Week 28 Q&A
A 6-month-old girl presents as a new patient for a well-child check.
Her length is below the third percentile and her weight is at the 10th percentile. Head circumference is greater than the 90th percentile. Her mother is 5 feet, 4 inches tall (50th percentile), and her father is 5 feet, 8 inches (25th percentile). She has normal eyes, a prominent forehead, and a prominent curve to her spine. She also demonstrates mild hypotonia.
The most likely diagnosis is:
B. Osteogenesis imperfecta.
C. Klippel-Feil syndrome.
D. Trisomy 21.
E. Familial short stature.
The correct answer is “A.” Achondroplasia is a skeletal dysplasia arising from a defect in fibroblast growth factor receptor 3 (FGFR3). Patients are of short stature with normal height through the trunk but shortening of the extremities. Although fingers may be short, the most pronounced shortening is of the upper arms and thighs (rhizomelic shortening). Macrocephaly with frontal bossing is often present, and midface hypoplasia predisposes to otitis media and obstructive apnea. Abnormalities at the base of the skull and cervical spine predispose to hydrocephalus, cervical instability, hypotonia, and central apnea. Orthopedic manifestations include excessive thoracic kyphosis, hyperlordosis, spinal stenosis, and bowed legs, often requiring surgical correction. Patients often have ulnar deviation of the fifth digits creating the appearance of a “trident hand.” Osteogenesis imperfecta is a genetic syndrome associated with frequent fractures, and while short stature may be present in more severe forms, the other findings are atypical. Klippel-Feil syndrome involves congenital fusion of the cervical spine and a short, webbed neck. Patients with trisomy 21 have short stature, short fingers, and hypotonia but demonstrate other characteristic findings (see Chapter 17). This patient’s length is below the fifth percentile, much lower than predicted by midparental height, making familial short stature less likely, especially when taking the other clinical findings into consideration.