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Achondroplasia is the most frequent form of short-limb dwarfism. As well as short stature due to shortening of limbs, affected individuals have characteristic facies with frontal bossing and mid-face hypoplasia, exaggerated lumbar lordosis, limitation of elbow extension, genu varum and trident-like hands. Incidence increases with paternal age.
Achondroplasia is caused, in virtually all of the cases, by a G380R mutation in fibroblast growth factor receptor 3 (FGFR3).FGFR3 is also important in craniofacial, vertebral and neurological development such that this mutation has multiple effects in an affected individual.
- Achondroplasia is apparent at birth and has a birth prevalence of 1 in 20,000-30,000 live-born infants. Although it is inherited as an autosomal dominant condition, 80% of cases occur sporadically.
- Incidence increases with paternal age.
- For couples of average stature who have a child with achondroplasia, the risk of recurrence is less than 1%, reflecting the risk of germline mosaicism.
- In the homozygous form the condition is severe and lethal.
- At birth or within the first year of life, with disparity between large skull, normal length trunk and short arms and legs.
- The chest is usually very narrow.
- Fingertips may only come down to the iliac crest.
- Shortness is particularly evident in the proximal segments of limbs.
- Limbs appear very broad with deep creases and trident-like hands with short fingers.
- There is often increased joint laxity.
- Skull shows a bulging vault, small face and a flat nasal bridge or 'scooped out' glabella.
- Spine shows marked lumbar lordosis.
- Frontal bossing, depressed nasal bridge.
- Hypochondroplasia: less pronounced skeletal disproportion and spinal abnormalities with the skull unaffected.
- Achondroplasia-like dwarfism (distinguish radiologically).
- Spondyloepiphyseal dysplasias: the spine is mainly affected, with the trunk short as well as the limbs.
- Proportionate dwarfism.
- The diagnosis is based on the typical clinical and X-ray features.
- Prenatal diagnosis is by ultrasound. However, prenatal sonographic diagnosis often fails as limb length is preserved until around 22 weeks of gestation, after the time of the routine fetal anomaly scan.
- Prenatal diagnosis of homozygous achondroplasia is also available in families at risk and in which the parents are heterozygous for either the 1138A or 1138C allele.
- Plasma can be analysed for the FGFR3 mutation in the mother when a short-limb skeletal dysplasia is diagnosed prenatally on ultrasound.
- A full skeletal survey should be undertaken if there is clinical suspicion of skeletal dysplasia, such as disproportionate short stature, limb malalignment or specific dysmorphic features.
- Confirmatory molecular analysis to detect the recurrent G380R FGFR3 mutations may be helpful where there is residual doubt about the diagnosis.
- X-rays show metaphyseal irregularity, flaring in the long bones, and late-appearing irregular epiphyses. The pelvis is narrow in anteroposterior diameter with deep sacroiliac notches and short iliac wings. The spine shows progressive narrowing of the interpedicular distance from top to bottom (reverse of normal).
- Investigation of possible cranial abnormalities and hydrocephalus includes ultrasound, and CT and MRI scans. Detailed imaging of the craniocervical junction is particularly important in infants in order to rule out spinal cord compression (see 'Complications', below).
- However, one recent study has shown that cervical cord lesions are observed in around 40% of people with achondroplasia and this are actually not associated with any clinical symptoms.
- Molecular genetic testing is the gold standard. Whilst clinical and X-ray features will identify the majority of patients, this is the only means of differentiating achondroplasia from the other forms of skeletal dysplasias. It is also a helpful investigation prior to considering therapeutic options and genetic counselling.
- It is recommended that all people with achondroplasia be followed up regularly to detect the significant complications that occur in approximately 10% of these children.
- There are condition-specific centile charts available to monitor the growth of children with achondroplasia.
- Upper to lower segment ratio should be monitored.
- Dental treatment for crowding of teeth.
- Management of frequent middle ear infections. Treatment of obstructive sleep apnoea - eg, adenotonsillectomy, weight loss and continuous positive airway pressure.
- As excess weight gain is often a significant issue for many affected children, regular height and weight measurement later in childhood and an emphasis on maintaining weight gain within acceptable limits from an early age are very important.
- Growth hormone therapy is not routinely given as there is no evidence of significant increases in final adult stature following the administration of growth hormone.
