Hypochondroplasia is a skeletal dysplasia characterized by short stature; stocky build; disproportionately short arms and legs; broad, short hands and feet; mild joint laxity; and macrocephaly. Radiologic features include shortening of long bones with mild metaphyseal flare; narrowing of the inferior lumbar interpedicular distances; short, broad femoral neck; and squared, shortened ilia. The skeletal features are very similar to those seen in achondroplasia but tend to be milder. Medical complications common to achondroplasia (e.g., spinal stenosis, tibial bowing, obstructive apnea) occur less frequently in hypochondroplasia but intellectual disability and epilepsy may be more prevalent. Children usually present as toddlers or at early school age with decreased growth velocity leading to short stature and limb disproportion. Other features also become more prominent over time.

The Al-Awadi/Raas-Rothschild/Schinzel phocomelia syndrome (AARRS) is a rare autosomal recessive disorder characterized by severe malformations of upper and lower limbs with severely hypoplastic pelvis and abnormal genitalia. The disorder is believed to represent a defect of dorsoventral patterning and outgrowth of limbs (summary by Kantaputra et al., 2010). Overlapping limb reduction syndromes, less severe in nature, that are also caused by homozygous mutation in the WNT7A gene include Fuhrmann syndrome (228930), characterized by fibular aplasia or hypoplasia, femoral bowing, and poly-, syn-, and oligodactyly, and Santos syndrome (228930), characterized by fibular agenesis/hypoplasia, oligodactylous clubfeet, and anonychia/nail hypoplasia. Al-Qattan et al. (2013) stated that AARRS and Fuhrmann syndrome can be differentiated by the following features, which are seen only in AARRS: complete aplasia of 1 or both lower limbs, and absent elbow with radiohumeral synostosis. In addition, the number of digits per hand is 1 to 3 in AARRS, whereas there are 4 to 5 digits in Fuhrmann syndrome. 'Phocomelia' refers to an intercalary limb defect with the hand or foot being directly attached to the humerus or femur (absent zeugopod) or directly attached to the trunk (absent stylopod and zeugopod). AlQattan et al. (2013) stated that the limb defect observed in Schinzel phocomelia syndrome represents 'true' phocomelia, whereas the limb defect in AARRS is an 'apparent' phocomelia, in which there is absent ulna with radiohumeral synostosis. The authors described 3 radiologic features that define 'apparent' phocomelia: a single arm/forearm bone that appears too long to be the humerus alone; a thicker cortex at the area of the radiohumeral synostosis, with or without slight angulation at the site of synostosis; and the apparently single bone resembling the humerus proximally and the radius distally. The authors also noted that phocomelia is not a feature of the allelic disorder Fuhrmann syndrome (228930). Other distinguishing features of Schinzel phocomelia syndrome include normal nails and dorsal hand skin; distoproximal gradient of lower limb defects, without a resultant stick-like appearance; and a characteristic large cranial defect. AlQattan et al. (2013) concluded that Schinzel phocomelia syndrome and AARRS are distinct phenotypes.