Diastrophic Dysplasia

Proteoglycans are considered to be among the chief constituents of cartilage. Undersulfation of proteoglycan in the cartilaginous matrix is responsible for the impairment of performance and load-bearing ability of physeal, epiphyseal, and articular cartilage throughout the body.

The DTDST (SLC26A2) protein acts as a sodium-independent sulfate/chloride transporter and belongs to the SLC26 anion transporter family. The Finnish founder mutation in DTDST (SLC26A2) has been identified.[5] Approximately 95% of affected Finnish patients have a rare ancestral haplotype that was found in only 4% of a Finnish control population. The founder mutation is a guanine-thymine (GT) to guanine-cytosine (GC) transition in the splice donor site of a 5'-untranslated exon of the DTDST gene. The mutation acts by severely reducing levels of mRNA of the DTDST transcript.[6]

Mutations in DTDST are responsible for a family of chondrodysplasias that include four recessively inherited conditions: diastrophic dysplasia, multiple epiphyseal dysplasia, atelosteogenesis type 2, and achondrogenesis type 1B. In addition to the intrinsic sulfate transport properties of the DTDST protein, other factors influence the phenotype in individuals with these mutated alleles.[7]

In 1994, Hastbacka et al identified the gene DTDST (SLC26A2). This gene, which codes for a sulfate transporter protein, has been mapped to distal end of chromosome bands 5q31-q34.[5]  DTDST is inherited in an autosomal recessive manner.[8] Diastrophic dysplasia and McKusick-type metaphyseal chondrodysplasia are the only skeletal dysplasias with autosomal recessive transmission.

About 5% of cases may involve sporadic new mutations. Both parents of an affected individual are carriers of the abnormal gene but are clinically healthy. For a carrier couple, each pregnancy entails a 25% risk of producing an affected child. Each unaffected full sibling of an affected individual has a 67% likelihood of being a carrier. The offspring of an affected individual is a carrier and therefore unaffected unless the other parent is a carrier or affected with the same condition.

Diastrophic dysplasia is a disorder with a wide range of clinical manifestations; this variation has important implications. Parents of a child with mild diastrophic dysplasia, which would previously have been called a diastrophic variant, must be informed that they are at 25% risk of having other children with disproportionate dwarfism. In addition, they should be made aware that the expression of the disorder may be more severe in subsequent children.

Although diastrophic dysplasia is extremely rare, the percentage of carriers in certain groups is high. A 2003 review reported a prevalence of 1 in 22,000 for Finland,[9] compared with an estimated prevalence of 1 in 100,000 elsewhere in the world.[4]  However, a 2021 registry-based study found that the frequency of this condition in Finland has been decreasing since 1960.[10]

Diastrophic dysplasia has been observed in most white populations.[11]

Diastrophic dysplasia is an autosomal recessive disorder and occurs with equal frequency in males and females.

Patients have a minimally (5%) increased rate of perinatal mortality due to cervical myelopathy or respiratory problems such as aspiration pneumonia and laryngotracheomalacia. Patients with severe spinal deformities are also predisposed to the development of respiratory problems. A lethal form of diastrophic dysplasia has been described that can cause death soon after birth as a consequence of cardiorespiratory insufficiency. Overall, life expectancy is not reduced, and patients are able to lead productive lives at work and with their families.

Patients may benefit from the information on the Web site for diastrophic dysplasia, Diastrophic Help.

An important resource for individuals with short stature is the Little People of America (LPA) Organization. The LPA is a national organization that addresses the social, physical, and medical needs of its constituency. It holds annual regional and national conventions. Philosophically, this organization emphasizes the positive aspects of their members' abilities and lives rather than presenting short stature as a disability.

The Dwarf Athletic Association of America (DAAA) is a member of the US Olympic Committee that promotes athletic participation for individuals with short stature.

In evaluating a patient, several aspects of his or her medical history should be investigated for coordination of care. Respiratory difficulties, neurogenic problems, family history, and history of any prior skeletal surgery should be investigated.

The diagnosis can be made at birth with the observation of pathognomic features or within the first few months of life, when the cystic swelling of the ears becomes apparent.[12]  Neonates with diastrophic dysplasia may be thought to have atypical achondroplasia at birth, but the diagnosis is relatively easy to establish in the older child or adult because of the unique combination of deformities. Birth length, head circumference, and weight should be recorded. The mental status and intelligence findings are normal.

