Inherited Metaphyseal Dysplasias | Spared Epiphyses & Spine | Not Rickets
- MCD affects metaphyses only — epiphyses and spine are spared (key vs SED)
- Schmid type: most common, AD, COL10A1, normal biochemistry — must differentiate from rickets
- Jansen type: PTH1R constitutive activation causes hypercalcaemia with suppressed PTH
- McKusick (cartilage-hair hypoplasia): AR, immune deficiency, lymphoma risk, Hirschsprung association
- Biochemistry is NORMAL in all types except Jansen (hypercalcaemia) — the critical discriminator from rickets
- “Metaphyseal dysplasia + normal biochemistry = NOT rickets, think MCD
- “Schmid = COL10A1, Jansen = PTH1R, McKusick = RMRP
- “McKusick type has EXTRA-SKELETAL features (hair, immunity, gut, malignancy)
- “Spine and epiphyses SPARED in all MCD types — if involved, consider SED instead
Normal biochemistry in Schmid and McKusick types. A child with metaphyseal irregularity and normal Ca, PO4, ALP, and vitamin D does NOT have rickets. This is the single most tested discriminator.
Epiphyses and spine are normal in all MCD types. If epiphyses are abnormal or spine shows platyspondyly, consider SED, not MCD. The "spine-sparing" pattern is pathognomonic for the MCD group.
Jansen type causes hypercalcaemia from constitutive PTH1R activation. PTH levels are suppressed (not elevated) because the receptor is active independently of PTH. This differentiates it from primary hyperparathyroidism.
Cartilage-hair hypoplasia has systemic features: fine sparse hair, cellular immune deficiency, increased malignancy risk (especially lymphoma), Hirschsprung disease. A child with metaphyseal dysplasia AND immune problems = McKusick type.
- Diagnosis
- Schmid metaphyseal chondrodysplasia (AD, COL10A1)
- Treatment
- Observation, corrective osteotomy for significant deformity
- Key Pearl
- Most common MCD; epiphyses and spine normal
- Diagnosis
- Jansen metaphyseal chondrodysplasia (AD, PTH1R)
- Treatment
- Bisphosphonates for hypercalcaemia, orthopaedic surveillance
- Key Pearl
- Constitutive PTH receptor activation; PTH suppressed
- Diagnosis
- McKusick / cartilage-hair hypoplasia (AR, RMRP)
- Treatment
- Immune monitoring, malignancy screening, orthopaedic care
- Key Pearl
- Extra-skeletal features (hair, immunity, gut) are key
NORMDifferentiating MCD from Rickets
Hook:NORM labs = NOT rickets — think Metaphyseal Chondrodysplasia!
HAIMMcKusick (CHH) Extra-Skeletal Features
Hook:HAIM: Hair-Anaemia-Immune-Malignancy — the systemic features that make McKusick unique!
Overview and Epidemiology
Metaphyseal chondrodysplasias are a heterogeneous group of inherited skeletal dysplasias characterised by abnormal endochondral ossification at the metaphyses. They are frequently misdiagnosed as rickets or child abuse (metaphyseal fractures) because of overlapping radiographic appearances. Correct identification hinges on recognising the pattern of isolated metaphyseal involvement with normal biochemistry and spared epiphyses and spine. Each subtype has distinct genetics, systemic features, and management implications.
- Group of genetically heterogeneous skeletal dysplasias
- Characterised by irregular metaphyseal ossification with normal epiphyses and spine
- Result from defects in genes controlling endochondral ossification at the growth plate
- All types cause short stature of varying severity
- Schmid type: most common MCD; exact incidence unknown but likely under-diagnosed
- Jansen type: extremely rare (fewer than 30 reported cases worldwide)
- McKusick / CHH: highest prevalence in Finnish (approximately 1 in 23,000) and Old Order Amish populations due to founder effects
- Overall MCD prevalence: approximately 1 in 50,000 to 1 in 100,000 live births
Pathophysiology
Metaphyseal chondrodysplasias result from disruption of the physis (growth plate), specifically the zone of hypertrophy and the transition from cartilage to bone. The growth plate has distinct zones: resting, proliferative, hypertrophic, and calcification. In Schmid type, type X collagen (produced by hypertrophic chondrocytes) is defective, impairing matrix mineralisation. In Jansen type, constitutive PTH1R signalling disrupts chondrocyte differentiation. In McKusick type, the RMRP gene affects ribosomal RNA processing, impairing chondrocyte proliferation. Despite different mechanisms, all three produce the characteristic pattern of metaphyseal irregularity with preserved epiphyses.
