Metaphyseal Chondrodysplasia

The metaphyseal chondrodysplasias are a group of genetically heterogeneous inherited skeletal dysplasias in which endochondral ossification at the metaphyses is abnormal. The defects lie in genes that control endochondral ossification at the growth plate, so the metaphyses ossify irregularly while the epiphyses and spine stay normal. All types cause short stature of varying severity.

Why they matter. They are frequently misdiagnosed as rickets, or as child abuse (metaphyseal fractures), because the radiographic appearances overlap. Correct identification hinges on recognising the pattern of isolated metaphyseal involvement with normal biochemistry and spared epiphyses and spine. Each subtype then carries its own genetics, systemic features and management.

Who. The three types are very unequal in frequency:

• Schmid type is the most common; the exact incidence is unknown and it is likely under-diagnosed
• Jansen type is extremely rare, with fewer than 30 reported cases worldwide
• McKusick type, cartilage-hair hypoplasia, has its highest prevalence in the Finnish (approximately 1 in 23,000) and Old Order Amish populations because of founder effects
• Overall MCD prevalence is approximately 1 in 50,000 to 1 in 100,000 live births

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The growth plate. The physis has distinct zones: resting, proliferative, hypertrophic and calcification. The metaphyseal chondrodysplasias disrupt it at the zone of hypertrophy and the transition from cartilage to bone, and each type gets there by its own route:

• Schmid: type X collagen, the product of hypertrophic chondrocytes, is defective and matrix mineralisation is impaired
• Jansen: constitutive PTH1R signalling disrupts chondrocyte differentiation
• McKusick: the RMRP gene affects ribosomal RNA processing, impairing chondrocyte proliferation

Despite the different mechanisms, all three produce the same pattern of metaphyseal irregularity with preserved epiphyses.

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Why the epiphyses are spared. The epiphysis develops through a secondary ossification centre that is mechanistically separate from the physis. The MCD gene defects primarily disrupt the physis, the primary growth plate between metaphysis and epiphysis, and not the epiphyseal ossification centre itself, which is why the metaphysis is abnormal and the epiphysis beside it is normal.

Why the spine is normal. For Schmid type the evidence gives the cleanest answer: type X collagen is expressed only by hypertrophic chondrocytes in regions of endochondral ossification, so the disease follows that expression and the vertebrae are untouched (Warman, in the evidence section). Whatever the type, platyspondyly suggests SED or a mucopolysaccharidosis, not MCD.

Three types share the metaphyseal pattern and differ in gene, inheritance, severity and what lies outside the skeleton. In one line each: Schmid is the common, purely skeletal type; Jansen is the rare, severe type with hypercalcaemia; McKusick is the recessive type in which the skeleton is the least of the problem.

SchmidJansenMcKusick

Schmid Metaphyseal Chondrodysplasia

Genetics. Autosomal dominant. COL10A1 on 6q21 encodes the alpha-1 chain of type X collagen, and the mutations are typically missense in the C-terminal NC1 domain.

Presentation. Often between 2 and 5 years, when weight-bearing begins and the bowing becomes apparent; familial cases may be detected earlier. The child has coxa vara, genu varum, anterior femoral bowing and a waddling gait, with widened metaphyses and flared ankles and wrists. Short stature is mild to moderate, with a final height typically of 135-155 cm.

What is not there. Biochemistry is completely normal, with calcium, phosphate, ALP and vitamin D all within the reference range, and the condition is purely skeletal: no extra-skeletal features, normal intelligence, normal lifespan. That normal biochemistry is the discriminator from rickets.

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Jansen Metaphyseal Chondrodysplasia

Genetics. Autosomal dominant. The mutation in PTH1R on 3p21 activates the PTH/PTHrP receptor constitutively, so it signals without ligand; H223R is the commonest, and most reported cases are sporadic de novo mutations.

Presentation. Congenital or early infancy, and severe from birth: severe short-limbed dwarfism with a final height often less than 120 cm, severe metaphyseal disorganisation, a prominent forehead, widely spaced eyes, hypertelorism and micrognathia. It is one of the rarest skeletal dysplasias.

Biochemistry. The receptor signals as if PTH were bound when it is not, so calcium rises and phosphate falls. Because the serum calcium is high, negative feedback suppresses PTH secretion from the parathyroid glands, so PTH is normal or undetectable rather than raised. That triad, high calcium, low phosphate and a low or normal PTH, is what separates Jansen from primary hyperparathyroidism, where PTH is high.

Beyond the skeleton. The hypercalcaemia brings its own complications, nephrocalcinosis and renal impairment; deafness from otosclerosis may occur.

McKusick Type — Cartilage-Hair Hypoplasia (CHH)

Genetics. Autosomal recessive. RMRP on 9p13 is the RNA component of the mitochondrial RNA-processing endoribonuclease (RNase MRP), and there are multiple founder mutations. Severity varies even within families.

Presentation. Usually apparent by 6-12 months: short-limbed, proportionate short stature with a final height typically of 110-135 cm, an adult height of approximately -4 to -6 SD. The skeletal features are short limbs, ligamentous laxity, genu varum, flared metaphyses, short fingers and cone-shaped epiphyses in the hands.

Beyond the skeleton. The hallmark extra-skeletal feature is fine, sparse, hypopigmented (silvery-blond) hair. With it come a combined T-cell and B-cell immune deficiency that is T-cell predominant, with recurrent infections and impaired vaccine responses; macrocytic anaemia that may be severe in infancy; Hirschsprung disease; an increased risk of malignancy, lymphoma (especially non-Hodgkin) and leukaemia; and impaired spermatogenesis. A child with metaphyseal dysplasia and immune problems has McKusick type.