- Anti-inflammatory drugs may be helpful in patients with degenerative joint disease.
- Surgical intervention includes enlargement of the foramen magnum in cases of severe stenosis, lengthening of the limb bones, tibial osteotomy or epiphysiodesis of the fibular growth plate to correct bowing of the legs, and lumbar laminectomy for spinal stenosis (typically presents in early adulthood).
The degree of complications and disability is variable:
- Gross motor skills in particular develop later in the child with achondroplasia; approximately 50% of children will sit alone by 9 months and just over 50% will walk alone by 18 months.
- Short arms, limited elbow and hip extension, and knee and leg deformities can cause disabilities in arm function and locomotion.
- A progressive, unresolving thoracolumbar kyphosis can occur.
- Hydrocephalus, a narrow foramen magnum, spinal deformity, and spinal canal stenosis can cause neurological problems (eg, progressive quadriparesis, pain, ataxia, incontinence) leading to disabilities in locomotion, communication, and learning.
- Skeletal disproportion can lead to early osteoarthritis, problems with childbirth in women, hydrocephalus and paraplegia.
- Narrowing of the spinal canal causes symptoms of spinal stenosis.
- Obesity is common; once the child has reached 75 cm there tends to be a disproportionate increase in weight compared with height.
- Ear, nose and throat abnormalities such as recurrent otitis media, upper respiratory tract obstruction, deafness, speech delay, and jaw malocclusion can also lead to disabilities in communication and learning.
- Respiratory complications may include apnoea (including obstructive sleep apnoea) and abnormalities of gas exchange.Children with respiratory dysfunction may be associated with cognitive deficit.
- The most severe complication results from craniocervical stenosis and medullary and upper spinal cord compression, which can have devastating and even lethal sequelae during early childhood.
- Life expectancy is normal in the majority of cases.
- Overall cognitive scores are normal but some children may exhibit mild deficits in visual-spatial tasks.
- Final height varies between 80 cm and 150 cm.
- Death in the first year of life can occur due to pressure on the spinal cord, caused by abnormalities at the craniocervical junction.
Further reading and references
; Molecular studies of achondroplasia. Indian J Orthop. 2009 Apr43(2):194-6.
; FGFR3 mutation causes abnormal membranous ossification in achondroplasia. Hum Mol Genet. 2014 Jan 20.
; Achondroplasia. Lancet. 2007 Jul 14370(9582):162-72.
; Neurologic manifestations of achondroplasia. Handb Clin Neurol. 2014119:551-63. doi: 10.1016/B978-0-7020-4086-3.00036-9.
; New evidence for positive selection helps explain the paternal age effect observed in achondroplasia. Hum Mol Genet. 2013 Oct 1522(20):4117-26. doi: 10.1093/hmg/ddt260. Epub 2013 Jun 4.
; Germline mosaicism in achondroplasia detected in sperm DNA of the father of three affected sibs. Am J Med Genet A. 2008 Mar 15146A(6):784-6. doi: 10.1002/ajmg.a.32228.
; New aids for the non-invasive prenatal diagnosis of achondroplasia: dysmorphic features, charts of fetal size and molecular confirmation using cell-free fetal DNA in maternal plasma. Ultrasound Obstet Gynecol. 2011 Mar37(3):283-9. doi: 10.1002/uog.8893. Epub 2011 Feb 1.
; FGFR3 targeting strategies for achondroplasia. Expert Rev Mol Med. 2012 Jan 1914:e11. doi: 10.1017/erm.2012.4.
; Clinical management of achondroplasia. Arch Dis Child. 2012 Feb97(2):129-34. doi: 10.1136/adc.2010.189092. Epub 2011 Apr 3.
; Cervical high-intensity intramedullary lesions in achondroplasia: aetiology, prevalence and clinical relevance. Eur Radiol. 2012 Oct22(10):2264-72. doi: 10.1007/s00330-012-2488-0. Epub 2012 May 26.
; Little People of America
; Lung function, diagnosis, and treatment of sleep-disordered breathing in children with achondroplasia. Am J Med Genet A. 2012 Aug158A(8):1987-93. doi: 10.1002/ajmg.a.35441. Epub 2012 Jun 18.
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