Melorheostosis

Overview

Melorheostosis is a very rare bone disorder where extra layers of bone grow on the surface of existing bones. This overgrowth often has a distinctive appearance on X-rays, resembling 'flowing wax' down the side of a candle [PMID:33678082]. It typically affects only one side of the body (unilateral) and commonly involves a single limb, although multiple bones or even bones in different parts of the body can be affected [PMID:33678082]. The condition is not inherited from parents; instead, it arises from changes that happen after conception.

While melorheostosis can be present at birth, symptoms often become noticeable in childhood or young adulthood. The exact number of people affected is not well established due to its rarity, but it is considered an exceptionally uncommon condition. The severity of symptoms can vary widely among individuals.

Symptoms & clinical features

The primary symptom of melorheostosis is pain, which can range from mild to severe and may worsen with activity. Stiffness and reduced range of motion in the affected limb are also common. The characteristic excessive bone growth (hyperostosis) can be felt as hard lumps under the skin and may cause visible limb deformities, such as shortening or lengthening of a limb, or bending of affected fingers or toes.

Over time, the affected bones can become thicker and denser, potentially leading to contractures, where joints become permanently bent and difficult to straighten. Skin changes over the affected bone, such as thickening or discolouration, have also been observed in some individuals. In rare cases, nerve compression can occur if the bone overgrowth puts pressure on nearby nerves, leading to numbness, tingling, or weakness.

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Affected organs

Melorheostosis primarily affects the bones, particularly the long bones of the arms and legs, as well as the small bones of the hands and feet. Any bone can potentially be involved, but involvement of the spine, skull, or ribs is less common. The excessive bone growth often appears on the outer surface of the bone, giving it a wavy or uneven texture.

While the condition is fundamentally a bone disorder, the secondary effects of bone overgrowth can impact surrounding soft tissues, such as muscles, tendons, and nerves. Joint involvement can also occur, leading to stiffness and reduced function in the affected joints.

Multiple body systems

Systemic involvement

Cellular impact

Mechanism at cellular level

Risks & severity

The severity of melorheostosis varies considerably between individuals. Some people may experience mild, intermittent pain and minimal functional limitation, while others may have severe pain, significant limb deformities, and substantial impact on their mobility and daily life. The condition is generally progressive, meaning the bone overgrowth can slowly worsen over time, but the rate of progression is unpredictable.

Complications can include chronic pain, joint contractures, limb length discrepancies requiring orthopaedic intervention, and in rare instances, nerve compression or vascular compromise. While melorheostosis is considered a benign (non-cancerous) condition, its chronic nature and potential for progressive symptoms mean that individuals often require ongoing medical management.

Genetic causes

Melorheostosis is understood to be caused by changes in the *MAP2K1* gene. These genetic changes are typically somatic, meaning they occur in some body cells after conception but are not present in every cell of the body. This is known as mosaicism, where an individual has two or more genetically different sets of cells [PMID:33678082].

The *MAP2K1* gene provides instructions for making a protein called MEK1. This protein is a crucial component of a signalling pathway within cells, known as the RAS/MAPK pathway. This pathway is essential for regulating many cellular processes, including cell growth, division, and specialisation [PMID:28076326]. Pathogenic changes in *MAP2K1* can lead to an overactive MEK1 protein, which in turn causes uncontrolled cell proliferation and excessive bone formation in the affected areas.

• MAP2K1
  mitogen-activated protein kinase kinase 1
  The MAP2K1 gene encodes the MEK1 protein, a key component of the RAS/MAPK signalling pathway, essential for cellular growth, division, and differentiation.

Inheritance pattern

Melorheostosis is primarily considered a somatic or mosaic condition. This means the genetic change responsible for the condition occurs randomly in a cell during early development, after fertilisation, rather than being inherited from a parent. As a result, only a subset of the body's cells carry the genetic change, typically those in the affected bone and surrounding tissues.

