There are several different types of spinal muscular atrophy (SMA) – the condition is classified according to the age at which symptoms develop and the level of muscle weakness it causes.

In general, SMA affects a person's physical abilities, such as moving, walking and breathing, but doesn't affect their mental development.

It causes the muscles throughout the body to become weak and shrink (atrophy). The muscles closest to the centre of the body, such as the shoulders, hips and back (proximal muscles) are usually affected first and most severely.

The most common forms of SMA are usually divided into four types, described below.

Type I

Type I SMA (also known as Werdnig-Hoffmann disease) is believed to be the most common form. It causes severe muscle weakness, which can result in problems moving, eating, breathing and swallowing.

These symptoms are usually apparent at birth or during the first few months of life.

The muscles of babies with Type I SMA are thin and weak, which makes their limbs limp and floppy. They're usually unable to raise their head or sit without support.

Breathing problems can be caused by weakness in the baby's chest muscles, and difficulty swallowing can be made worse by weakness of the muscles in the tongue and throat.

Because of the high risk of serious respiratory problems, most children with Type I SMA die in the first few years of life.

Type II

Symptoms of Type II SMA usually appear when an infant is 7-18 months old. The symptoms are less severe than Type I, but become more noticeable in older children.

Infants with Type II SMA are usually able to sit, but cannot stand or walk unaided. They may also have the following symptoms:

In some cases, deformities of the hands, feet, chest and joints develop as the muscles shrink.

As they grow, many children with Type II SMA develop scoliosis. This is an abnormal curvature of the spine caused by the muscles supporting the bones of the spine becoming weaker.

A child with Type II SMA has weak respiratory muscles, which can make it difficult for them to cough effectively. This can make them more vulnerable to respiratory infections.

Although Type II SMA may shorten life expectancy, improvements in care standards mean most people can live long, fulfilling and productive lives. The majority of children with Type II SMA are now expected to survive into adulthood.

Type III

Type III SMA (also known as Kugelberg-Welander disease) is the mildest form of childhood SMA. Symptoms of muscle weakness usually appear after 18 months of age, but this is very variable and sometimes the symptoms may not appear until late childhood or 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
• difficulty walking
• difficulty running or climbing steps
• a slight tremor (shaking) of their fingers

Over time, the muscles of children with Type III SMA become weaker, resulting in some children losing the ability to walk when they get older.

Breathing and swallowing difficulties are very rare and the condition doesn't usually affect life expectancy.

Type IV (adult-onset)

Type IV SMA is a less common form that begins in adulthood. The symptoms are usually mild to moderate, and may include:

• muscle weakness in the hands and feet
• difficulty walking 
• muscle tremor (shaking) and twitching  

Type IV SMA doesn't affect life expectancy.

Rarer types

As well as the four more common forms of SMA, there are a number of rare types, which have slightly different characteristics and causes.

Spinal muscular atrophy with respiratory distress

Spinal muscular atrophy with respiratory distress (SMARD) is a very rare form of SMA that severely affects the muscles used in breathing. It's usually diagnosed within the first year of life.

Kennedy's syndrome

Kennedy's syndrome, or spinobulbar muscular atrophy (SBMA), is a rare type of adult SMA.

SBMA only affects men. It usually develops very gradually between the ages of 20 and 40. Rarely, it can affect teenage boys or sometimes only become obvious after 40.

The initial symptoms of Kennedy's syndrome may include tremor (shaking) of the hands, muscle cramps on exertion, muscle twitches and weakness of the limb muscles.

As the condition progresses, it may cause other symptoms, including:

• weakness of the facial and tongue muscles, which may cause difficulty swallowing (dysphagia) and slurred speech
• recurring pneumonia (infection of lung tissue)

Some people with Kennedy's syndrome also develop enlarged male breasts (gynaecomastia), diabetes, and a low sperm count or infertility.

Kennedy's syndrome doesn't usually affect life expectancy.

Distal SMA

Distal spinal muscular atrophy (DSMA) is a rare form of SMA that affects the distal muscles, such as the hands, feet, lower arms and lower legs. This leads to reduced mobility and range of movement.

Some types of DSMA can affect the muscles used for speaking or swallowing.