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Exostoses (Hereditary Multiple Exostoses)

Overview

Hereditary Multiple Exostoses (HME), also called multiple osteochondromas, is a rare genetic disorder characterized by the growth of multiple benign bone tumors called exostoses (or osteochondromas). These growths arise from the outer surface of long bones, especially near the growth plates of the arms and legs, and can cause pain, deformity, and functional limitation.

• Who it affects: HME is inherited in an autosomal‑dominant pattern, meaning a child has a 50 % chance of inheriting the mutation from an affected parent. Both males and females are equally affected.
• Prevalence: The condition occurs in approximately 1 in 50,000 – 1 in 100,000 individuals worldwide, making it one of the more common inherited skeletal dysplasias (CDC, Mayo Clinic).
• Typical age of onset: Exostoses become apparent in early childhood, usually between ages 3 and 5, and continue to grow until skeletal maturity (around 16‑20 years).

Symptoms

Symptoms vary widely depending on the number, size, and location of the exostoses. Common manifestations include:

• Painless lump(s): Hard, bony protrusions that can be felt under the skin, most often near the knees, elbows, wrists, and ankles.
• Localized pain: Usually due to irritation of surrounding muscles, tendons, or nerves; pain may worsen with activity.
• Limited range of motion: Exostoses near joints can restrict movement, leading to difficulty with activities such as climbing stairs or reaching overhead.
• Leg length discrepancy: Unequal growth of the femur or tibia can cause one leg to be shorter, resulting in a limp or scoliosis.
• Deformities: Bowing of long bones, angular deformities of the forearm (e.g., ulnar shortening), or malalignment of the wrist and ankle.
• Cosmetic concerns: Prominent outward growths may cause self‑esteem issues, especially in children and adolescents.
• Neurologic symptoms: Nerve compression (e.g., ulnar nerve palsy at the elbow) can produce numbness, tingling, or weakness.
• Vascular compression: Rarely, large exostoses compress blood vessels, leading to swelling, discoloration, or claudication.
• Fracture through an exostosis: Trauma can cause a break that may be painful and may require surgical stabilization.
• Potential malignant transformation: In ~1–5 % of patients, an exostosis can evolve into a chondrosarcoma, presenting with rapid growth, night pain, or a new palpable mass.

Causes and Risk Factors

Genetic basis

HME is caused by pathogenic variants in either the EXT1 (≈55 % of cases) or EXT2 (≈40 % of cases) genes. These genes encode glycosyltransferases essential for the synthesis of heparan sulfate, a molecule important for normal bone growth regulation. Loss‑of‑function mutations disrupt cartilage development at the growth plate, leading to exostosis formation.

Inheritance pattern

• Autosomal‑dominant: A single altered copy of EXT1 or EXT2 is sufficient to cause disease.
• De novo mutations: Approximately 10–15 % of cases arise from a new mutation in a child whose parents are not affected.

Risk factors

• Having a parent or sibling with a confirmed diagnosis.
• Carrying a known pathogenic EXT1/EXT2 variant (even if asymptomatic).
• Early skeletal growth spurts, which may accelerate exostosis size.

Diagnosis

Diagnosis relies on a combination of clinical evaluation, imaging studies, and, when needed, genetic testing.

Clinical assessment

• Physical exam to identify palpable bony protrusions, assess limb length, and evaluate joint range of motion.
• Family history to detect hereditary patterns.

Imaging

• Plain radiographs (X‑ray): First‑line; shows characteristic sessile or pedunculated bony outgrowths with a continuous cortex and medullary cavity.
• Magnetic Resonance Imaging (MRI): Useful for evaluating cartilage caps (thicker caps >2 cm in adults raise suspicion for malignancy) and soft‑tissue involvement.
• CT scan: Provides detailed bone anatomy, helpful for pre‑operative planning.

Genetic testing

Targeted sequencing or multi‑gene panels that include EXT1 and EXT2 confirm the diagnosis, aid in family counseling, and differentiate HME from sporadic solitary osteochondromas.

Diagnostic criteria (per NIH consensus)

1. Two or more osteochondromas (or one osteochondroma plus a first‑degree relative with HME).
2. Presence of a pathogenic variant in EXT1 or EXT2.

