achondroplasia

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Related to Congenital osteosclerosis: osteosclerosis congenita, Chondrodystrophy syndrome, Achondrodysplasia, Achondroplasia tarda

Achondroplasia

 

Definition

Achondroplasia is the most common cause of dwarfism, or significantly abnormal short stature.

Description

Achondroplasia is one of a number of chondodystrophies, in which the development of cartilage, and therefore, bone is disturbed. The disorder appears in approximately one in every 10,000 births. Achondroplasia is usually diagnosed at birth, owing to the characteristic appearance of the newborn.
Normal bone growth depends on the production of cartilage (a fibrous connective tissue). Over time, calcium is deposited within the cartilage, causing it to harden and become bone. In achondroplasia, abnormalities of this process prevent the bones (especially those in the limbs) from growing as long as they normally should, at the same time allowing the bones to become abnormally thickened. The bones in the trunk of the body and the skull are mostly not affected, although the opening from the skull through which the spinal cord passes (foramen magnum) is often narrower than normal, and the opening (spinal canal) through which the spinal cord runs in the back bones (vertebrae) becomes increasingly and abnormally small down the length of the spine.

Causes and symptoms

Achondroplasia is caused by a genetic defect. It is a dominant trait, meaning that anybody with the genetic defect will display all the symptoms of the disorder. A parent with the disorder has a 50% chance of passing it on to the offspring. Although achondroplasia can be passed on to subsequent offspring, the majority of cases occur due to a new mutation (change) in a gene. Interestingly enough, the defect seen in achondroplasia is one of only a few defects known to increase in frequency with increasing age of the father (many genetic defects are linked to increased age of the mother).
People with achondroplasia have abnormally short arms and legs. Their trunk is usually of normal size, as is their head. The appearance of short limbs and normal head size actually makes the head appear to be oversized. The bridge of the nose often has a scooped out appearance termed "saddle nose." The lower back has an abnormal curvature, or sway back. The face often displays an overly prominent forehead, and a relative lack of development of the face in the area of the upper jaw. Because the foramen magnum and spinal canal are abnormally narrowed, nerve damage may occur if the spinal cord or nerves become compressed. The narrowed foramen magnum may disrupt the normal flow of fluid between the brain and the spinal cord, resulting in the accumulation of too much fluid in the brain (hydrocephalus). Children with achondroplasia have a very high risk of serious and repeated middle ear infections, which can result in hearing loss. The disease does not affect either mental capacity, or reproductive ability.

Diagnosis

Diagnosis is often made at birth due to the characteristically short limbs, and the appearance of a large head. X-ray examination will reveal a characteristic appearance to the bones, with the bones of the limbs appearing short in length, yet broad in width. A number of measurements of the bones in x-ray images will reveal abnormal proportions.

Treatment

No treatment will reverse the defect present in achondroplasia. All patients with the disease will be short, with abnormally proportioned limbs, trunk, and head. Treatment of achondroplasia primarily addresses some of the complications of the disorder, including problems due to nerve compression, hydrocephalus, bowed legs, and abnormal curves in the spine. Children with achondroplasia who develop middle ear infections (acute otitis media) will require quick treatment with antibiotics and careful monitoring in order to avoid hearing loss.

Key terms

Cartilage — A flexible, fibrous type of connective tissue which serves as a base on which bone is built.
Foramen magnum — The opening at the base of the skull, through which the spinal cord and the brainstem pass.
Hydrocephalus — An abnormal accumulation of fluid within the brain. This accumulation can be destructive by pressing on brain structures, and damaging them.
Mutation — A new, permanent change in the structure of a gene, which can result in abnormal structure or function somewhere in the body.
Spinal canal — The opening that runs through the center of the column of spinal bones (vertebrae), and through which the spinal cord passes.
Vertebrae — The individual bones of the spinal column which are stacked on top of each other. There is a hole in the center of each bone, through which the spinal cord passes.

Prognosis

Achondroplasia is a disease which causes considerable deformity. However, with careful attention paid to the development of dangerous complications (nerve compression, hydrocephalus), most people are in good health, and can live a normal lifespan.

Prevention

The only form of prevention is through genetic counseling, which could help parents assess their risk of having a child with achondroplasia.

Resources

Books

Krane, Stephen M., and Alan L. Schiller. "Achondroplasia." In Harrison's Principles of Internal Medicine, edited by Anthony S. Fauci, et al. New York: McGraw-Hill, 1997.

