Types of MND
Motor neurone disease can be classified into four main types depending on the pattern of motor neurone involvement and the part of the body where the symptoms begin. There can be an overlap between all of the different types so, while it is useful to classify the various types of the disease, in practice it is not always possible to be so specific.
Amyotrophic lateral sclerosis (ALS)
ALS is the most common type of MND, with both upper and lower motor neurone involvement. ALS is characterised by muscle weakness and stiffness, over-active reflexes and, in some cases, rapidly changing emotions. Initially the limbs cease to work properly. The muscles of speech, swallowing and breathing are usually also later affected.
Progressive bulbar palsy (PBP)
Mixed bulbar palsy and pseudo-bulbar palsy are forms of progressive bulbar palsy that involve the muscles of speech and swallowing. The nerves that control these functions are located in the bulb (the lower part of the brain), hence the term bulbar palsy (paralysis). The limb muscles may also later be affected. PBP affects about a quarter of people diagnosed, and involves both the upper and lower motor neurones.
Progressive muscular atrophy (PMA)
Progressive muscular atrophy affects only a small proportion of people, and has a slower rate of progression and longer survival compared to ALS and PBP types. It mainly causes damage to the lower motor neurones. Early symptoms may be noticed as weakness or clumsiness of the hand. Characterised by absent reflexes, muscle wasting and weakness.
Primary lateral sclerosis (PLS)
PLS is a rare form of MND involving the upper motor neurones only, causing mainly weakness in the lower limbs, although some people may experience clumsiness in the hands, or speech problems. Life span is usually more than 10 years from onset of symptoms, although in some cases PLS develops into ALS. Diagnosis is often provisional.
Upper vs lower motor neurons
Motor neurons act as transmitters that provide a chain of command for voluntary movement to muscles through the body. In MND this chain of command is broken as neurons degenerate.
Normally, messages from nerve cells in the brain (called upper motor neurons) are transmitted to nerve cells in the brain stem and spinal cord (called lower motor neurons) and from them to particular muscles.
Lower motor neurons originate in the spinal cord. Lower motor neurons control movement in the arms, legs, chest, face, throat, and tongue. When there are disruptions in the signals from the lower motor neurons, the muscles do not work properly. The muscles gradually weaken, and may begin wasting away and develop uncontrollable twitching (called fasciculations) and muscle cramps.
Upper motor neurons originate in the cortex of the brain. Upper motor neurons direct the lower motor neurons to produce movements such as walking or chewing. When there are disruptions in the signals from the upper motor neurons, the limb muscles develop stiffness (called spasticity), movements become slow and effortful, and tendon reflexes such as knee and ankle jerks become overactive. Over time, the ability to control voluntary movement can be lost.
Most motor neurone disease (MND) is sporadic. This means it appears for no apparent reason. Only 5 - 10% of MND is estimated to be inherited.
In this small number of cases, there is a family history of MND and/or frontotemporal dementia. Where this occurs, the disease is caused by a mistake in the genetic code which can be passed down, although other triggers are still necessary for the disease to emerge.
If you are concerned about the possibility of a family history of MND and what that could mean for those close to you (in terms of inheriting the genetic code), you may wish to seek genetic counselling.
Genetic counselling in New Zealand
The Genetic Health Service NZ sees people where a mutation is already identified in their family, or to assess a family that has not had any testing. Testing needs to start in a family member with a clinical diagnosis of MND or dementia, as this gives the most information for a family.
Read more about genetic testing for MND in New Zealand on page 4 of our Autumn 2017 issue of MND News.
Genes that cause inherited MND
We know approximately 70% of the genes known to cause inherited MND. Currently, testing is only available for four of the genes that play a part in inherited MND:
- SOD1, discovered in 1993, accountable for 20% of cases of inherited MND
- TARDBP, discovered in 2008, accountable for 3-5% of cases of inherited MND
- FUS, discovered in 2009, accountable for 3-5% of cases of inherited MND
- C9ORF72, discovered in 2011, accountable for around 40% of cases of inherited MND
Other extremely rare causative genes have also been identified. These discoveries represent major breakthroughs because they can provide important clues as to how motor neurons are damaged in MND and may advance our understanding of all types of the disease.
- Latest genetic study on MND - on page 17 of our Autumn 2019 issue of MND News (PDF)
- Genetic testing for MND in New Zealand on page 4 of our Autumn 2017 issue of MND News (PDF).
- Introduction to inherited MND (MND Association of England, Wales and Northern Ireland fact sheet)
- Familial MND and Genetic Testing (MND Australia fact sheet)