Height is related to severity of involvement; taller people are less severely affected than shorter people. The mean height is 136 cm in men and 129 cm in women. The pubertal growth spurt is diminished; therefore, the overall growth failure is progressive. The other components of puberty occur normally. This fact suggests that the bones do not respond to hormonal influences.

The patient's height percentile for age should be determined by using specific growth charts. In relation to trunk length, the limbs are shorter than normal. Short-limbed stature can be confirmed by measurement of the upper-to-lower segment ratio. The normal ratio is 1.6 at birth and decreases to 0.93 during adolescence. Kyphoscoliosis and hip and knee contractures further accentuate the apparent dwarfism. Difficulty in walking is common, and some patients are wheelchair-bound.[13, 14, 15]

The expression and severity of diastrophic dysplasia vary greatly. Patients may present with complaints pertaining to the head and neck, spine, major joints, and hands and feet (see the image below).

Head and neck

Typical facial features are present in most patients.[16]  Prominent cheek and circumoral fullness inspired the historical term cherub dwarf. The nasal bridge is narrow, without flattening or depression. Because the eyes are normally spaced, the distance between the bridge of the nose and the inner canthus of the eye appears to be increased. In contrast to the bridge, the mid-nose is broad. The nostrils flare, and the columella dips low between them.

The arch of the palate is wide, and many teeth are visible when the patient smiles. The face has a long, full appearance with a high, broad forehead and square jaw. Midline hemangiomas are sometimes present but fade with age.

Approximately 50% of patients have a cleft palate, either complete or partial. Other abnormalities, such as a high and arched palate, bifid uvula, or submucous clefts, are common. This cleft palate may also contribute to aspiration pneumonia. The cartilage of the larynx and trachea is abnormally soft, and this may contribute to narrowing of respiratory passages.

The abnormal palate may play a part in the production of the somewhat characteristic voice heard in patients with this condition. Patients also have a hoarse cry. Hyponasality or misarticulation of R, S, or L sounds is observed in 50% of these patients, possibly due to defects in the palate or vocal cords or due to laryngeal cartilage affection in a process similar to that seen in the ears.[17]

The pinnae, though unaffected at birth, develop acute swelling, usually bilateral, at 3-6 weeks of age (see the image below). They are reddened, swollen, hot, and often fluctuant and may feel cystic. After these findings resolve, the cartilage thickens and deforms, giving rise to cauliflower ear, a pathognomonic feature of diastrophic dysplasia. This cauliflower ear is present in 85% of patients. Eventually, the pinnae may become calcified or even ossified. Typically, older patients have rock-hard masses in the superior portions, which are easily palpable.

Pinnal calcification may also be present in other conditions, such as Addison disease, ochronosis, acromegaly, hypertension, diabetes mellitus, and hyperthyroidism. Abnormal hearing has been reported in patients with diastrophic dysplasia, as well as in patients with other skeletal dysplasias.[18]  Although deafness is rare, it may be present as a consequence of external auditory canal stenosis or fusion of the ossicles.

Spine

One half to one third of patients develop cervical kyphosis. Cervical kyphosis may be present in infancy; its course is variable. Though spontaneous resolution has been reported, the possibility of progression with neurologic involvement should always be considered. Progressive kyphosis of the cervical spine causing quadriplegia with respiratory muscle paralysis and neurogenic bladder has been described in young children.[19, 20, 21, 22, 23, 24]

Generally, external signs of cervical abnormalities are absent; however, 80% of patients present with some degree of spinal curvature. Scoliosis is not present at birth, but it tends to develop within the first year of life; it becomes progressively severe with weightbearing.

Spinal stenosis is not common, in contrast to its presence in achondroplasia. Mild stenosis may be masked by a patient's relative inactivity. Most patients have significant lumbar lordosis, most likely due to associated hip flexion contracture. Kyphoscoliosis can cause imbalance of the trunk, leading to difficulties in gait and a reduction in already short standing height.

Early and rapid progression of spinal degenerative changes is a normal finding in patients, regardless of age, and leads to stiffness of the cervical spine on physical examination. One case of atlantoaxial instability has been reported, to the authors' knowledge.

Scoliosis is common, with a reported prevalence of 37-88%. It can be divided into the following three subtypes:

• Early progressive
• Idiopathic
• Mild nonprogressive (see the image below)

In early progressive scoliosis, dysplastic curves are sharply angular and segment curves are short with associated kyphosis at the same level. Neurologic deficits may be present from severe kyphosis.

Lumbar lordosis is usually significant (see the image below). Spinal stenosis is less common. Degenerative changes are common at all levels, and in patients of all ages. These changes lead to spinal stiffness that is evident on clinical examination.

