Spinal muscular atrophy (SMA) affects a person’s physical abilities, such as moving, walking and breathing, but does not affect their mental development.
SMA can affect the muscles throughout the body, but it is the proximal muscles (those closest to the centre of the body, such as the shoulders, hips and back) that are usually the most seriously affected.
The proximal muscles are usually affected sooner than the muscles that are further away from the centre of the body. For example, the thigh muscles usually become weaker before the muscles of the lower legs and feet.
Weakening and atrophy (shrinking) of the muscles around the chest and abdomen can cause skeletal deformities, such as scoliosis of the spine (see below).
SMA can also affect the muscles that are involved in breathing and coughing. If they are affected, there is an increased risk of fatal respiratory problems developing, such as pneumonia.
The symptoms of the different types of SMA are outlined in more detail below.
Childhood SMA
Type 0 (develops before birth)
Type 0 SMA is a very severe form of SMA that develops before birth. It causes reduced movement of the foetus, which is usually first noticed between weeks 30 and 36 of the pregnancy.
Babies with type 0 SMA are hardly able to move and have problems breathing and swallowing.
Type I
Babies with type I SMA (known as Werdnig-Hoffmann disease) also have severe muscle weakness, which can cause problems with moving, eating, breathing and swallowing.
Symptoms may be apparent at birth or during the first few months of life. They usually occur before the baby is six months old.
The muscles of babies with type I SMA are thin and weak, which makes their limbs limp and floppy. They are usually unable to raise their head or sit without support.
Breathing problems are caused by a weakness in the baby’s chest muscles, and difficulty swallowing can be made worse by the tongue shrinking and weakness of the muscles in the throat.
Most children who have type I SMA are unlikely to live past their second birthday.
Type II
Type II SMA usually develops when an infant is 6–18 months old. Infants have less severe symptoms than those with types 0 and I, but will usually become weaker over time.
In time, infants with type II SMA will usually be able to sit, but it is unlikely that they will be able to stand or walk unaided.
Infants with type II SMA may also have the following symptoms:
- breathing problems
- floppy arms and legs
- reduced or absent tendon reflexes
- twitching of the muscles in the arms, legs or tongue
They may also develop deformities of the hands, feet and chest, and their joints may be affected by shortening of the muscles, reducing limb movement.
As they grow and develop, most children with type II SMA will develop scoliosis of their spine. Scoliosis is an abnormal curvature of the spine to one side. It occurs as a result of the muscles that support the bones of the spine becoming weaker.
Type III
Type III SMA (known as Kugelberg-Welander disease) is the mildest form of childhood SMA. However, the symptoms and their severity can vary greatly.
The condition often develops later than types I and II, with a diagnosis usually only being made after the age of two.
The symptoms of type III SMA usually develop between early childhood (after one year old) and early adulthood.
Most children with type III SMA are able to stand unaided and walk, although many find walking or getting up from a sitting position difficult. They may also have:
- balance problems
- an abnormal way of walking
- difficulty running or climbing steps
- a slight tremor (shaking) of their fingers
Over time, the muscles of children with type III SMA will become weaker, and those who are able to walk may be unable to do so when they get older.
Breathing and swallowing difficulties are not as common as in other types of SMA.
Adult SMA
Type IV
Type IV SMA begins in adulthood. The symptoms are usually mild to moderate and start to develop after the age of 35. The symptoms usually progress very slowly.
In most cases, muscle weakness initially affects just one side of the body, before eventually progressing to both sides. Symptoms may include:
- muscle weakness in the hands, feet and tongue
- muscle tremor (shaking) and twitching
- speech impairment
Kennedy’s syndrome
Kennedy's syndrome, or spinal-bulbar muscular atrophy (SBMA), is another type of adult SMA.
SBMA only affects men. It usually develops between the ages of 20 and 40, although it can sometimes affect teenage boys and those aged over 40.
The initial symptoms of Kennedy’s syndrome may include:
- tremor (shaking) of the hands
- muscle cramps on exertion
- muscle twitches
- weakness of the muscles of the limbs
As the condition progresses further, other symptoms may be experienced, including:
- weakness of the facial and tongue muscles, which may cause difficulty swallowing (dysphagia) and slurred speech
- recurring pneumonia (inflammation of lung tissue)
In some cases of Kennedy’s syndrome, the following symptoms may occur:
- enlargement of the male breasts (gynecomastia)
- a low sperm count or infertility
- diabetes mellitus, an excess of glucose (sugar) in the blood
