Achondroplasia

Alternate names

ACH; Achondroplastic dwarfism

Definition

Achondroplasia is a disorder of bone growth that prevents the changing of cartilage (particularly in the long bones of the arms and legs) to bone. File:Achondroplasia.webm

Summary

• Achondroplasia is a form of short-limbed dwarfism.
• The word achondroplasia literally means "without cartilage formation."
• Cartilage is a tough but flexible tissue that makes up much of the skeleton during early development.
• However, in achondroplasia the problem is not in forming cartilage but in converting it to bone (a process called ossification), particularly in the long bones of the arms and legs.
• Achondroplasia is similar to another skeletal disorder called hypochondroplasia, but the features of achondroplasia tend to be more severe.

Three sisters with achondroplasia Wellcome L0061377

Epidemiology

Achondroplasia is the most common type of short-limbed dwarfism. The condition occurs in 1 in 15,000 to 40,000 newborns.

Female achondroplasic dwarf Wellcome L0062548

Cause

• Mutations in the FGFR3 gene cause achondroplasia.
• The FGFR3 gene provides instructions for making a protein that is involved in the development and maintenance of bone and brain tissue.
• Two specific mutations in the FGFR3 gene are responsible for almost all cases of achondroplasia.
• Researchers believe that these mutations cause the FGFR3 protein to be overly active, which interferes with skeletal development and leads to the disturbances in bone growth seen with this disorder.

Kyle Pacek

Inheritance

Autosomal dominant pattern, a 50/50 chance.

Achondroplasia is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. About 80 percent of people with achondroplasia have average-size parents; these cases result from new mutations in the FGFR3 gene. In the remaining cases, people with achondroplasia have inherited an altered FGFR3 gene from one or two affected parents. Individuals who inherit two altered copies of this gene typically have a severe form of achondroplasia that causes extreme shortening of the bones and an underdeveloped rib cage. These individuals are usually stillborn or die shortly after birth from respiratory failure.

Signs and symptoms

It is characterized by dwarfism, limited range of motion at the elbows, large head size (macrocephaly), small fingers, and normal intelligence. Achondroplasia can cause health complications such as interruption of breathing (apnea), obesity, recurrent ear infections, an exaggerated inward curve of the lumbar spine (lordosis). More serious problems include a narrowing of the spinal canal that can pinch (compress) the upper part of the spinal cord (spinal stenosis) and a buildup of fluid in the brain (hydrocephalus). Some people with achondroplasia may have delayed motor development early on, but cognition is normal.

For most diseases, symptoms will vary from person to person. People with the same disease may not have all the symptoms listed. 80%-99% of people have these symptoms

• Bowing of the legs(Bowed legs)
• Thoracolumbar kyphosis

30%-79% of people have these symptoms

• Abnormality of the shape of the midface
• Anteverted nares(Nasal tip, upturned)
• [[Brachydactyly](Short fingers or toes)
• Central sleep apnea
• Cervical spinal canal stenosis
• Depressed nasal bridge(Depressed bridge of nose)
• Frontal bossing
• Functional abnormality of the middle ear
• Hearing impairment(Deafness)
• Hip joint hypermobility
• Infantile muscular hypotonia(Decreased muscle tone in infant)
• Knee joint hypermobility(Knee joint over-flexibility)
• Limited elbow extension(Decreased elbow extension)
• Lumbar hyperlordosis(Excessive inward curvature of lower spine)
• Macrocephaly(Increased size of skull)
• Obstructive sleep apnea
• Parietal bossing
• Short long bone(Long bone shortening)
• Short middle phalanx of finger(Short middle bone of finger)
• Short nasal bridge(Decreased length of bridge of nose)
• Short proximal phalanx of finger(Short innermost finger bones)
• Trident hand

5%-29% of people have these symptoms

• Abnormality of the wing of the ilium
• [[Acanthosis nigricans)
• Flat acetabular roof
• Hydrocephalus(Too much cerebrospinal fluid in the brain)
• Hypoxemia(Low blood oxygen level)
• Narrow greater sciatic notch
• Obesity(Having too much body fat)
• Restrictive ventilatory defect(Stiff lung or chest wall causing decreased lung volume)
• Rhizomelia(Disproportionately short upper portion of limb)
• Thoracic hypoplasia(Small chest)
• Wide anterior fontanel(Wider-than-typical soft spot of skull)

Diagnosis

The diagnosis of achondroplasia should be suspected in the newborn with proximal shortening of the arms, large head, narrow chest, and short fingers. When there is clinical suspicion, radiographic features can confirm the diagnosis; neonatal radiographs will show square ilia and horizontal acetabula, narrow sacrosciatic notch, proximal radiolucency of the femurs, generalized metaphyseal abnormality, and decreasing interpedicular distance caudally.

Peter Dinklage 2008

Features that may be seen at any age

• Disproportionate short stature
• Macrocephaly with frontal bossing
• Midface retrusion and depressed nasal bridge
• Rhizomelic (proximal) shortening of the arms with redundant skin folds on limbs
• Limitation of elbow extension
• Brachydactyly
• Trident configuration of the hands
• Genu varum (bowlegs)
• Thoracolumbar kyphosis (principally in infancy)
• Exaggerated lumbar lordosis, which develops when walking begins

Radiographic findings

• Short, robust tubular bones
• Narrowing of the interpedicular distance of the caudal spine
• Square ilia and horizontal acetabula
• Narrow sacrosciatic notch
• Proximal femoral radiolucency
• Mild, generalized metaphyseal changes

Establishing the Diagnosis Those with typical findings generally do not need molecular confirmation of the diagnosis. In those in whom there is any uncertainty, identification of a heterozygous pathogenic variant in FGFR3 by molecular genetic testing can establish the diagnosis.

Treatment

Ventriculoperitoneal shunt may be required for increased intracranial pressure Suboccipital decompression as indicated for signs and symptoms of craniocervical junction compression Adenotonsillectomy, positive airway pressure, and, rarely, tracheostomy to correct obstructive sleep apnea Pressure-equalizing tubes for middle ear dysfunction Monitor and treat obesity Evaluation and treatment by an orthopedist if progressive bowing of the legs arises Spinal surgery may be needed for severe, persistent kyphosis Surgery to correct spinal stenosis in symptomatic adults Modification in the school and work setting to optimize function Educational support in socialization and school adjustment.[1].

Referaneces

Achondroplasia Resources
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NIH genetic and rare disease info

• NIH info on Achondroplasia

Achondroplasia is a rare disease.