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McKusick Malignancy Risk

Patients with cartilage-hair hypoplasia have a significantly increased risk of non-Hodgkin lymphoma, leukaemia and basal cell carcinoma, and malignancy is a leading cause of early mortality. The Finnish registry cohort (Taskinen, 123 patients, mean 19.2 years of follow-up) puts numbers on it: 14 cancers against 2.0 expected — a standardised incidence ratio of 7.0 overall, driven by non-Hodgkin lymphoma (9 cases, SIR 90.2) and basal cell carcinoma (10 patients, SIR 33.2 against 0.3 expected). Quote those as orders of magnitude rather than precise figures — the intervals are wide. Two practical consequences. Nine of the fourteen cancers appeared before the age of 45, so surveillance must extend well beyond paediatric care rather than stopping at transition. And because the skin risk is as striking as the lymphoid risk, examine the skin at every review, not just the blood count. Investigate lymphadenopathy, unexplained fever or weight loss promptly rather than attributing it to the known immune deficiency (impaired T-cell and NK-cell function), which contributes to both infection susceptibility and malignancy risk. Note that the registry study did not identify any immune parameter that predicts which patients develop cancer — the mechanism remains unexplained, so surveillance applies to everyone.

History. Birth weight is often normal and growth falls off the curves post-natally. The family history sorts the dominant types (Schmid, Jansen) from the recessive one (McKusick), and the age of onset sorts them again: Schmid at 2-5 years, Jansen neonatal, McKusick at 6-12 months. Then ask about:

• Gait: a waddle from coxa vara, and whether the bowing is progressing
• Infections: recurrent sinopulmonary infections point to the McKusick immune deficiency

Examination. Measure and plot the stature and decide whether the child is short-limbed or short-trunked. The spine should be normal; if it is not, reconsider the diagnosis and think of SED. Then work through:

• Lower limbs: genu varum, coxa vara, anterior femoral bowing, waddling gait
• Upper limbs: wide wrists and limited elbow extension (Schmid)
• Hair: fine, sparse, hypopigmented (McKusick)
• Joints: ligamentous laxity (McKusick), and the range of motion

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Radiographs first. PA and lateral views of the knees, hips and wrists, a full-length standing lower-limb alignment film and a lateral thoracolumbar spine. Look for metaphyseal irregularity, flaring, sclerosis and cysts, coxa vara, femoral and tibial bowing, and cone-shaped epiphyses in the hands (McKusick). Then critically assess: are the epiphyses truly normal, and is the spine truly normal? If not, reconsider the diagnosis.

The pattern differs by type:

• Schmid: irregular, widened, flared metaphyses with sclerotic and cystic change; growth plates that appear widened, especially at the hips, knees and ankles; coxa vara with horizontal growth plates; anterior bowing of the femora and tibiae; normal epiphyses and spine
• Jansen: severely expanded, irregular, "crumpled" or "fragmented" metaphyses with markedly disorganised growth plates; thin diaphyseal cortices and osteopenia from the hypercalcaemia; a thickened cranial vault with hyperostosis; normal epiphyses and spine. The changes may mimic hyperparathyroidism (subperiosteal resorption)
• McKusick: cupped, flared, scalloped metaphyses with irregular ossification; short tubular bones in the hands with the distinctive cone-shaped epiphyses; anterior flaring of the ribs ("cupped" costochondral junctions); normal spine, and normal epiphyses apart from the cone-shaped ones in the hands; ligamentous laxity that may cause joint subluxations

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MCD or rickets on the film. Rickets produces metaphyseal cupping, fraying and splaying with Looser zones and generalised osteopenia, and the bone changes are diffuse. MCD produces metaphyseal irregularity that may include sclerosis and cysts rather than only cupping and fraying, with no Looser zones and no osteomalacia, and the changes are isolated to the metaphyses; clinically there is no craniotabes and no widened fontanelle.

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Biochemistry. Serum calcium, phosphate, alkaline phosphatase, PTH, 25-hydroxyvitamin D, creatinine and urea. In Jansen type the panel shows hypercalcaemia with suppressed PTH, and there may be hypercalciuria. Any abnormality of calcium, phosphate or ALP means working up rickets, renal osteodystrophy or metabolic bone disease first.

Genetic testing. Confirms the suspected diagnosis, separates the subtypes and enables genetic counselling and prenatal diagnosis:

• Schmid: targeted COL10A1 sequencing
• Jansen: PTH1R sequencing
• McKusick: RMRP sequencing, including the promoter region
• Unclear phenotype: a skeletal dysplasia gene panel

The McKusick work-up. Lymphocyte subsets (CD4, CD8, CD19, NK cells), immunoglobulin levels and vaccine-response testing characterise the immune deficiency; a full blood count and reticulocyte count look for the macrocytic anaemia; constipation prompts consideration of rectal biopsy to exclude Hirschsprung disease. FBC and immune monitoring then continue annually, for life.

Schmid has the best prognosis of the three: a normal lifespan and normal intelligence, with the morbidity orthopaedic (coxa vara, bowing). Jansen has the worst: severe dwarfism, the metabolic complications of hypercalcaemia and possible deafness. McKusick is variable; the mortality risk is malignancy (especially lymphoma) and severe immune deficiency, and with modern immune support and malignancy surveillance many patients reach adulthood.