Hypochondroplasia
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A 5-year-old presents for evaluation of short stature (–2 SD). On exam, limbs appear mildly short relative to trunk, hands are short, and lumbar lordosis is mildly increased. Skull and facial features are normal (no frontal bossing, no macrocephaly). Father is also short with similar proportions and was never diagnosed.
AD; FGFR3 (fibroblast growth factor receptor 3), the same gene as achondroplasia, with milder mutations:
- N540K (~70%): the recurrent hotspot variant in the tyrosine kinase domain
- N540T, N540S, K650N: other hotspot variants
Compare with achondroplasia (~98% G380R in the transmembrane domain). The hypochondroplasia variants cause weaker constitutive activation of FGFR3 → milder phenotype.
About 50–70% of cases have an identifiable FGFR3 mutation; the remainder are clinically diagnosed without molecular confirmation (presumed FGFR3-negative or non-coding variants).
- Mild short-limbed short stature: adult height typically –2 to –4 SD (≈ 130–145 cm); much milder than achondroplasia
- Disproportion subtle: limbs only mildly short relative to trunk
- Normal facies: distinguishes from achondroplasia (no frontal bossing, no midface hypoplasia, normal head size)
- Lumbar lordosis mild
- Brachydactyly mild
- Cognition usually normal; a small subset has learning difficulties or epilepsy
- No medullary stenosis / cervico-medullary junction issues of the kind seen in achondroplasia
- Skeletal survey: short long bones, square iliac wings, short broad femoral necks; interpedicular distance does NOT narrow caudally in lumbar spine (helpful contrast with achondroplasia which DOES narrow)
- FGFR3 sequencing: N540K and other hotspots first; if negative and clinical picture is strong, the diagnosis can still stand
- Prenatal: difficult to detect; mild short long bones in 3rd trimester at most
- Achondroplasia: same gene, much more severe; macrocephaly, frontal bossing, characteristic radiographic findings, lumbar interpedicular narrowing; G380R hotspot
- Idiopathic short stature: proportionate; no skeletal findings
- Pseudoachondroplasia (COMP): short stature with normal facies (similar to hypochondroplasia) but early-onset osteoarthritis and characteristic vertebral findings; presents in childhood with waddling gait
- SHOX deficiency: short stature + Madelung deformity (forearm) + mesomelic shortening
- Familial short stature: proportionate, normal radiographs
- Endocrine evaluation if growth velocity drops
- Growth hormone considered case-by-case (modest gains)
- Orthopedic: occasional limb-lengthening surgery
- Developmental screening; brain MRI if seizures/cognitive issues (FGFR3 has been associated with temporal lobe abnormalities in some)
- AD; ~50% recurrence; mild adult phenotype means parents may be undiagnosed (examine them)
"Hypochondroplasia = Mild Achondroplasia minus the head": same FGFR3 gene, milder mutation, similar limb proportions, but NO macrocephaly and NO characteristic facies. If the head is normal and the kid is just shorter than expected, hypochondroplasia is on the differential.
Compare the two: achondroplasia uses G380R (transmembrane); hypochondroplasia uses N540K (tyrosine kinase domain). Both gain-of-function, different mutation strength.