Because the genetic changes are somatic, melorheostosis is generally not passed down through families. There are very few documented cases of familial inheritance, and these are considered extremely rare exceptions. For most individuals, there is no increased risk of their children inheriting the condition.

Diagnosis & testing

Diagnosis of melorheostosis typically begins with clinical examination and imaging studies. X-rays often show the characteristic 'flowing wax' appearance of excessive bone growth. Further imaging, such as CT scans or MRI, may be used to assess the extent of bone involvement and any impact on surrounding soft tissues. A bone biopsy may occasionally be performed to confirm the diagnosis and rule out other bone conditions, though this is less common now with advanced imaging techniques.

Genetic testing for somatic *MAP2K1* variants can confirm the diagnosis, particularly in challenging cases or for research purposes. This type of testing usually requires a tissue sample from the affected bone, rather than a blood sample, due to the mosaic nature of the genetic change. In the UK, genetic testing would typically be coordinated through the NHS Genomic Medicine Service, following referral to a clinical genetics service, and may fall under relevant R-codes for rare constitutional disorders.

Management & lifestyle

Management of melorheostosis focuses on alleviating symptoms and improving function, as there is currently no cure for the condition. Pain management is a key aspect, often involving a combination of pain medications and physical therapy to maintain joint mobility and muscle strength. Occupational therapy can help individuals adapt to any functional limitations caused by the condition.

Orthopaedic interventions may be necessary to address complications such as limb length discrepancies, joint contractures, or severe deformities. This could involve surgical procedures to remove excess bone, correct deformities, or lengthen/shorten limbs. Regular follow-up with orthopaedic specialists and pain management clinics is typically part of the ongoing care pathway within the NHS. There is ongoing research into targeted therapies that might modulate the overactive signalling pathways in affected cells, but these are not yet standard treatment options.

UK care pathway

In the UK, individuals suspected of having a rare bone condition like melorheostosis would typically be referred by their GP or another specialist to a clinical genetics service within the NHS Genomic Medicine Service. A clinical geneticist or a specialist orthopaedic consultant would then assess the need for genetic testing. Genetic counsellors are available to provide information and support regarding the condition, its inheritance, and the implications of genetic testing results. Diagnostic pathways for rare diseases are typically supported by the NHS National Genomic Test Directory.

Related conditions

Melorheostosis shares some features with other conditions involving bone overgrowth or developmental anomalies. For instance, osteopoikilosis and Buschke-Ollendorff syndrome are conditions associated with sclerotic bone lesions, though their genetic causes and clinical presentation differ. Progressive diaphyseal dysplasia (Camurati-Engelmann disease) also involves bone thickening but typically affects the shafts of long bones more symmetrically. Maffucci syndrome and Ollier disease are conditions characterised by benign cartilage tumours and can also cause limb deformities. Differentiating between these conditions is important for accurate diagnosis and management.

Frequently asked questions

References

1. Nielsen GP, Jarraya M. Bone Tumor Mimics. Surgical pathology clinics. 2025. PMID: 40716925
2. Rozencwaig R, Wilson MR, McFarland GB Jr. Melorheostosis. American journal of orthopedics (Belle Mead, N.J.). 1997. PMID: 9040882
3. Deshmukh NS. Melorheostosis (Leri's Disease): A Review. Cureus. 2024. PMID: 38978887
4. Watts NB, Chesnut CH 3rd, Genant HK. History of etidronate. Bone. 2020. PMID: 31911206
5. Wordsworth P, Chan M. Melorheostosis and Osteopoikilosis: A Review of Clinical Features and Pathogenesis. Calcified tissue international. 2019. PMID: 30989250
6. Leong AWS, Langdon J, Malhotra V. A case of spinal melorheostosis. Journal of clinical imaging science. 2022. PMID: 35251764
7. Vo TT, To K, Dang TN. Late-Onset Melorheostosis: A Case Report. Case reports in oncology. 2023. PMID: 38130895
8. Dhillon MS, Saibaba B. Melorheostosis-Foot and ankle perspective. Foot (Edinburgh, Scotland). 2017. PMID: 28544914