Hypotonia (decreased muscle tone) is a symptom rather than a condition. It can be caused by a number of underlying problems, which can either be neurological or non-neurological.
Neurological conditions are those that affect the nerves and nervous system. Hypotonia is most commonly linked to neurological control of muscle tone.
To function normally, muscles depend on signals from motor nerves. These signals can become disrupted at the level of the brain and spinal cord (central hypotonia), or as a result of nerve damage between the spinal cord and muscle (peripheral hypotonia).
Neurological conditions that affect the central nervous system and can cause central hypotonia include:
- cerebral palsy – neurological problems present at birth that affect a child's movement and co-ordination
- brain and spinal cord injury – including bleeding in the brain
- serious infections – such as meningitis (an infection of the outside membrane of the brain) and encephalitis (an infection of the brain itself)
Neurological conditions that affect the peripheral nervous system and can cause peripheral hypotonia include:
- muscular dystrophy – a group of genetic conditions that gradually cause the muscles to weaken, leading to increasing but variable levels of disability
- myasthenia gravis – a condition that causes weakness and increased fatigue rather than hypotonia in adults; babies born to mothers with myasthenia gravis may also be affected and, if so, will usually have hypotonia
- spinal muscular atrophy – a genetic condition that causes muscle weakness and a progressive loss of movement
- Charcot-Marie-Tooth disease – an inherited condition that affects a substance called myelin, which covers nerves and helps carry messages to and from the brain
Non-neurological problems that can cause hypotonia in newborn babies and young children include:
- Down's syndrome – a genetic disorder present at birth that affects a person's normal physical development and causes learning difficulties
- Prader-Willi syndrome – a rare genetic syndrome that causes a wide range of symptoms, including permanent hunger, restricted growth and learning difficulties
- Tay-Sachs disease – a rare and usually fatal genetic disorder that causes progressive damage to the nervous system
- congenital hypothyroidism – where a baby is born with an underactive thyroid gland; in the UK, babies are screened for this and treated early
- Marfan syndrome and Ehlers-Danlos syndrome – inherited syndromes that affect the connective tissues that provide structure and support to other tissue and organs
- connective tissue disorders – connective tissue, such as collagen, provides the body's tissues with strength and support, and is found in ligaments and cartilage
- being born prematurely (before week 37 of pregnancy) – premature babies sometimes have hypotonia because their muscles aren't fully developed by the time they're born
Hypotonia can sometimes occur in older children and adults, although this is less common.
It can be caused by some of the problems mentioned above, but other possible causes include:
- multiple sclerosis – where the myelin covering nerve fibres is damaged, interfering with their ability to transfer electrical signals from the brain and spinal cord to the rest of the body
- motor neurone disease – a rare condition that progressively damages the motor nerves and causes the muscles to waste away
Weakness and problems with mobility and balance are also common with these conditions.