- Schmid Type
- COL10A1 (chromosome 6q21)
- Jansen Type
- PTH1R (chromosome 3p21)
- McKusick / CHH
- RMRP (chromosome 9p13)
- Schmid Type
- Type X collagen (hypertrophic chondrocyte matrix)
- Jansen Type
- PTH/PTHrP receptor (chondrocyte differentiation)
- McKusick / CHH
- Mitochondrial RNA-processing endoribonuclease
- Schmid Type
- Autosomal dominant
- Jansen Type
- Autosomal dominant
- McKusick / CHH
- Autosomal recessive
- Schmid Type
- Hypertrophic zone (matrix mineralisation)
- Jansen Type
- Proliferative to hypertrophic transition
- McKusick / CHH
- Chondrocyte proliferation (all zones)
- Schmid Type
- None (skeletal only)
- Jansen Type
- Hypercalcaemia (constitutive receptor activation)
- McKusick / CHH
- Hair, immune system, gut, haematopoietic
The epiphyses develop through a secondary ossification centre that is mechanistically separate from the physis. MCD gene defects primarily disrupt the physis (primary growth plate between metaphysis and epiphysis), not the epiphyseal ossification centre itself. This explains the characteristic pattern of metaphyseal abnormality with normal epiphyses.
Vertebral body growth occurs through a ring apophysis rather than a conventional physis. The columnar architecture of the vertebral growth plate differs from that of long bones, making it relatively resistant to the metabolic effects of COL10A1, PTH1R, and RMRP mutations. Platyspondyly suggests SED or a mucopolysaccharidosis, NOT MCD.
Classification and Types
Schmid Metaphyseal Chondrodysplasia
Inheritance: Autosomal dominant | Gene: COL10A1 | Protein: Type X collagen
Epidemiology: Most common MCD subtype. Often presents between ages 2 and 5 years when the child begins walking and bowing becomes apparent.
- Detail
- AD; COL10A1 on 6q21; encodes alpha-1 chain of type X collagen; typically missense mutations in the C-terminal NC1 domain
- Detail
- Age 2-5 years (when weight-bearing begins); may be detected earlier in familial cases
- Detail
- Mild to moderate short stature (final height typically 135-155 cm)
- Detail
- Coxa vara, genu varum (bow legs), anterior femoral bowing, waddling gait, widened metaphyses, flared ankles and wrists
- Detail
- COMPLETELY NORMAL — Ca, PO4, ALP, vitamin D all within reference range
- Detail
- NONE — purely skeletal condition; normal intelligence, normal lifespan
The key exam discriminator: a child with metaphyseal irregularity and completely normal biochemistry does NOT have rickets. Consider Schmid MCD.
The Schmid/Jansen/McKusick trio are metaphyseal chondrodysplasias (defective endochondral ossification), but examiners also expect the non-chondrodysplastic metaphyseal dysplasias caused by failure of metaphyseal remodelling/modelling - recognised by the Erlenmeyer flask deformity (under-tubulated, widened, flask-shaped metaphyses, classically of the distal femur):
- Pyle disease (metaphyseal dysplasia): autosomal recessive (SFRP4), with gross Erlenmeyer flask widening of the metaphyses but little or no skull involvement and often surprisingly mild symptoms (genu valgum, fragility) - the "metaphyses look dramatic, the patient is well" entity.
- Craniometaphyseal dysplasia (CMD): autosomal dominant (ANKH) - Erlenmeyer flask metaphyses plus craniofacial hyperostosis/sclerosis that can cause cranial-nerve compression (facial palsy, hearing/visual loss) and nasal obstruction - the skull, not the limb, drives morbidity.
- The broader Erlenmeyer flask differential also includes osteopetrosis, Gaucher disease, thalassaemia and fibrous dysplasia - so an Erlenmeyer flask femur is a pattern to work up, not a single diagnosis.