Treatment Options

There is no cure for HME; management focuses on symptom control, functional preservation, and prevention of complications.

Non‑surgical approaches

• Analgesia: Acetaminophen or NSAIDs (ibuprofen, naproxen) for mild‑to‑moderate pain. Use caution with long‑term NSAID therapy (gastrointestinal, renal risks).
• Physical therapy: Stretching and strengthening exercises improve joint mobility and muscle balance, especially after surgical removal.
• Orthotics: Shoe lifts or custom braces can address leg‑length discrepancy and improve gait.
• Activity modification: Encourage low‑impact sports (swimming, cycling) and avoid repetitive trauma to areas with large exostoses.

Surgical interventions

Surgery is indicated when exostoses cause pain, functional limitation, neurovascular compromise, or have suspicious features for malignancy.

• Exostectomy: Removal of the osteochondroma; the procedure preserves surrounding structures and typically has a low recurrence rate (<10 %).
• Corrective osteotomy: Realigns bones when exostoses cause deformities (e.g., forearm angular deformity).
• Limb‑lengthening techniques: Circular external fixators (Ilizarov) or internal lengthening nails can address significant leg‑length difference.
• Malignancy management: Wide excision with negative margins for chondrosarcoma, sometimes followed by radiation or chemotherapy per oncologic guidelines.

Emerging therapies

Research into modulating heparan‑sulfate synthesis (e.g., small‑molecule inhibitors of EXT pathways) is ongoing, but no agents are yet approved for routine clinical use (NIH).

Living with Exostoses (Hereditary Multiple Exostoses)

Effective self‑management can improve quality of life and reduce the need for repeated surgeries.

Daily management tips

• Regular self‑exams: Feel for new or enlarging lumps, especially after growth spurts.
• Exercise: Maintain flexibility and strength; a balanced routine of aerobic activity, stretching, and resistance training is ideal.
• Joint protection: Use padded sleeves or cushions when kneeling or leaning on elbows to prevent irritation.
• Weight management: Keeping a healthy body weight reduces stress on joints and exostoses.
• Dental care: Some patients develop mandibular exostoses; routine dental exams can detect oral lesions early.
• Psychosocial support: Join patient‑support groups (e.g., HME Alliance) to share experiences and coping strategies.
• School & work accommodations: Request ergonomic seating or modified physical‑education activities if needed.

Monitoring schedule

Age range	Recommended follow‑up
0‑5 years	Physical exam & X‑ray every 1‑2 years
6‑12 years	Annual exam; radiographs if new symptoms
13‑18 years	Every 6‑12 months; MRI if cartilage cap >1 cm
Adults	Annual exam; MRI if pain or rapid growth

Prevention

Because HME is genetic, primary prevention is not possible. However, secondary prevention—reducing the impact of the disease—can be achieved through:

• Genetic counseling for families planning children.
• Early detection of problematic exostoses to allow timely surgical removal.
• Avoiding high‑impact or contact sports that increase trauma risk.
• Routine monitoring for malignant transformation.

Complications

If left unmanaged, HME can lead to several serious outcomes:

• Functional limitation: Persistent pain and joint deformity may impair walking, sports, or daily tasks.
• Neurovascular injury: Nerve compression (e.g., peroneal, radial) can cause chronic weakness or sensory loss.
• Fracture through an exostosis: May require complex orthopedic fixation.
• Malignant transformation: Chondrosarcoma carries a 5‑year survival of ~70 % when diagnosed early, but drops sharply if detected late (Cleveland Clinic).
• Spinal involvement: Rarely, cervical spine exostoses can cause cord compression, presenting with neck pain, paresthesia, or weakness.
• Psychological impact: Chronic pain and cosmetic concerns may lead to anxiety or depression.

When to Seek Emergency Care

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Sources: Mayo Clinic, CDC, National Institutes of Health (NIH), World Health Organization (WHO), Cleveland Clinic, peer‑reviewed articles in Journal of Bone & Joint Surgery and Orphanet Journal of Rare Diseases.

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