Motor neuron disease is a progressive neurological condition that attacks the motor nerves of the brain and spinal cord. Therefore it can affect any function in the body such as walking, talking, breathing and eating to name a few. The underlying cause of it is unknown. It is thought that there may be environmental factors at play and in some people genetic factors. It can happen to people of any age but more commonly presents from the 60’s onwards.
Within the brain and spinal cord we have what are referred to as upper motor neurons and lower motor neurons. The upper motor neurons are the nerve cells that leave the surface of the brain and travel down as far as the spinal cord. When they reach the spinal cord they communicate with the lower motor neuron and this is the nerve pathway that then carries signals out to the rest of the body (as in to the face, arms, legs etc). In motor neuron disease there is degeneration along the upper or lower motor neuron pathways or both and this gives a characteristic set of clinical findings.
Damage to the upper motor neuron pathways can cause muscles to become stiff or “spastic” and this can lead to difficulty with walking. In the hands it can lead to loss of dexterity such as difficulty with doing buttons or using a knife and fork. It can also cause the speech to take on a nasal quality and some people find it hard to project their voice clearly when this happens. It can also make swallowing difficult.
Damage to the lower motor neurons can cause muscles to become weak and wasted- the medical term for this is atrophy. It can also lead to the muscles twitching or “fasciculating” (this on its own is often a benign symptom- described below). When the muscles involved in speaking and eating become affected this can lead people to have slurred speech and have trouble eating because they cannot move food around their mouth. The muscles involved in swallowing and breathing can be affected in a similar manner. When this happens there is a high chance of developing respiratory tract infections as the food can go down the wrong passage and in to the lungs where it can cause further difficulty with breathing as well as infections.