Exam point: distinguish the metaphyseal chondrodysplasias (Schmid/Jansen/McKusick - endochondral defect, growth-plate irregularity) from the remodelling metaphyseal dysplasias that give an Erlenmeyer flask deformity - Pyle disease (SFRP4, no skull) and craniometaphyseal dysplasia (ANKH, with cranial hyperostosis and nerve compression).
SJMMCD Types and Genetics
Hook:SJM: Schmid-Jansen-McKusick — learn the gene for each type!
Clinical Assessment
- Family history: AD (Schmid, Jansen) vs AR (McKusick) pattern
- Age of onset: Schmid age 2-5, Jansen neonatal, McKusick 6-12 months
- Gait: Waddling (coxa vara), bowing progression
- Hair: Fine, sparse, light-coloured (McKusick)
- Infections: Recurrent sinopulmonary infections (McKusick immune deficiency)
- Growth: Birth weight often normal; growth falls off curves post-natally
- Stature: Measure and plot (short-limbed vs short-trunk)
- Lower limbs: Genu varum, coxa vara, anterior femoral bowing, waddling gait
- Upper limbs: Wide wrists, limited elbow extension (Schmid)
- Hair: Inspect for fine, sparse, hypopigmented hair (McKusick)
- Spine: Normal (if abnormal, reconsider diagnosis — think SED)
- Joints: Ligamentous laxity (McKusick), range of motion
If any of the following are present, reconsider MCD and broaden the differential:
- Epiphyseal involvement (irregular, fragmented epiphyses) = consider SED, multiple epiphyseal dysplasia, or mucopolysaccharidosis
- Spinal abnormalities (platyspondyly, odontoid hypoplasia) = consider SED, mucopolysaccharidosis, or spondylo-metaphyseal dysplasia
- Abnormal biochemistry (low Ca, low PO4, high ALP) = likely rickets, not MCD
- Craniofacial dysmorphism with rhizomelia = consider achondroplasia (large head, trident hand, frontal bossing, FGFR3 mutation)
- Periosteal reaction / multiple fractures = consider non-accidental injury or osteogenesis imperfecta
- Metaphyses
- Irregular, flared, sclerotic
- Epiphyses
- Normal
- Spine
- Normal
- Biochemistry
- Normal
- Key Discriminator
- AD family history, COL10A1, coxa vara
- Metaphyses
- Cupped, frayed, splayed
- Epiphyses
- Normal (late widening)
- Spine
- Normal
- Biochemistry
- Abnormal (low Ca/PO4, high ALP)
- Key Discriminator
- Biochemistry is ALWAYS abnormal
- Metaphyses
- Metaphyseal changes similar to MCD
- Epiphyses
- Normal early
- Spine
- Normal
- Biochemistry
- Low PO4, normal Ca, normal PTH
- Key Discriminator
- Renal phosphate wasting, PHEX mutation (X-linked)
- Metaphyses
- Normal
- Epiphyses
- Delayed, irregular, fragmented
- Spine
- Platyspondyly
- Biochemistry
- Normal
- Key Discriminator
- Epiphyses AND spine involved
- Metaphyses
- Flared, 'ball-in-socket' hip
- Epiphyses
- Normal (may appear delayed)
- Spine
- Lumbar canal stenosis (not platyspondyly)
- Biochemistry
- Normal
- Key Discriminator
- Rhizomelic, macrocephaly, trident hand, FGFR3
- Metaphyses
- May show metaphyseal changes
- Epiphyses
- Normal
- Spine
- Bowing, compression fractures
- Biochemistry
- Normal (except type VI with elevated ALP)
- Key Discriminator
- Blue sclerae, fractures, COL1A1/COL1A2
Biochemistry. In Schmid and McKusick MCD, calcium, phosphate, alkaline phosphatase, parathyroid hormone, and vitamin D levels are all completely normal. In rickets, at least one parameter is always abnormal. If a child has metaphyseal changes on X-ray and normal biochemistry, rickets is excluded and you should consider MCD.
A metaphyseal abnormality in a young child sits at a medicolegally critical crossroads, and examiners test both directions of error:
- The classic metaphyseal lesion (CML) of abuse: the "corner" or "bucket-handle" fracture is a planar fracture through the primary spongiosa created by shearing/twisting forces. It is focal and often asymmetric, highly specific for non-accidental injury in an infant, and is a different lesion from the symmetric growth-plate-wide change of a dysplasia.
