Connective Tissue Abnormalities
Connective tissue abnormalities produce hypotonia by decreasing the spring-like properties of the collagen fibers. The hallmark of connective tissue abnormalities is the presence of lax joints. Lax joints can be assessed by gently bending the joints and observing their maximal angulation. Lax joints bend farther than normal joints. Dynamic muscle tone is not significantly affected by connective tissue abnormalities. Several connective tissue diseases may produce hypotonia in the neonatal period. Ehlers-Danlos syndrome is the most frequent. Ehlers-Danlos syndrome type I is a hereditary (autosomal dominant) connective tissue disorder.
Neonates with Ehlers-Danlos syndrome type I are usually born prematurely due to friability of the amniotic sac. Evidence of collagen dysfunction is especially noticeable in the skin, blood vessels, and tendons. Neonates with Ehlers-Danlos syndrome type I usually have signs of joint hyperelasticity, bilateral hip dislocation, and hyprextensible skin.
Costello syndrome is an unusual and complex syndrome. The characteristic features of neonates with Costello syndrome are hypotonia, feeding difficulty, deep palmar and plantar creases (Figure 102.1), loose skin of the hand and feet, and cardiac arrhythmia. Coarse features and hypertrophic cardiomyopathy usually develop after the neonatal period. There is usually a history of polyhydramnios and large size for gestational age. In Costello syndrome chondroitin sulfate-bearing proteoglycans accumulates in the heart producing hypertrophic cardiomyopathy and in other mesenchymal tissue producing a Hurler's like appearance later in life. Mutation in the HRAS geneon chromosome 11p13.3 is present in most patients with Costello syndrome. Blood DNA testing for HRAS mutation detects most cases.

Figure 101.1.— Costello syndrome: typical plantar creases (deep and excessive).