# Achondroplasia Pathology > Achondroplasia pathology for MBBS Pathology: FGFR3 gain of function, rhizomelic short stature, endochondral defect and the thanatophoric spectrum. - Canonical URL: https://prepelephant.com/topics/mbbs/pathology/achondroplasia-pathology - Exam / course: MBBS · Subject: Pathology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Achondroplasia Pathology", PrepElephant, https://prepelephant.com/topics/mbbs/pathology/achondroplasia-pathology ## Direct answer Rhizomelic short stature — short limbs relative to the trunk, a large head with frontal bossing and a depressed nasal bridge, trident hands and lumbar gibbus — is achondroplasia, the commonest form of disproportionate short stature, caused by a gain-of-function mutation in FGFR3 (fibroblast growth factor receptor 3) that constitutively inhibits chondrocyte proliferation in the growth plate. Endochondral ossification fails while intramembranous ossification proceeds normally, so bones with a cartilage template (long bones, skull base) are short while the calvarium, which ossifies from membrane, grows normally — the disproportion written into the skeleton. Over 80% of cases arise from a new mutation on a paternal allele, its incidence rising with advanced paternal age. ## What you must remember - **Molecular genetics:** FGFR3 on 4p16.3; the classic G380R substitution in the transmembrane domain accounts for the overwhelming majority of cases; autosomal dominant, with about 80% de novo mutations associated with increasing paternal age — the archetype of the paternal-age effect along with Apert and Marfan. - **Growth-plate pathology:** disordered endochondral ossification — reduced chondrocyte proliferation and maturation in the proliferative and hypertrophic zones; the enchondral column formation is disorganised, so long bones lengthen poorly while width (subperiosteal, intramembranous) is spared, giving short, thick bones with flared metaphyses. - **Radiographic-clinical set:** rhizomelia (proximal segment shortest), trident hand (fingers equal length with a gap between middle and ring), genu varum, lumbar lordosis with gibbus, narrow interpedicular distances decreasing down the spine (spinal stenosis), small foramen magnum, and a skull-base-chondrocranial mismatch producing macrocephaly with midface hypoplasia. - **Life-threatening complications in infancy:** foramen magnum stenosis with cervicomedullary compression — apnoea, hypotonia, sudden death; jugular foramen stenosis with raised intracranial pressure; hydrocephalus from impaired venous return; sleep apnoea surveillance is standard paediatric care. - **Later complications:** lumbar spinal stenosis with neurogenic claudication (recurrent pregnancy-like presentation in adults), recurrent otitis media from Eustachian dysfunction with conductive hearing loss, obesity aggravating the skeleton. - **Homozygous achondroplasia:** lethal perinatally — the severity lesson of gene dosage; two affected parents therefore face a 25% risk of a lethal homozygous child, a standard genetics-counselling viva. - **The FGFR3 spectrum:** hypochondroplasia (milder, N540K, often only short stature with lumbar lordosis) at the mild end; thanatophoric dysplasia (R248C or K650E) at the severe, lethal-perinatal end with telephone-receiver femurs, cloverleaf skull and pulmonary hypoplasia — one gene, a severity gradient. - **Indian context:** disproportionate short stature in Indian clinics must also be separated from the far commoner nutritional and endocrine (growth hormone, thyroid) causes of proportionate short stature — the disproportion, not the height, is the diagnostic hinge. ## Working from the neonate to the adult A term newborn has a long trunk, short limbs, macrocephaly with frontal bossing and hypoplastic midface; radiographs confirm short ribs, small foramen magnum and narrowing interpedicular distances. The neonatal tasks are the dangerous ones: monitor breathing (foramen-magnum compression causes apnoea and even sudden death in infancy), watch the head circumference trajectory for hydrocephalus, and protect hearing with ear surveillance through childhood. Growth through childhood follows achondroplasia-specific charts — final height near 120-130 cm in untreated individuals — with human growth hormone largely ineffective and limb-lengthening controversial; vosoritide, a C-type natriuretic peptide analogue that inhibits FGFR3 signalling, improves growth velocity. The adult then presents twice: once with recurrent discharging ears and mixed hearing loss, and once at 45 with leg numbness on walking that eases on stooping — neurogenic claudication from a congenitally stenotic lumbar canal, treated by decompressive laminectomy when conservative measures fail. ## Where students slip The first slip is calling achondroplasia "dwarfism of all bones": the trunk is near-normal and the calvarium large — the membrane-versus-cartilage ossification distinction is the whole point and the examiner's next question. The second is confusing it with the two neighbours: pituitary (proportionate, GH-deficient) short stature and thanatophoric dysplasia (lethal, with telephone-receiver femurs and cloverleaf skull) — anchoring achondroplasia in the middle of the FGFR3 severity gradient prevents both errors. The third is forgetting the mechanism of the paternal-age effect: new mutations accumulate in the spermatogonial stem-cell line with advancing paternal age, not in oocytes — a meiosis-versus-mitosis point that separates a memorised answer from an understood one. ## Frequently asked questions ### Why are the limbs short but the skull large in achondroplasia? Endochondral ossification (long bones, skull base) is inhibited by overactive FGFR3, while intramembranous ossification (calvarium, face width) proceeds — hence rhizomelic limbs with a relatively large head. ### What is the inheritance and mutation origin of achondroplasia? Autosomal dominant FGFR3 (G380R) mutation, with about 80% of cases arising de novo, the risk increasing with advanced paternal age. ### Why can achondroplasia be lethal in infancy? A stenotic foramen magnum compresses the cervicomedullary junction, causing apnoea, hypotonia and sudden infant death; hydrocephalus adds to the risk. ### What is thanatophoric dysplasia? The lethal-perinatal severe end of the FGFR3 spectrum, with telephone-receiver-shaped femurs, cloverleaf skull, severe pulmonary hypoplasia and death soon after birth. ### How does homozygous achondroplasia present? Severe neonatal lethal skeletal dysplasia, because two mutant alleles double FGFR3 inhibition — hence the 25% recurrence risk when both parents are affected.