- The discriminator: metaphyseal chondrodysplasia (and rickets) produce symmetric, bilateral, growth-plate-wide metaphyseal irregularity affecting multiple bones in a patterned way - whereas CMLs of abuse are focal corner/bucket-handle lesions at specific sites, often with other features of inflicted injury (rib fractures, differing fracture ages, retinal haemorrhages).
- Both errors harm: missing genuine abuse is catastrophic, but wrongly attributing a dysplasia's or rickets' metaphyseal changes to abuse devastates a family - so the work-up is a full skeletal survey, biochemistry (Ca/PO4/ALP/PTH/vitamin D) and genetic/specialist (paediatric radiology + safeguarding) review before any conclusion.
- Rickets caveat: rickets both mimics CMLs (fraying/cupping) and predisposes to true fractures, compounding the diagnostic difficulty - hence biochemistry is mandatory.
Exam point: a metaphyseal abnormality in an infant requires you to distinguish the symmetric, biochemistry-checked pattern of dysplasia/rickets from the focal corner/bucket-handle lesions of abuse - get a skeletal survey, biochemistry and expert review, because both missing abuse and falsely accusing a dysplasia/rickets child are serious harms.
Investigations
Investigation Protocol
Views: PA and lateral of knees, hips, wrists; full-length standing lower limb alignment; lateral thoracolumbar spine
Look for: Metaphyseal irregularity, flaring, sclerosis, cysts; coxa vara; femoral and tibial bowing; CONE-SHAPED epiphyses in hands (McKusick)
Critically assess: Are the epiphyses truly normal? Is the spine truly normal? If not, reconsider diagnosis
Essential tests: Serum calcium, phosphate, alkaline phosphatase, PTH, 25-hydroxyvitamin D, creatinine, urea
Schmid / McKusick: All normal (Ca, PO4, ALP, PTH, vitamin D)
Jansen: Hypercalcaemia with suppressed PTH; may have hypercalciuria
If any biochemical abnormality in Ca/PO4/ALP: Work up for rickets, renal osteodystrophy, or metabolic bone disease first
Indication: Confirm suspected diagnosis; differentiate between MCD subtypes; enable genetic counselling and prenatal diagnosis
Schmid: COL10A1 sequencing (targeted gene testing)
Jansen: PTH1R sequencing
McKusick: RMRP sequencing (includes promoter region mutations)
Multigene panel: Skeletal dysplasia gene panel if phenotype unclear
Immunology: Lymphocyte subsets (CD4, CD8, CD19, NK cells), immunoglobulin levels, vaccine response testing
Haematology: Full blood count (macrocytic anaemia), reticulocyte count
Gastrointestinal: Consider rectal biopsy if constipation (exclude Hirschsprung disease)
Ongoing: Annual FBC and immune monitoring lifelong
The two most important initial investigations are plain radiographs (to characterise the pattern of metaphyseal involvement and confirm that epiphyses and spine are normal) and biochemistry (to exclude rickets and identify Jansen hypercalcaemia). Genetic testing confirms the specific subtype and guides counselling.
Management Algorithm
Schmid Type Management
Principle: Orthopaedic management of deformity; no medical treatment required (biochemistry is normal, no systemic disease)
Management Timeline
Confirm: Normal biochemistry + genetic testing (COL10A1)
Counsel: Autosomal dominant inheritance (50 percent transmission risk); normal intelligence and lifespan
Baseline: Document deformity severity with full-length standing radiographs and gait analysis
Physiotherapy: Gait training, strengthening, joint range of motion
Orthotics: May help with gait instability from coxa vara or severe bowing
Monitoring: Annual clinical and radiographic review during growth; monitor coxa vara angle (Hilgenreiner-epiphyseal angle)
Activity: No specific restrictions; encourage normal activity
Coxa vara: Proximal femoral valgus osteotomy if Hilgenreiner-epiphyseal angle greater than 60 degrees or progressive deformity
Genu varum: Proximal tibial or distal femoral corrective osteotomy; consider guided growth (hemiepiphysiodesis) if skeletally immature
Timing: Ideally after growth plates have closed or near skeletal maturity to minimise recurrence
Post-operative: Protected weight-bearing, physiotherapy, long-term follow-up for recurrence
Corrective osteotomies in Schmid MCD have a significant risk of recurrence during remaining growth. Surgical correction is ideally deferred until near skeletal maturity unless deformity is severe or progressive. Guided growth techniques (hemiepiphysiodesis with plates or staples) may be used in younger children to gradually correct angular deformity without osteotomy.
