Metatropic Dysplasia

The word “Metatropic” is derived from the Greek word “Metatropos”, meaning "changing form." Clinically, this dysplasia is one that progresses over time. Metatropic dysplasia is a rare disorder. Because it is so uncommon, the exact incidence is not known.

 
How Metatropic Dysplasia Is Inherited

Metatropic dysplasia has an autosomal dominant inheritance.
This means that those with metatropic dysplasia have a 50/50 chance of passing this condition on to their children, either males or females. Metatropic dysplasia can also happen for the first time in a child; in cases when both parents are of typical stature, the chance to have another child with metatropic dysplasia is low (2-3%).

 
Causes of Metatropic Dysplasia

A change in a gene called TRPV4.

 
Physical Characteristics

Initially, individuals have shortened limbs with a relatively average-sized trunk (short-limbed dwarfism). As the child gets older and the condition progresses, kyphoscoliosis of the spine develops that decreases trunk height (short-trunk dwarfism). Apparent shortening of the limbs also occurs over time, due to progressive joint contractures.

Face & Skull
Trunk, Chest, & Spine:
  • Small and narrow chest
  • Pectus carinatum (chestbone sticking out more than average) or pectus excavatum (depressed breast bone)
  • Severe kyphoscoliosis
  • In infancy, “coccygeal tail” can be apparent, which is a prolongation of the normal tailbone consisting of cartilage material
Arms & Legs:
  • Signficantly shortened limbs with a characteristic dumbbell shape bone
  • Enlarged joints
  • Progressive joint contractures during childhood
What are the X-ray characteristics?

The radiographic features of Metatropic Dysplasia include small, flat, diamond-shaped vertebral bodies in early infancy due to defective ossification.

Later, platyspondyly and anterior wedging of vertebral bodies are characteristic. Appearance of a hump-like build-up of bone in both the central and posterior portions of vertebral end plates in the lower posterior and upper lumbarspine. The thorax is narrow and ribs are short in both infancy and early childhood. Limbs are also short with marked metaphyseal flare and epiphyseal dysplasia. Deformed capital femoral epiphyses. Hyperplasia of proximal femoral metaphyses. The capital femoral epiphyses are typically deformed. Hyperplasia of proximal femoral metaphyses is usually observed.

Finally, hypoplasia of basilar pelvis with crescent-shaped iliac crests and low-set anteriosuperior iliac spines is characteristic.

 
Making the Diagnosis

Metatropic dysplasia is diagnosed by its characteristic clinical features such as the coccygeal tail, normal facies, spinal issues, and limb shape. Radiographic features help with diagnosis and genetic testing can also help confirm a diagnosis.

Being a rare disorder with few reports in the medical literature, consultation with an experienced clinical geneticist may be required before a diagnosis is made.

 
Musculoskeletal Problems

Metatrophic Dyplasia varies in severity. Some infants die from severe respiratory problems whereas others survive with only minor changes.

Neck

Atlantoaxial instability is almost universally present in metatropic dysplasia. X-rays of the neck should be performed at diagnosis and at periodical intervals thereafter. Progressive instability in this region will lead to spinal cord compression and is potentially life threatening. Signs of cord compression have been listed elsewhere.

If instability is progressive or symptomatic, early surgical fusion of the affected bones is essential. In cases of diagnostic doubt, further information can be obtained by means of an MRI scan (with flexion-extension views and CSF flow studies). It allows accurate determination of the degree of spinal cord compression and space available for the cord.

Spinal fusion may be supplemented by instrumentation (metal implants) to support the bones until the fusion mass consolidates. Usually extra bone is taken from a rib or from the pelvis to help the healing process. Immobilization of the neck is achieved by a halo vest or body cast, for at least 3 months.

Spine

Kyphoscoliosis is commonly seen in early childhood. It is often severe and rapidly progressive. Spinal curves should be diagnosed early and followed-up at regular intervals. Bracing may be of some benefit in younger children with smaller curves (400 to 500).

The timing of spinal decompression and fusion for scoliosis in metatropic dysplasia is dependent upon the severity of the curve, curve progression, age and risk of injury to the spinal cord. Instrumentation of the spinal fusion may not be possible due to the size and structure of the vertebral column. Prolonged immobilization in a halo body cast may be necessary
following surgery.

The status of the respiratory system may dictate the timing of surgery, especially in the younger, more severely affected children. In the lower back, spinal stenosis may occur requiring decompression and spinal fusion.

Limb Deformities

The limbs are short with significant joint contractures. The treatment of bony deformities and joint contractures is dictated by walking ability, amount of functional compromise and symptoms. Common problems include hip and knee flexion contractures and genu valgus. Some individuals may have signs of ligamentous laxity. Premature degenerative arthritis invariably occurs, requiring joint replacement surgery.

 
Problems Elsewhere in the Body
Respiratory Problems

Respiratory problems are the result of a poorly developed, stiff rib cage. Prolonged breathing difficulties may warrant a tracheostomy and long-term ventilatory support. This is a frequent cause of death in infancy.

Other serious but preventable causes of breathing impairity are spinal cord compression and hydrocephalus. Lung function tests and sleep studies are frequently used to diagnose breathing problems in skeletal dysplasias. Regular review by a pulmonologist is recommended.

Hydrocephalus

Hydrocephalus has been reported in metatropic dysplasia. Regular measurement of head circumference will facilitate early diagnosis. Headache, vomiting, visual disturbances, and loss of consciousness are signs of increased pressure around the brain.

 
What to Look For

In metatropic patients, any change in walking ability, endurance or
breathing should merit further assessment by a physician to rule out
spinal cord compression. Specific neurological symptoms such as
tingling or numbness in the arms or legs, weakness, shooting leg or
arm pain, or problems controlling bladder/bowel function should be investigated further.

One should also watch out for progressive curvature of the spine.
Headache, vomiting, visual disturbances, and loss of consciousness
are signs of increased pressure around the brain; possibly due to
worsening hydrocephalus.

 
References
  1. Jones, Kenneth L. Recognizable Patterns of Human Malformation. Philadelphia, PA: Elsevier Saunders. 2006.
  2. Krakow D, Vriens J, Camacho N, Luong P, Deixler H, Funari TL, Bacino CA, Irons MB, Holm IA, Sadler L, Okenfuss EB, Janssens A, Voets T, Rimoin DL, Lachman RS, Nilius B, Cohn DH. Mutations in the gene encoding the calcium-permeable ion channel TRPV4 produce spondylometaphyseal dysplasia, Kozlowski type and metatropic dysplasia. Am J Hum Genet. 2009 Mar;84(3):307-15.
  3. Scott, Charles I. Dwarfism. Clinical Symposium, 1988; 40(1):17-18
  4. Spranger, Jurgen W. Brill, Paula W. Poznanski, Andrew. Bone Dysplasias: An Atlas of Genetic Disorder of Skeletal Development. Oxford: Oxford University Press. 2002.

Trusted Insights from Nemours' KidsHealth