Melorheostosis

What it is, symptoms and effects

From: MedlinePlus Genetics, National Library of Medicine

Melorheostosis is a rare bone disease. It causes the abnormal growth of new bone tissue on the surface of existing bones. The new bone has a characteristic appearance on x-rays, often described as "flowing" or like dripping candle wax. The excess bone growth typically occurs on the bones in one arm or leg, although it can also affect the pelvis, breastbone (sternum), ribs, or other bones. (The term "melorheostosis" is derived from the Greek words "melos," which means limb; "rheos," which means flow; and "ostosis," which refers to bone formation.) The abnormal bone growth associated with melorheostosis is noncancerous (benign), and it does not spread from one bone to another.

The signs and symptoms of melorheostosis usually appear in childhood or adolescence. The condition can cause long-lasting (chronic) pain, permanent joint deformities (contractures), and a limited range of motion of the affected body part. In some people, the limb may appear thickened or enlarged, and the skin overlying the affected area can become red, thick, and shiny.

Another rare disease, Buschke-Ollendorff syndrome, can include melorheostosis. Buschke-Ollendorff syndrome is characterized by skin growths called connective tissue nevi and areas of increased bone density called osteopoikilosis. A small percentage of affected individuals also have melorheostosis or other bone abnormalities. Scientists originally speculated that melorheostosis that occurs without the other features of Buschke-Ollendorff syndrome might have the same genetic cause as that syndrome. However, it has since been determined that Buschke-Ollendorff syndrome and melorheostosis that occurs alone are caused by mutations in different genes, and the two conditions are considered separate disorders.

Causes and biological mechanisms

From: MedlinePlus Genetics, National Library of Medicine

Mutations in the MAP2K1 gene are estimated to cause about half of all cases of melorheostosis. The MAP2K1 gene provides instructions for making a protein called MEK1 protein kinase. This protein is active in many kinds of cells, including bone cells. It is part of a signaling pathway called RAS/MAPK. This signaling pathway helps control the growth and division (proliferation) of cells, the process by which cells mature to carry out specific functions (differentiation), and cell movement (migration). RAS/MAPK signaling is critical for normal development, including the formation of bones.

The MAP2K1 gene mutations that cause melorheostosis are somatic, which means that they occur during a person's lifetime and are present only in certain cells, in this case, bone cells in a particular area of the body. The mutations lead to the production of a version of MEK1 protein kinase that is overactive, which increases RAS/MAPK signaling in bone tissue. The increased signaling disrupts the regulation of bone cell proliferation, allowing new bone to grow abnormally. Studies suggest that increased RAS/MAPK signaling also stimulates excess bone remodeling, a normal process in which old bone is broken down and new bone is created to replace it. These changes in bone growth and turnover underlie the bone abnormalities characteristic of melorheostosis.

In cases of melorheostosis without an identified mutation in the MAP2K1 gene, the cause of the condition is usually unknown. Studies suggest that somatic mutations in other genes, particularly genes related to the RAS/MAPK signaling pathway, may also cause the disorder.

Inheritance and family implications

From: MedlinePlus Genetics, National Library of Medicine

This condition is not inherited from a parent, and it cannot be passed down to children. It arises from somatic mutations in bone cells that occur during an individual's lifetime.

How common is it?

From: MedlinePlus Genetics, National Library of Medicine

Melorheostosis affects about 1 in 1 million people. Approximately 400 cases have been reported 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.

Abnormality of the skeletal system · Very frequent (99-80%)

An abnormality of the skeletal system.

Arthralgia · Very frequent (99-80%)

Joint pain.

Bone pain · Very frequent (99-80%)

An unpleasant sensation characterized by physical discomfort (such as pricking, throbbing, or aching) localized to bone.

Ectopic ossification in muscle tissue · Very frequent (99-80%)

Formation of abnormal bony tissue within muscle tissue.

Failure to thrive · Very frequent (99-80%)

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

Hyperostosis · Very frequent (99-80%)

Excessive growth or abnormal thickening of bone tissue.

Increased bone mineral density · Very frequent (99-80%)

An abnormal increase of bone mineral density, that is, of the amount of matter per cubic centimeter of bones which is often referred to as osteosclerosis. Osteosclerosis can be detected on radiological examination as an increased whiteness (density) of affected bones.

Joint stiffness · Very frequent (99-80%)

Joint stiffness is a perceived sensation of tightness in a joint or joints when attempting to move them after a period of inactivity. Joint stiffness typically subsides over time.

Other findings in the same source

From: Orphanet

Additional reported features include Lymphedema (Very frequent (99-80%)); Skeletal dysplasia (Very frequent (99-80%)); Skeletal muscle atrophy (Very frequent (99-80%)); Cranial nerve paralysis (Very frequent (99-80%)); Lower limb asymmetry (Frequent (79-30%)); Upper limb asymmetry (Frequent (79-30%)); Arthritis (Occasional (29-5%)); Atypical scarring of skin (Occasional (29-5%)); Peripheral arteriovenous fistula (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 Melorheostosis 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 Melorheostosis. 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.