Complications
- MCD Type
- Schmid
- Risk Factors
- Severe Hilgenreiner-epiphyseal angle, continued growth
- Management
- Valgus osteotomy; monitor angle during growth
- MCD Type
- Schmid
- Risk Factors
- Surgery performed too early (active growth plates)
- Management
- Time surgery near skeletal maturity; use guided growth in younger children
- MCD Type
- Jansen
- Risk Factors
- Persistent hypercalcaemia, hypercalciuria
- Management
- Aggressive calcium control, bisphosphonates, renal monitoring
- MCD Type
- McKusick
- Risk Factors
- Profound T-cell deficiency, null mutations in RMRP
- Management
- HSCT referral, infection prophylaxis, immunoglobulin replacement
- MCD Type
- McKusick
- Risk Factors
- Immune surveillance failure, cumulative risk increases with age
- Management
- Annual surveillance, family education, prompt investigation of symptoms
- MCD Type
- McKusick
- Risk Factors
- Macrocytic anaemia in infancy, may be life-threatening
- Management
- Transfusion support, usually self-limiting after early childhood
The most common clinical error is treating MCD patients with vitamin D and calcium supplementation for presumed rickets. In Schmid type, this is unnecessary and may cause iatrogenic hypercalcaemia or hypercalciuria. In Jansen type, additional calcium is actively harmful as the patient is already hypercalcaemic. Always confirm biochemistry before treating metaphyseal changes as rickets.
Outcomes and Prognosis
- Final Height
- 135-155 cm (mild-moderate short stature)
- Function
- Good; most achieve independent ambulation and normal activity
- Life Expectancy
- Normal
- Key Determinant
- Severity of coxa vara and bowing
- Final Height
- Often less than 120 cm (severe dwarfism)
- Function
- Variable; limited by severe skeletal dysplasia and metabolic complications
- Life Expectancy
- Reduced (renal complications, hypercalcaemia)
- Key Determinant
- Degree of hypercalcaemia control
- Final Height
- 110-135 cm (moderate-severe short stature)
- Function
- Good with appropriate immune and orthopaedic management
- Life Expectancy
- Reduced (malignancy, severe immune deficiency)
- Key Determinant
- Immune function and malignancy surveillance
Schmid: Best prognosis of all MCD types. Normal lifespan and intelligence. Main morbidity is orthopaedic (coxa vara, bowing).
Jansen: Worst prognosis. Severe dwarfism, metabolic complications from hypercalcaemia, possible deafness.
McKusick: Variable. Main mortality risk is malignancy (especially lymphoma) and severe immune deficiency. With modern immune support and malignancy surveillance, many patients reach adulthood.
Guidelines, Registries & Global Practice
- Schmid type: worldwide distribution; likely under-diagnosed as mild cases may be attributed to physiological bowing or familial short stature
- Jansen type: extremely rare globally; no ethnic or geographic concentration
- McKusick / CHH: founder mutations in Finnish (approximately 1 in 23,000 births) and Old Order Amish (Pennsylvania, USA) populations; cases reported worldwide across all ethnicities
- Overall: MCD accounts for a small proportion of skeletal dysplasias; accurate diagnosis requires genetic confirmation
- High-resource: multidisciplinary care with geneticists, paediatric orthopaedic surgeons, immunologists, and haematologists; skeletal dysplasia gene panels available
- Limited-resource: clinical and radiographic diagnosis with basic biochemistry; genetic testing may require sample shipment to reference laboratories; orthopaedic management prioritised
- Universal principle: biochemistry (Ca, PO4, ALP) is universally available and is the most important first-line test to differentiate MCD from rickets in all settings
- McKusick surveillance: malignancy and immune monitoring requires infrastructure that may be limited in resource-constrained settings
- Diagnosis
- Biochemistry first (exclude rickets); then genetic testing for MCD subtype
- Medical Management
- Jansen: bisphosphonates for hypercalcaemia; McKusick: immune monitoring and prophylaxis
- Orthopaedic Management
- Referral to paediatric orthopaedics for deformity correction
- Diagnosis
- Standing lower limb radiographs + pelvis; classify pattern of metaphyseal involvement
- Medical Management
- No role for vitamin D/calcium supplementation in MCD with normal biochemistry
- Orthopaedic Management
- Corrective osteotomy for progressive coxa vara (Hilgenreiner angle greater than 60 degrees); guided growth for genu varum
