Skeletal dysplasia

A child (or fetus) with disproportionate stature, abnormally shaped bones, or shortened limbs. The skeletal dysplasias are a family of roughly 450 named conditions in which the underlying biology of cartilage, bone matrix, or growth-plate signaling has gone wrong. "Disproportionate" is the operative word: the trunk and limbs are not in the expected ratio. Recognition usually begins prenatally on a third-trimester ultrasound with short femurs, or postnatally with a chunky-looking infant whose limbs look too short for the trunk.

There is no single test that sorts this group. The reasoning is built from three orthogonal splits:

1. Onset: prenatal (visible by 20-week anatomy scan) vs postnatal (growth-plate disorders that present later).
2. Lethality: lethal perinatally (narrow thorax, severe limb shortening, cloverleaf skull, hypoplastic vertebrae) vs non-lethal.
3. Pattern of limb shortening: which segment is shortest?
   • Rhizomelic (proximal: humerus, femur): achondroplasia family
   • Mesomelic (middle: radius/ulna, tibia/fibula): Robinow, Leri-Weill dyschondrosteosis
   • Acromelic (distal: hands, feet): trichorhinophalangeal syndrome

A fourth, radiographic split organizes the rest: which part of the bone is abnormal? Spondylar (vertebrae), metaphyseal (the flared ends), epiphyseal (the joint surfaces), or some combination.

Rhizomelic, non-lethal

• Achondroplasia (FGFR3, c.1138G>A in >99%, AD): the most common non-lethal skeletal dysplasia. Rhizomelic shortening, frontal bossing, midface retrusion, trident hand, lumbar lordosis, normal cognition. Recurrent mutation makes testing essentially binary.
• Hypochondroplasia (FGFR3, milder activating variants): a milder FGFR3 phenotype; often diagnosed later when the child plots below the curve without dramatic disproportion.

Rhizomelic, lethal perinatally

• Thanatophoric dysplasia (FGFR3, AD, almost always de novo): the lethal end of the FGFR3 spectrum. Severe micromelia, narrow thorax with respiratory failure at birth, cloverleaf skull (type II), platyspondyly. Most infants die in the first hours to days.
• Achondrogenesis type II (COL2A1) and type IB (SLC26A2): severe limb shortening, poor ossification, lethal.

Spondyloepiphyseal / collagen II

• Spondyloepiphyseal dysplasia congenita (SEDC) (COL2A1): short trunk, short limbs, flat face, myopia, cleft palate, atlantoaxial instability. Part of the type II collagenopathies along with Kniest and Stickler.

Multiple epiphyseal / pseudoachondroplasia

• Pseudoachondroplasia (COMP): the trap. Normal length and normal facies at birth; growth deceleration around age 2 unmasks short-limbed short stature with significant joint laxity. The face is not dysmorphic, distinguishing it from achondroplasia.
• Multiple epiphyseal dysplasia (MED): mild short stature with early joint pain.

Bone fragility group

• Osteogenesis imperfecta (COL1A1/COL1A2, mostly AD): fractures dominate the picture, but bowing and short stature can mimic a dysplasia.
• Hypophosphatasia (ALPL): the must-not-miss because it is treatable. Low serum alkaline phosphatase, defective bone mineralization. Asfotase alfa enzyme replacement therapy changes the natural history. Always check ALP in any suspected severe perinatal dysplasia.

Sex-reversal / curved-bone group

• Campomelic dysplasia (SOX9, AD): bowed femurs and tibiae, hypoplastic scapulae, small chest. In 46,XY infants, sex reversal is common because SOX9 also drives testis determination. Often lethal in infancy from respiratory failure.
• Stuve-Wiedemann syndrome (LIFR, AR): bowed long bones, episodic hyperthermia, feeding difficulties in infancy.

1. Skeletal survey: head (skull shape, foramen magnum), spine (platyspondyly, hemivertebrae), chest (rib length, thoracic narrowness), pelvis (iliac flare, acetabular angle), and long bones (which segment is shortest, metaphyseal vs epiphyseal pattern). The radiograph is the diagnostic core of this field.
2. Limb-segment measurements: upper-to-lower segment ratio and arm span vs height localize the shortening.
3. Serum alkaline phosphatase: screens for hypophosphatasia. Always.
4. Family history and parental exam: AD conditions (achondroplasia, SEDC, pseudoachondroplasia) often have an affected parent; new dominants (thanatophoric, severe OI) usually do not.
5. Targeted gene testing when the phenotype is classic: FGFR3 for achondroplasia/hypochondroplasia/thanatophoric, COL2A1 for SEDC/Stickler, COL1A1/COL1A2 for OI, ALPL for hypophosphatasia.
6. Skeletal dysplasia gene panel when the phenotype is atypical or radiographs do not pin a single diagnosis. Exome sequencing is increasingly the default for unrevealing panels.
7. Prenatal management: suspected lethal dysplasia changes delivery planning (hospital with NICU, family counseling, comfort-care discussions before delivery).