Major joints

The hips maintain a persistent flexion contracture. Patients may present with hip subluxation. Arthritic changes develop by early-to-middle adulthood. Hip dysplasia is usually progressive and may result in a significant decrease in range of motion.[25, 26]  

The knees, too, have flexion contractures. Excessive valgus with lateral dislocation of the patella may occur. Genu valgum with patellar dislocation may occur in patients with diastrophic dysplasia. Genu varum is uncommon. Similar to hip changes, arthritic changes develop by early-to-middle adulthood.

Rarely, the patient may manifest joint laxity and instability rather than limitation of motion. The symptoms may involve hips, knees, ankles, shoulders, or elbows.[27]

In the major joints, motion is markedly limited, and there is a tendency toward dislocation or subluxation. The hips, knees, and elbows are usually affected. At birth, the hips are normal, but with walking, coxa vara becomes apparent, often with subluxation or dislocation of the femoral heads. Bilateral hip dislocations occur in 25% of patients. The proximal femoral epiphyses deform. Hinge abduction may develop in some patients.

Valgus may develop at the knee joint, along with subluxation of the patella and flexion contracture. The elbows and shoulders are occasionally subluxated, and the radial head may be dislocated. Degenerative joint disease of the hips and knees develops in early-to-mid adulthood. The joints may be affected by webbing, in addition to the contractures.

Hands and feet

The hands are short, broad, and ulnarly deviated due to shortening of the ulna. The fingers appear widely spaced. About 95% of patients have an abducted, hypermobile, hitchhiker thumb (see the image below). The first metacarpal is characteristically short, leading to the clinical appearance of proximal insertion of the thumb. Flexion is absent at multiple proximal interphalangeal (PIP) joints of the fingers. This finding, termed symphalangism, is associated with the absence of normal flexural creases and suggests an antenatal onset.

Foot deformities are stiff and involve bony malformations, contracture, and malalignment, and they are difficult to correct passively. A wide spectrum of deformities exist, some of which are specific for the condition. Deformities may range from equinovarus to pure equinus or valgus. In a large series of 102 patients, the various deformities included tarsal valgus deformity with metatarsus adductus (43%), equinovarus adductus (29%), metatarsus adductus (13%), and pure equinus (8%).

The great toe may be in additional varus, analogous to hitchhiker thumb. A frequent finding is a wide gap between the great toe and the second toe. Clubfoot, the expression generally used for the foot deformity, is a misnomer.[28, 29]

Other findings

Associated hernia is another possible manifestation of the basic mesodermal defect. Cases of inguinal hernia have been reported in patients with diastrophic dysplasia.

Morbidity occurs due to the following associated conditions:

• Cleft palate
• Respiratory insufficiency due to laryngotracheobronchomalacia
• Progressive deformities and contractures of joints
• Progressive hip dysplasia and dislocation
• Severe foot deformities ^[30]
• Severe hand deformities
• Progressive spinal deformities (scoliosis, kyphosis, lordosis)
• Cervical kyphosis with neurologic involvement
• Early degenerative changes in joints

Laboratory tests may include a determination of serum calcium, phosphate, alkaline phosphatase, and protein to exclude hypophosphatasia and hypophosphatemia. Urine should be screened for storage products in cases of progression.

Cytogenetic studies can be used for DNA and mutational analyses to confirm the diagnosis, either antenatally or postnatally.

Radiography

A skeletal survey should be requested, including lateral imaging of the head and neck as well as anteroposterior (AP) imaging of the entire spine, pelvis, arms, hands, and legs.[31]

Skull findings are normal initially, though calcifications develop in the pinna of the ear and later in the cranium. Precocious calcification of laryngeal and costal cartilages is a typical feature. A double-layered manubrium has been reported in 81% of patients. The most common finding is an accessory ossification center located ventral to the cranial part of the manubrium. In older patients, this ossification center may fuse with the main part of the manubrium, giving the appearance of an anteriorly bulging manubrium.

The spine has normal vertebral body height and breadth, and scoliosis is not present at birth. Developmental anomalies of the vertebra are unusual. Kyphosis and subluxation may be evident in the cervical spine (see the images below). Cervical spina bifida occulta is present in almost all patients.

Progressive scoliosis usually begins in the first year of life and may become severe. It may occur in the form of either a sharp angular curve (see the image below) or a gradual idiopathic curve. The interpedicular distances narrow only slightly at descending levels of the lumbar spine, unlike the findings in achondroplasia.