- Diagnosis
- Nosology-based classification; genetic confirmation for all MCD subtypes
- Medical Management
- Multidisciplinary management; McKusick requires immunology and oncology input
- Orthopaedic Management
- Individualised surgical planning; recurrence risk high if surgery before skeletal maturity
- Diagnosis
- McKusick CHH is a recognised combined immune deficiency; lymphocyte subset analysis mandatory
- Medical Management
- HSCT for severe combined immune deficiency phenotype; IVIG for antibody deficiency
- Orthopaedic Management
- Coordinate orthopaedic surgery with immunological status; avoid live vaccines if T-cell deficient
There is no dedicated international registry for metaphyseal chondrodysplasias, though the Finnish CHH cohort represents one of the best-characterised single-gene skeletal dysplasia populations worldwide. The ISDS maintains a nosology of skeletal dysplasias that classifies MCD subtypes. Evidence is predominantly from case series, cohort studies, and expert reviews rather than randomised controlled trials, owing to the rarity of these conditions.
In any child with metaphyseal irregularity on radiographs:
- ALWAYS check biochemistry (Ca, PO4, ALP) before prescribing vitamin D or calcium
- If biochemistry is normal, the diagnosis is NOT rickets — do not treat as rickets
- Consider MCD and refer for genetic testing
- In Jansen type, additional calcium is actively dangerous (patient is already hypercalcaemic)
Misdiagnosis of MCD as rickets leads to unnecessary treatment, delayed appropriate management, and potential harm from iatrogenic hypercalcaemia or hypercalciuria.
Controversies & Areas of Uncertainty
The role of GH in MCD remains uncertain. Small series suggest modest benefit in McKusick CHH (improved growth velocity without significant side effects), but there is no evidence of benefit in Schmid or Jansen types. GH does not address the underlying growth plate defect. Most guidelines do not routinely recommend GH for MCD.
The ideal timing of corrective osteotomy in Schmid MCD is debated. Operating during active growth carries significant recurrence risk, but delaying surgery until skeletal maturity may allow prolonged deformity with joint compensation issues. Guided growth techniques offer an intermediate solution but long-term data are limited.
Calcimimetics and PTH1R antagonists have been proposed as targeted therapies for Jansen hypercalcaemia, but clinical data are extremely limited. Current management relies on bisphosphonates and supportive care. A true targeted approach requires development of a functional PTH1R inhibitor, which does not yet exist in clinical practice.
The optimal malignancy surveillance protocol for CHH patients is not standardised. Annual clinical examination and FBC is widely practised, but the role of routine imaging (ultrasound, CT) for lymphoma screening is debated due to radiation risk and low positive predictive value in asymptomatic patients. Current practice relies on clinical vigilance and patient education.
MCQ Practice Points
Q: Which gene is mutated in Schmid metaphyseal chondrodysplasia? A: COL10A1, located on chromosome 6q21. This gene encodes the alpha-1 chain of type X collagen, which is specifically expressed by hypertrophic chondrocytes in the growth plate. Mutations (typically in the NC1 domain) disrupt trimer assembly and impair endochondral ossification at the metaphysis, producing the characteristic metaphyseal irregularity while sparing the epiphyses and spine.
Q: What is the biochemical profile in Schmid metaphyseal chondrodysplasia? A: Completely normal. Calcium, phosphate, alkaline phosphatase, parathyroid hormone, and 25-hydroxyvitamin D are all within normal limits. This is the critical discriminator from rickets, where at least one parameter is always abnormal. Treating a child with Schmid MCD with vitamin D is unnecessary and potentially harmful.
Q: Why is PTH suppressed in Jansen metaphyseal chondrodysplasia despite hypercalcaemia? A: Constitutive activation of the PTH1R receptor. The mutation causes ligand-independent receptor signalling, producing the biological effects of PTH (calcium release from bone, renal calcium reabsorption) without requiring PTH binding. The resulting hypercalcaemia then suppresses parathyroid gland PTH secretion via negative feedback, producing the characteristic pattern of high calcium with low PTH.