Organizations

Little People of America, c/o Mary Carten. 7238 Piedmont Drive, Dallas, TX 75227-9324. (800) 243-9273.

achondroplasia

 [ah-kon″dro-pla´zhah]
a disorder of cartilage formation in the fetus, leading to achondroplastic dwarfism. adj., adj achondroplas´tic.
 A young child with achondroplasia. From Mueller and Young, 2001.

a·chon·dro·pla·si·a

(ā-kon'drō-plā'zē-ă), [MIM*100800 *134934]
This chondrodystrophy, characterized by an abnormality in conversion of cartilage to bone, is the most common form of short-limb dwarfism; characterized by short stature with rhizomelic shortening of the limbs, large head with frontal bossing and midface hypoplasia, exaggerated lumbar lordosis, limitation of elbow extension, genu varum, trident hand, characteristic radiographic skeletal findings, and neurologic symptoms complicating hydrocephalus and spinal canal stenosis. Autosomal dominant inheritance with most cases sporadic, caused by mutation in the fibroblast growth factor receptor 3 gene (FGFR3) on chromosome 4p.
[G. a- priv. + chondros, cartilage, + plasis, a molding]

achondroplasia

/achon·dro·pla·sia/ (-pla´zhah) a hereditary, congenital disorder of cartilage formation, leading to a type of dwarfism.

achondroplasia

(ā-kŏn′drō-plā′zhə)
n.
Improper development of cartilage at the ends of the long bones, resulting in a form of congenital dwarfism.

a·chon′dro·plas′tic (-plăs′tĭk) adj.

achondroplasia

[ākon′drōplā′zhə]
Etymology: Gk, a + chondros, without cartilage, plassein, to form
a disorder of the growth of cartilage in the epiphyses of the long bones and skull. It results in premature ossification, permanent limitation of skeletal development, and dwarfism typified by a protruding forehead and short, thick arms and legs on a normal trunk. Onset is in fetal life and the diagnosis can be made on prenatal ultrasound. It is inherited as an autosomal-dominant gene with most cases occurring as a sporadic mutation (affecting a fibroblast growth factor receptor). The majority of affected individuals die during gestation or the first year of life. Those who survive have relatively normal longevity. Also called chondrodystrophy, fetal rickets.

achondroplasia

An autosomal dominant MIM (100800, 85% are due to random mutations of the FGFR3 gene) short-limbed dystrophy characterised by upper arms and thighs that are disproportionately smaller than forearms and legs. It is classically linked to advanced paternal age.

Features
Bowing of extremities, waddling gait, limited ROM of major joints, frontal bossing, short stubby fingers, moderate hydrocephalus, depressed nasal bridge, lumbar lordosis.
 
Clinical findings
Short, thick tubular bones; irregular epiphyseal plates; broad, cupped diaphyses; delayed epiphyseal ossification.
 
Management
Osteotomies for severe deformities; Ilizarov procedure.
 
Complications
Disc herniation may lead to acute paraplegia.

achondroplasia

Pediatrics An AD –80% are due to random mutations–short-limbed dystrophy characterized by upper arms and thighs that are disproportionately smaller than forearms and legs Features Bowing of extremities, waddling gait, limited ROM of major joints, frontal bossing, short stubby fingers, moderate hydrocephalus, depressed nasal bridge, lumbar lordosis Imaging Short, thick tubular bones, irregular epiphyseal plates; broad, cupped diaphyses; delayed epiphyseal ossification Management Osteotomies for severe deformities; Ilizarov procedure Complications Disk herniation may lead to acute paraplegia. See Frontal bossing, Ilizarov procedure, Osteotomy.

a·chon·dro·pla·si·a

(ā-kon-drō-plā'zē-ă)
A hereditary type of chondrodystrophy characterized by an abnormality in conversion of cartilage into bone, predominantly affecting long bones, in which epiphysial growth is retarded and ceases early, resulting in dwarfism apparent at birth, with short limbs but normal trunk.
Synonym(s): achondroplastic dwarfism.
[G. a- priv. + chondros, cartilage, + plasis, a molding]

achondroplasia

A dominant genetic defect that interferes with the growth of the cartilage at the growing sites at the end of long bones, resulting in a characteristic form of dwarfism. Achondroplasia has no effect other than on growth.

achondroplasia

the commonest form of human dwarfism. It results from abnormal cartilage development, producing numerous bone defects: stunted trunk, shortened and deformed limbs, and bulging skull. The condition is controlled by an autosomal dominant gene (see DOMINANCE, GENETIC) and thus affected children born to normal parents must result from a mutation in one or other parent. The majority (around 80%) of achondroplastics die in early childhood but those surviving are fertile, have the normal range of intelligence and can live more or less normal lives. The typical circus dwarf is achondroplastic.

achondroplasia

autosomal-dominant congenital chondrodystrophy characterized by restricted limb growth but normal head and trunk size

a·chon·dro·pla·si·a

(ā-kon-drō-plā'zē-ă) [MIM*100800 *134934]
Hereditary chondrodystrophy characterized by an abnormality in conversion of cartilage into bone.
[G. a- priv. + chondros, cartilage, + plasis, a molding]

achondroplasia (ākon´drōplā´zēə, -zhə),

n a hereditary disturbance of endochondral bone formation transmitted as a mendelian dominant factor and resulting in dwarfism. Malocclusion and prognathism may occur.

achondroplasia

a failure of growth of cartilage in the young, leading to a type of dwarfism. Several breeds of dogs display this in their standard conformation, e.g. Dachshund, Basset. See also chondrodysplasia.

inherited congenital achondroplasia
see achondroplastic dwarfism.
inherited achondroplasia dwarfism
see achondroplastic dwarfism.
inherited achondroplasia with hydrocephalus
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