Hypotonia is a condition where the muscles lose structure and tone.
Healthy muscles are never completely relaxed and are always tense to a certain degree. This is referred to as muscle tone. When muscles have muscle tone, there is always some resistance to movement. In hypotonia, this muscle tone is reduced and muscles become floppy and difficult to use. It is usually a symptom of another underlying condition.
Hypotonia may be congenital (present since birth) or acquired later in life, but it is most common among babies and detectable shortly after birth.
The loss of muscle tone seen in hypotonia can usually be detected by time a baby is six months old. The symptoms and signs of this condition include the following:
- Difficulty feeding
- Little or no control of neck muscles leading to a floppy head
- The infant’s body feels limp when held
- The infant cannot place any weight on their legs or shoulders
- The baby’s arms and legs lie straight against their sides rather than bending at the joints
- There is delayed achievement of developmental milestones such as sitting up, crawling, walking, talking or independent feeding
- Slipping out of high chair
- If there are severe congenital myopathies, there may be accompanying skeletal or bone and joint abnormalities such as hip dislocation or spine deformities
- Ptosis or drooping eyelids
- Wing-like outward protrusion of the scapula bones of the back
- Thin body
- Protrusion of the abdomen due to weak abdominal muscles
- Many patients with hypotonia develop insufficiency of breathing and 50% of patients between the ages of 3 and 55 years need some form of breathing device to help them breathe at night.
Congenital hypotonia is generally caused by inherited conditions that affect how the brain, muscles and nerves develop. Some of the inherited conditions known to cause congenital hypotonia include the following:
Down’s syndrome: This is the most common cause of hypotonia. Aside form hypotonia, features of Down’s Syndrome include a short neck, excess skin folds on back of neck, eyes that slant upwards, a small head and flat nose and facial profile. Intelligence level is also affected and there is an increased risk of a congenital heart defect and problems with vision and hearing.
Marfan syndrome: Here, the connective tissue that maintains body structure and supports organs, bone and muscle is affected. Features include being thin and tall, a curved spine, abnormally long legs, fingers and arms and a protruding breast bone
Dyspraxia: This condition is characterized by difficulties with movement, language and coordination.
Fragile X syndrome: This is the most common inherited condition to cause mental retardation. Features of the condition include behavioral, physical and developmental problems such as delayed language development, hyperactivity, tantrum throwing and autism. Physical features include large head (macrocephaly, large ears and a long face.
Prader Willi syndrome: this is a genetic condition characterized by a constant urge to eat, weight gain, learning difficulties, limited growth, short stature, tantrums and stubbornness.
In cases where doctors are unable to identify the underlying cause of congenital hypotonia, the term “benign congenital hypotonia” is sometimes used.
Hypotonia can develop as a result of injury, illness or infection that damages the nervous system. Some examples of when this acquired hypotonia may develop include the following:
Muscular dystrophy: In cases of muscular dystrophy, which is the most common cause of acquired hypotonia, the muscles gradually become progressively weaker and damaged
Infection: Severe nervous system infections such as meningitis and encephalitis
Injury: A severe head injury that affects the brain can lead to acquired hypotonia
How long symptoms and signs of acquired hypotonia take to develop depends on what has caused it. Infection, for example, can cause hypotonia to develop in just a matter of hours or days, whereas hypotonia caused by muscular dystrophy can take many years to develop.
Decreased muscle tone is the main sign of acquired hypotonia, as is the case with congenital hypotonia. However, the symptoms of hypotonia that develops later in life often differ from those seen in congenital hypotonia and include the following:
- Sudden clumsiness
- Frequent falling
- Joints feeling unusually flexible
- Difficulty stretching to reach things
- Difficulty getting up after lying or sitting down
The symptoms of hypotonia may or may not improve, depending on what the underlying cause is. The majority of congenital hypotonia cases cannot be cured and last throughout a person’s life, although they tend to remain the same in terms of severity and not worsen with age. Although the hypotonia cannot be cured, it is possible to improve symptoms using physiotherapy and occupational therapy.
In cases of acquired hypotonia that are caused by infection, symptoms may resolve when treatment is given, but acquired hypotonia caused by a chronic condition such as muscular dystrophy does not usually improve and symptoms may worsen as a person ages.
Edited by Sally Robertson, BSc