Pelvic configuration findings are abnormal, with some distortion of the iliac bones and posterior tilting of the sacrum. The acetabular roofs are widened and irregular, but unlike the findings in achondroplasia, the sacrosciatic notches are normal.

The long bones are short and thick and appear relatively massive. The mild metaphyseal broadening and flaring seen in the neonatal period increases markedly with growth (see the image below). Both the ulna and the fibula are shortened, contributing to the valgus of the knees and radial head subluxation. Epiphyseal development is delayed in the major limb articulations, and these epiphyses, when they do appear, are flattened and irregular. In the newborn, these flattened epiphyses may have a stippled appearance.

At birth, the distal and capital femoral epiphyses are absent. In one study, the proximal femoral ossific nuclei had not appeared in 20% of patients by the age of 12 years. In childhood, the femoral necks are short and broad with a prominent trochanteric region. The capital femoral epiphyses show flattening, irregularity, and inferomedial bulking. The acetabulum is shallow, with a double-hump configuration.

The changes in the hip lead to secondary osteoarthritis before early middle age (see the images below). The distal femoral epiphyses appear to be medially displaced, perhaps as a consequence of underossification of the lateral portions.

Excessive valgus is present in the knee joint, with a mean tibiofemoral angle of 14°. Most patients experience some instability in early life. Signs of early degeneration and deformation of the bony epiphyses develop in patients younger than 6 years. The lateral femoral condyle is hypoplastic. The patellofemoral joint is abnormal with a marked patella infera, and the patella is in a lateral position with bony fragmentation. (See the image below.)

Characteristic radiologic features of the hands (see the images below), and feet exist.

Carpal centers appear early, in contrast to the epiphyseal delay observed elsewhere, and opaque borders and osteopenic centers, as well as some irregularity of shape, are seen. The tubular bones of the hands are short, broad, and irregularly ossified, with metaphyseal flaring associated with flat epiphyses. The first metacarpal is particularly short, ovoid, or triangular, and the proximal phalanx of the thumb may be subluxated or dislocated. Although the proximal interphalangeal (PIP) joints of the digits are ankylosed, a radiolucent space is present early on; this later fuses.

The tubular bones of the feet are similar to those of the hands, and metatarsus adductus with medial twisting of metatarsals is evident (see the first image below). The first metatarsal may be particularly shortened. Equinus deformity is usually present (see the second and third images below).

Magnetic resonance imaging

Magnetic resonance imaging (MRI) may be helpful in the evaluation of patients with diastrophic dysplasia.[23, 32, 33, 34]

In cervical kyphosis with neurologic deficit, MRI performed in neutral position and in extension helps determine the degree of anterior cord compression. In all age groups, degenerative changes in the intervertebral disks and decreases in disk heights are common. No stenosis of the foramen magnum is present, but the spinal canal is narrowed, especially with advanced age. These findings also explain the stiffness of the cervical spine on clinical examination.

In a study of MRI analysis for the thoracic and lumbar spine, the transverse dural tube area at all levels was below the reference range. All patients had narrowed disk heights and a decrease in signal intensity on T2-weighted images, suggesting disk degeneration. All patients had muscular atrophy of psoas muscles and back muscles on T1-weighted images and degeneration-like facet joint hypertrophy.

MRI of the hip joint can depict the flattened femoral ossific nuclei, acetabuli, thickness of articular cartilage, and joint congruity.

Ultrasonography

Plain radiographs do not provide adequate information in patients with diastrophic dysplasia, because of abnormal ossification and severe deformation. In one study, ultrasonography (US) of the knee joint revealed lateral deviation of the anatomic complex of the femoral condyles and patella, deformed patellofemoral joint, hypoplastic lateral femoral condyle, deep trochlear groove, and a distally aligned patella or patella infera.[35]

Antenatal US may be used to diagnose this condition in utero during the second trimester.[36, 37, 38, 39] Long bone measurements at least three standard deviations below reference ranges, along with abnormalities of feet and hands, may be diagnostic. Parents may opt to terminate the pregnancy after genetic counseling.

Arthrography

Arthrograms of major joints depict flattening of the epiphyses and other joint abnormalities.

Computed tomography

Computed tomography (CT) with or without three-dimensional (3D) reconstruction may be useful in preoperative planning and for improving the definition of bony anatomy in cases of severe deformities (see the image below).

Antenatal diagnosis is possible in the first trimester via mutation analysis of chorionic villus DNA.

