occasional (29-5%)); Abnormality of the cardiovascular system (Occasional (29-5%)); Abnormality of the cervical spine (Occasional (29-5%)); Cleft palate (Occasional (29-5%)); Conductive hearing impairment (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 Larsen syndrome 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 Larsen syndrome. 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.