A word that seems to be “thrown around” a lot is Hypotonia.I’d like to dedicate the next two blog posts towards understanding what exactly hypotonia means and its implications in Pediatric Physical Therapy.
Hypotonia is a condition of abnormally low muscle tone, often involving reduced muscle strength. Hypotonia is not a specific medical disorder, but a potential manifestation of many different diseases and disorders that affect motor nerve control by the brain or muscle strength. Recognizing hypotonia, even in early infancy, is usually relatively straightforward, but diagnosing the underlying cause can be difficult and often unsuccessful. The long-term effects of hypotonia on a child’s development and later life depend primarily on the severity of the muscle weakness and the nature of the cause. Some disorders have a specific treatment but the principal treatment for most hypotonia of idiopathic or neurologic cause is physical therapy to help the person compensate for the neuromuscular disability.
All of the muscles in our bodies have a resting muscle tone. Muscle tone is a muscle’s potential ability to respond or counter an outside force, a stretch, or a change in direction of the muscle. Muscle tone is involuntary and directly related to a child’s inherent neural wiring in which the brain perceives the positions of the muscles. With an intact neuromuscular system, the muscles can adjust to immediate postural or limb changes and maintain a child’s posture or arm and leg position upright against gravity.
Appropriate muscle tone enables a child to respond quickly to an outside force either through balance responses/righting reactions or protective reactions. It also allows a child’s muscles to quickly relax once the perceived change is gone. A child with hypotonia has muscles that are slow to initiate a contraction against an outside force, and also cannot sustain a muscle contraction as long. The muscles “relax” quicker despite the fact that the outside force may still be present. Muscle strength is different in that it increases with voluntary repetitions of movement (e.g. 10 repetitions of biceps curls). Muscle tone improves with a more comprehensive treatment approach directed to increase the “alert state” of the muscles themselves.
Hypotonia or low muscle tone can be based in the muscles themselves or within the central nervous system. If the low muscle tone is derived from the muscles (e.g. muscular dystrophy), then there is a dysfunction within the muscles. The receptors within the muscles are not getting the proper signal to the brain to alert the child’s system of changes in position. If the hypotonia is based within the central nervous system, complex feedback loops of sensory processing and motor output are implicated. There are often sensory processing deficits (vestibular, proprioceptive and tactile) that are not alerting the brain of changes in body position. Also, the child may have a praxis or motor planning deficit in which the child’s body cannot formulate the proper motor response.
A child with hypotonia may range from mild and benign developmental delays to a more involved muscular, genetic or neurological condition(e.g. Down’s syndrome, Cerebral Palsy, Prader Willi syndrome, Tay-Sachs disease). Regardless of the cause, these children are at risk for developing poor balance and coordination, permanent orthopedic abnormalities, learning disabilities, and language delays. The most appropriate treatment for a child with hypotonia is immediate early intervention and therapy.
HYPOTONIA THROUGH THE YEARS
An infant with hypotonia may have poor head control and resist propping on their tummies. Babies will often seem to “slip out of your hands” and have trouble “stiffening” their bodies when you carry them. When lying on their backs, babies with hypotonia will often rest with their arms and legs extended outward and sometimes resist bearing weight on their feet when held upright on your legs. Younger children with hypotonia may tend to lean excessively forward when they are sitting up. Older toddlers and children will tend to “W-sit” to reduce the challenge to their postural muscles. Children with low muscle tone may have difficulty learning to roll, sit, crawl and walk independently. Because these gross motor skills are the building blocks for later higher level gross and fine motor skills, it is recommended that children receive therapy to facilitate appropriate motor development. Muscle tone improves with a comprehensive treatment approach to improve postural control and address issues within the nervous system that help the brain perceive where the body is in space.
GOALS OF TREATMENT
Therapy often focuses on improving the delayed postural responses and protective reactions that are typical in children with hypotonia. Therapy will also focus on improving sensory processing in order to improve body awareness, balance and motor planning. Because low muscle tone is also associated with laxity or looseness in a child’s ligaments, therapy also aims to improve overall muscle strength to increase the support around the joints in the arms and legs.