Q: How do you differentiate metaphyseal chondrodysplasia from rickets on clinical and biochemical grounds? A: Biochemistry is the key. In rickets, calcium and/or phosphate are low, ALP is elevated, and vitamin D is low (nutritional) or phosphate is low with normal Ca (hypophosphataemic). In MCD (Schmid and McKusick), ALL biochemical parameters are normal. Radiologically, both show metaphyseal changes, but rickets also produces looser zones, craniotabes, and generalized osteopenia, which are absent in MCD.
Q: What extra-skeletal features distinguish McKusick (cartilage-hair hypoplasia) from other MCD types? A: Fine, sparse, hypopigmented hair; combined immune deficiency (T-cell and B-cell); increased malignancy risk (especially non-Hodgkin lymphoma); macrocytic anaemia; and Hirschsprung disease. These systemic features are unique to the McKusick type and reflect the broader role of the RMRP gene in cell proliferation across multiple tissues, not just cartilage.
Q: What are the key imaging features that distinguish MCD from spondyloepiphyseal dysplasia (SED)? A: In MCD, the metaphyses are abnormal but the epiphyses and spine are normal. In SED, the epiphyses are irregular, delayed, or fragmented AND the spine shows platyspondyly or other vertebral anomalies. The pattern of involvement (metaphyses only vs epiphyses plus spine) is the fundamental radiographic discriminator.
Clinical Imaging
Radiographic Features of Metaphyseal Chondrodysplasia
Key imaging findings that differentiate MCD from other skeletal dysplasias and rickets:
- Metaphyses: irregular, widened, flared with sclerotic and cystic changes
- Growth plates: appear widened (especially at hips, knees, ankles)
- Femoral necks: coxa vara with horizontal growth plates
- Long bones: anterior bowing of femurs and tibiae
- Epiphyses: normal (key discriminator from SED)
- Spine: normal (key discriminator from SED and mucopolysaccharidoses)
- Metaphyses: severely expanded, irregular, "crumpled" or "fragmented" appearance
- Growth plates: markedly disorganised
- Diaphyses: thin cortices, osteopenia from hypercalcaemia
- Skull: thickened cranial vault, hyperostosis
- Epiphyses and spine: normal
- Changes may mimic hyperparathyroidism (subperiosteal resorption)
- Metaphyses: cupped, flared, scalloped with irregular ossification
- Hands: short tubular bones, cone-shaped epiphyses (distinctive)
- Ribs: anterior flaring ("cupped" costochondral junctions)
- Spine and epiphyses: normal
- Ligamentous laxity: may cause joint subluxations
- Rickets: metaphyseal cupping, fraying, splaying + looser zones + generalized osteopenia
- MCD: metaphyseal irregularity but no looser zones, no osteomalacia, localised to growth plates
- Key: MCD changes are isolated to metaphyses; rickets has diffuse bone changes
- Biochemistry confirms: rickets shows abnormal Ca, PO4, ALP, vitamin D; Schmid MCD is normal
Exam Viva Scenarios
Practise clinical reasoning and management decisions out loud
“A 4-year-old boy presents with progressive bilateral genu varum, a waddling gait, and short stature. His parents are unaffected. Biochemistry reveals normal calcium, phosphate, alkaline phosphatase, PTH, and vitamin D. Pelvis and knee radiographs show coxa vara with irregular, flared metaphyses. The epiphyses and spine appear normal. What is the diagnosis and how would you manage this child?”
“An 18-month-old girl of Finnish descent presents with short-limbed short stature, recurrent lower respiratory tract infections, and chronically loose stools. Examination reveals fine, sparse, light-coloured hair, ligamentous laxity, and bilateral genu varum. Hand radiographs show short metacarpals with cone-shaped epiphyses. Knee radiographs show flared metaphyses with normal epiphyses. What is the diagnosis, what investigations are needed, and how would you manage her?”
