Osteochondrodysplasias (Osteochondrodysplastic Dwarfism)

(Genetic Skeletal Dysplasias)

By Frank Pessler, MD, PhD, Helmholtz Centre for Infection Research, Braunschweig, Germany

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Connective Tissue Disorders in Children
Introduction to Connective Tissue Disorders in Children
Chondromalacia Patellae
Cutis Laxa
Ehlers-Danlos Syndrome
Infrapatellar Tendinitis
Marfan Syndrome
Nail-Patella Syndrome
Osteochondrodysplasias (Osteochondrodysplastic Dwarfism)
Osteogenesis Imperfecta
Pseudoxanthoma Elasticum

Osteochondrodysplasias involve abnormal bone or cartilage growth, leading to skeletal maldevelopment, often short-limbed dwarfism. Diagnosis is by physical examination, x-rays, and, in some cases, genetic testing. Treatment is surgical.

The basic genetic defects have been identified in most of the osteochondrodysplasias. The mutations typically cause perturbation of function in proteins involved in growth and development of connective tissue, bone, or cartilage (see Table: Types of Osteochondrodysplastic Dwarfism).

Dwarfism is markedly short stature (adult height < 4 ft 10 in) frequently associated with disproportionate growth of the trunk and extremities. Achondroplasia is the most common and best-known type of short-limbed dwarfism, but there are many other distinct types, which differ widely in genetic background, course, and prognosis (see Table: Types of Osteochondrodysplastic Dwarfism). Lethal short-limbed dwarfism (thanatophoric dysplasia, caused by mutations in the same gene as achondroplasia) causes severe chest wall deformities and respiratory failure in neonates, resulting in death.

Types of Osteochondrodysplastic Dwarfism

Disorder	Symptoms and Signs	Usual Mode of Inheritance	Defective Gene Product
Achondroplasia	Bulky forehead, saddle nose, lumbar lordosis, bowlegs	AD	Fibroblast growth factor receptor 3 (FGFR)
Chondrodysplasia punctata	Variable extraskeletal manifestations X-rays show epiphyseal stippling in infancy due to calcifications	See below	See below
Chondrodysplasia punctata (rhizomelic form)	Marked proximal limb shortening Death during infancy	AR	Peroxisomal type 2 targeting signal receptor (PTS2)
Chondrodysplasia punctata (Conradi-Hünermann form)	Mild, asymmetric limb shortening Benign	AD or XL dominant	Delta(8)-delta(7)-sterol isomerase emopamil-binding protein (EBP)
Chondroectodermal dysplasia (Ellis-van Creveld [EVC] syndrome)	Distal limb shortening, postaxial polydactyly, structural cardiac defects	AR	EVC, EVC2
Diastrophic dysplasia	Severe dwarfism with rigid hitchhiker thumb and fixed talipes equinovarum	AR	Solute carrier family 26 (sulfate transporter), member 2 (SLC26A2)
Hypochondroplasia	Symptoms of achondroplasia but milder	AD	Fibroblast growth factor receptor 3 (FGFR3—not all patients)
Mesomelic dysplasia*	Predominantly, shortening of the forearms and shanks Normal facies and spine	AD or AR	Not defined
Metaphyseal chondrodysplasia^†	In some forms, malabsorption, neutropenia, thymolymphopenia	AR or AD	Parathyroid hormone receptor (PTHR), type X collagen (COL10A1)
Multiple epiphyseal dysplasia	Mild dwarfism, normal spine and facies, sometimes stubby digits, hip dysplasia (often as 1st symptom) Very heterogeneous	AR or AD	Solute carrier family 26 (sulfate transporter), member 2 (SLC26A2; AR form)
Pseudoachondroplasia	Normal facies, various degrees of dwarfism and kyphoscoliosis Heterogeneous	AD or AR	Cartilage oligomeric matrix protein (COMP)
Spondyloepiphyseal dysplasia	Predominantly, kyphoscoliosis Sometimes myopia and a flat facies Heterogeneous	AD, AR, or XL	Type II collagen (COL2A1), tracking protein particle complex, subunit 2 (TRAPPC2, also known as SEDL)
*There are several eponymous forms (eg, Nievergelt, Langer).
^†There are many different eponymous forms (eg, Jansen, Schmid, McKusick).
AD = autosomal dominant; AR = autosomal recessive; XL =X-linked.

Diagnosis

X-rays

Characteristic x-ray changes may be diagnostic. A whole-body x-ray of every affected neonate, even if stillborn, should be taken because diagnostic precision is essential for predicting prognosis.

Prenatal diagnosis by fetoscopy or ultrasonography is possible in some cases (eg, when fetal limb shortening is severe).

Standard laboratory tests do not help, but molecular diagnosis is feasible for chondrodysplasias with known molecular defects. Genetic testing is advised if a diagnosis cannot be made based on clinical grounds or if genetic counseling is desired.

Treatment

Sometimes surgical limb-lengthening or joint replacement

In achondroplasia, treatment with human growth hormone is generally not effective. An increase in adult height may be achieved by surgical limb lengthening. In this and other nonlethal osteochondrodysplasias, surgery (eg, hip replacement) can help improve joint function. Hypoplasia of the odontoid process can predispose to subluxation of the 1st and 2nd cervical vertebrae and compression of the spinal cord. Therefore, the odontoid process should be evaluated preoperatively and, if it is abnormal, the patient’s head should be carefully supported when hyperextended for endotracheal intubation during anesthesia.

Because the inheritance pattern and gene mutations in most types are known, genetic counseling can be effective. Organizations such as Little People of America provide resources for affected people and act as advocates on their behalf. Similar societies are active in other countries.

Key Points

Osteochondrodysplasias are inherited abnormalities of growth and development of connective tissue, bone, and/or cartilage.
There are many types, which differ widely in genetic background, course, and prognosis, but all cause markedly short stature and often disproportionate growth of the trunk and extremities.
Diagnosis is by clinical manifestations and identification of characteristic x-ray changes.
Growth hormone treatments are typically ineffective.