Infants with very stiff muscles may clench their hands and feet, such as clenching their fingers into a fist or curling their toes, or cross their legs in a scissor-like fashion when picked up. Hypertonia is the medical term for high muscle tone, meaning the baby's muscles constantly contract, even when the muscle is not in use; it can be a symptom of a wide variety of diseases and conditions.
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Cerebral Palsy -- often abbreviated CP -- is a movement disorder that starts in the brain. An abnormality in this region leaves children unable to control or coordinate their muscular system. Of CP's three different presentations -- spastic, ataxic and athetotic -- only the first, spastic, includes hypertonic symptoms. Hypertonia is the most common symptom of this type of CP. Treatment for hypertonia in infants with CP includes muscle massage, yoga and other physical therapy. As children get older, botulism toxin -- better known as Botox -- as well as muscle relaxants can help stiff muscles relax.
When an infant's sternocleidomastoid muscle -- the muscle that connects the collarbone and breastbone to the skull -- is chronically tight or stiff, the condition is called torticollis. This conditions can present at birth, when it's known as congenital muscular torticollis, CMT, or later on, when it's called acquired torticollis. CMT is caused by the position of an infant within its mother's womb, damage to the muscle during delivery or due to abnormalities in the muscle or the bones around it. Acquired torticollis occurs when an infant spends too much time looking in one direction, causing the muscle on the other side to weaken; an injury can also damage this muscle, causing torticollis. Treatment for both the acquired and congenital versions of this condition include physical therapy such as stretching, as well as changing the baby's position frequently to ensure the neck muscles develop properly.
Babies born prematurely -- before 37 weeks gestation -- are at higher risk for muscle problems, including hypertonia. Not all preemies will exhibit hypertonic symptoms. For those that do, the hypertonia may be a sign of CP; it could also be just high muscle tone that will remedy itself as the child grows. At 4 months old, a pediatrician will diagnose a preemie with hypertonia as having abnormal muscle movement; hypertonia is extremely rare past eight months, except in cases with another diagnosis — such as CP or torticollis — or with extremely premature infants.
Schwartz-Jampel Syndrome -- abbreviated SJS -- is an autosomal recessive condition. In order for an infant to have SJS, both of its parents must have the recessive gene and pass it on to the child. There are two types of SJS, both of which are very rare. The first variety -- SJS type I -- presents as subtype IA and subtype IB. Both subtypes have similar symptoms; the key difference is the earlier and more severe onset of subtype IB. Symptoms of SJS subtype IA include muscle stiffness as well as muscle weakness and other motor skill abnormalities. These symptoms present within the first year of a child's life. Subtype IB symptoms present as early as birth. Unlike other causes of hypertonia in infants, physical therapy is not an affective treatment for SJS type I; surgery and medication are more often prescribed.