Spirometry may be helpful. Rigid spinal deformities can reduce the mobility of the chest cage. In one study, at least one abnormally low spirometric finding was found in 33% of children and in 54% of adults. The angle of thoracic or thoracolumbar scoliosis and the age of the patient are correlated with the measured parameters.

The histologic characteristics of the cartilage appear to be similar in the fetus and the older patient and therefore may be useful in differentiating diastrophic dysplasia from other osteochondrodysplasias in the second trimester.

The degenerative changes are primarily seen in the resting cartilage. At low magnification, the changes appear as irregular myxoid degeneration with small cystic areas. At higher magnification, the chondrocytes are surrounded by a halo of dense-appearing cartilage matrix. The chondrocytic nuclei are larger than normal, with some lacunae containing two nuclei. The chondrocytes appear to be irregularly distributed in the surrounding densely staining matrix.

Scattered foci are acellular, and within these foci, residual thickened collagen fibers form an irregular lacy pattern within amorphous material. The physeal growth zones have normal-appearing hypertrophic and proliferating zones with normal columnization.

Nutritional counseling may be appropriate. As a result of progressive contractures, spine deformities, and joint involvement, patients are relatively inactive. Many patients have difficulty in ambulation, and some are wheelchair-bound. Obesity can be an issue in patients with decreased activity levels, and dietary therapy should be initiated early in life.

In a large survey of patients with chondrodysplasias, the highest percentage of patients treated surgically were those with diastrophic dysplasia (87.5%). As a part of the same survey, 88.8% of patients judged the outcome after surgery to be an improvement. Various operations may be performed for patients with diastrophic dysplasia.[20, 40, 41]

Palatoplasty may be required during the initial years of life.

A lateral radiograph should be obtained during the first 2 years of life, and if cervical kyphosis is present, the patient should be monitored clinically and radiographically every 6 months. If the kyphosis progresses with no neurologic deficit, a Milwaukee brace may be used on a full-time basis.

If the curve progresses despite bracing or if neurologic deficit occurs, posterior fusion should be performed. Care should be taken during exposure because of bifid lamina, and instrumentation usually is not technically possible. Immobilization with a halo and vest is required for 2-4 months. If anterior compression is evident on magnetic resonance imaging (MRI), corpectomy and strut grafts may be indicated.

The results of brace treatment for scoliosis in diastrophic dysplasia have been poor, and bracing is not recommended for progressive curves or curves larger than 45°. Early surgical intervention has been proposed. Posterior fusion is the mainstay of treatment. For young patients or those with associated kyphosis of more than 50°, anterior fusion may be added.[42]

Hip flexion contractures and knee flexion contractures should be assessed together. If they are significant (>40°), release may be considered if the potential for gaining motion is good. If epiphyseal flattening is present, releases should be avoided because recurrences are common.

Acetabular augmentation or femoral osteotomy can be performed for the treatment of hip dysplasia. Supracondylar osteotomies may be performed for knee flexion contractures, but these frequently recur. The hip contractures should be corrected first, as these may influence the recurrence of the knee deformity. Patellar subluxation may be corrected to help improve extensor power. Degenerative changes in the hip may require hip joint arthroplasty.

The feet are rigid, and cast treatment is usually futile. A plantigrade foot is the goal of treatment. The recurrence rate for deformity of the foot treated with tenotomy, lengthening of calcaneal tendon, or posteromedial release is more than 80%. Thus, surgical release should be as extensive as necessary to correct the deformity; this goal may require release of inferior tibiofibular ligament to bring the dome of talus into the mortise. Salvage procedures include talectomy, talocalcaneal decancellation, and arthrodesis in the older child.

Consultations should involve the following specialists:

• Otolaryngologist - Cleft palate and laryngeal abnormalities should be identified and treated early in life
• Geneticist - A clinical geneticist may be of help in counseling the family and may also be a valuable resource for the pediatrician seeking additional information or consultation; proper establishment of the mode of inheritance not only aids in genetic counseling but also enables the orthopedist to distinguish diastrophic dysplasia from other forms of dwarfism
• Pulmonologist - Pulmonary function should be evaluated, and respiratory complications such as hypopnea and cyanosis should be prevented
• Pediatrician - The American Academy of Pediatrics and the Committee on Genetics have issued guidelines to assist the pediatrician in caring for children with dwarfism and in advising their families; occasionally, the pediatrician is asked to advise pregnant women who have been informed of an antenatal diagnosis of diastrophic dysplasia