Key Genetics
- Schmid = COL10A1 (type X collagen), AD, chromosome 6q21 — most common MCD
- Jansen = PTH1R (PTH/PTHrP receptor), AD, chromosome 3p21 — rarest, most severe
- McKusick / CHH = RMRP (RNase MRP RNA component), AR, chromosome 9p13 — Finnish/Amish founder effect
- All three affect the growth plate (physis) but spare epiphyses and spine
Clinical Features
- Schmid: coxa vara, genu varum, bowing, waddling gait, mild-moderate short stature, onset age 2-5
- Jansen: severe dwarfism, hypercalcaemia, prominent forehead, onset neonatal/infancy
- McKusick: fine sparse hair, immune deficiency, short limbs, cone-shaped epiphyses in hands, malignancy risk
- Spine and epiphyses are NORMAL in all MCD types — if abnormal, consider SED
Diagnosis
- Biochemistry is NORMAL in Schmid and McKusick — the critical discriminator from rickets
- Jansen has hypercalcaemia with SUPPRESSED PTH (constitutive receptor activation)
- Radiographs: metaphyseal irregularity, flaring, sclerosis; normal epiphyses and spine
- Genetic testing confirms subtype: COL10A1, PTH1R, or RMRP sequencing
Management
- Schmid: orthopaedic only — corrective osteotomy for coxa vara (angle greater than 60 degrees) or severe genu varum
- Jansen: bisphosphonates for hypercalcaemia, renal monitoring, orthopaedic support
- McKusick: multidisciplinary — immune monitoring, malignancy surveillance, orthopaedic care, genetic counselling
- DO NOT treat with vitamin D or calcium unless biochemistry confirms rickets
Differential Diagnosis
- Rickets: abnormal biochemistry (low Ca, low PO4, high ALP) — MCD has normal labs
- SED: epiphyses AND spine involved — MCD spares both
- Achondroplasia: rhizomelic, macrocephaly, FGFR3 — different gene and pattern
- Hypophosphataemic rickets: low phosphate with renal wasting, PHEX mutation — biochemistry abnormal
Evidence Base and Key Trials
A type X collagen mutation causes Schmid metaphyseal chondrodysplasia
- First identification of COL10A1 mutations as the cause of Schmid metaphyseal chondrodysplasia
- Linkage analysis mapped the disease locus to chromosome 6q21-q22.3
- Demonstrated that mutations in the C-terminal NC1 domain of type X collagen disrupt trimer assembly
- Established the molecular basis for the most common metaphyseal chondrodysplasia
A constitutively active mutant PTH-PTHrP receptor in Jansen-type metaphyseal chondrodysplasia
- Identified activating mutations in PTH1R as the molecular cause of Jansen metaphyseal chondrodysplasia
- Mutations cause ligand-independent (constitutive) signalling through the PTH/PTHrP receptor
- Explained the paradox of hypercalcaemia with suppressed PTH levels
- Demonstrated that the receptor is active in both kidney and bone, driving calcium release
Mutations in the RNA component of RNase MRP cause a pleiotropic human disease, cartilage-hair hypoplasia
- Identified RMRP mutations as the cause of cartilage-hair hypoplasia (McKusick type MCD)
- The same founder mutation (A70G) is present in both Finnish and Amish populations
- RMRP encodes the RNA component of the mitochondrial RNA-processing endoribonuclease (RNase MRP)
- Mutations impair ribosomal RNA processing and cell cycle control in chondrocytes
Extended follow-up of the Finnish cartilage-hair hypoplasia cohort confirms high incidence of non-Hodgkin lymphoma and basal cell carcinoma
- Extended Finnish CHH cohort follow-up confirmed a 90-fold increased risk of non-Hodgkin lymphoma compared to the general population
- Basal cell carcinoma incidence was also significantly elevated
- Immune deficiency severity correlated with malignancy risk
- Combined immune deficiency (T-cell and B-cell defects) was the strongest predictor of adverse outcomes
A novel parathyroid hormone (PTH)/PTH-related peptide receptor mutation in Jansen's metaphyseal chondrodysplasia
- Identified a novel PTH1R mutation (H223R) in Jansen metaphyseal chondrodysplasia, expanding the mutational spectrum
- Confirmed that heterozygous activating mutations in PTH1R are the universal cause of Jansen MCD across all reported families
- Hypercalcaemia severity correlates with the degree of constitutive receptor activation
- Demonstrated that the mutant receptor signals through both cAMP and inositol phosphate pathways