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Updated 06 February 2017

Motor neuron disease

This term refers to a group of disorders that destroy the nerve cells which control the actions of voluntary muscles like those used in breathing, swallowing and walking.

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Description

Motor neuron disease (MND) is not one disease. This term refers to a group of disorders that destroy motor neurons, the nerve cells which control the actions of voluntary muscles used in breathing, swallowing, walking etc. MND is a relentless, progressive disorder, with only 10% surviving 10 years. There are notable exceptions though, like Prof. Steven Hawkins.

Neuroanatomy

Nerves conduct electrical impulses which control activity in muscles. The distance between the brain and the muscle concerned may be great, for example the nerves to the toes. To ensure effective conduction and prevent signal loss, nerve conduction uses a system of relays, where the signal from the brain is relayed across a junction (synapse) to the next nerve, which eventually connects to the muscle. This last, or “final pathway” nerve is called the lower motor neuron, and the one before it (higher up in the chain and closer to the brain) is called the “upper motor neuron”.

Damage to the upper motor neuron causes

  • Spasticity
  • Very brisk reflexes
  • Abnormal reflexes like Babinski's sign

Lower motor neuron damage results in

  • Muscle weakness
  • Muscle wasting
  • Muscle twitching (fasciculation)

Patients with MND show a mixture of upper and lower motor neuron problems.

Incidence and causes

Motor neuron disease (MND) occurs in 1 - 5 per 100,000 people, with typical onset at 50 - 70years. Early onset cases affect men more than women, with rare cases appearing in patients in their 20's.

90% of cases are sporadic, while 10% have a family history of motor neuron disease.

The recognized types of motor neuron disease include

  • Amyotrophic Lateral Sclerosis (ALS, or Lou Gehrig's disease)
  • Progressive muscular atrophy
  • Primary lateral sclerosis
  • Progressive bulbar palsy

There is no known cause for MND, though there has been speculation about nerve toxins in some diets, or having received a bad electrical shock. Neither has been proved.

Symptoms

Initial symptoms will vary according to which set of motor neurons is first affected: muscle groups of the upper extremity, such as hand, arm or shoulder, may be affected first, with symptoms of:

  • Weakness of the hand
  • Difficulty with fine movements like fastening buttons, using a knife and fork, writing
  • Difficulty lifting things
  • Stiffness and tremor

Muscle groups in the lower body affected by MND cause:

  • Spastic walk or foot drop
  • Poor balance
  • Muscle twitching and spasms
  • Stumbling (if knees give way, or tripping easily)

Bulbar functions can be affected in MND of the swallowing muscles:

  • Speech slurring - often worsening as the day progresses
  • Difficulty swallowing, especially thin liquids - causes choking
  • Muscle spasm preventing mouth opening - if severe, called trismus

Pseudo-bulbar affect is common in MND, producing:

  • Inappropriate emotional responses - laughing or crying
  • Frequent yawning

Involvement of the diaphragm causes difficulty breathing - at first breathlessness when speaking, but later even at rest.

Eye muscles may be affected, causing visual difficulties, though this usually happens late in the disease, and is often found in patients who are being mechanically ventilated. These patients may survive long enough for almost every muscle group in the body to be affected, making it impossible for them to move any voluntary muscle - this is called a “locked-in syndrome”, in which patients are awake and alert, but cannot communicate in any way.

Cognitive: 15-50% develop fronto-temporal dementia.

Autonomic symptoms may occur, mainly constipation, which is worsened by the inability to effectively swallow liquids. Delayed stomach emptying can cause bloating. Urinary incontinence is NOT common.

Progression, treatment and survival

Symptoms gradually worsen in whichever muscle group is first affected. The disease spreads in an organized way until all four limbs are affected, and finally bulbar muscles (swallowing and breathing) are also included. Involvement of these bulbar muscles eventually causes life-threatening problems, such as respiratory failure or severe aspiration pneumonia.

The average survival of MND patients is 3-5 years from diagnosis. 10% live for 10 years or more.

There is no cure for MND, but aggressive management may relieve the problems of daily living, and extend the quality of life for some time.

Tube feeding via tube surgically implanted directly into the stomach, together with assistance in disposing of saliva, can maintain life and ensure nutrition and hydration. Mechanical ventilation - a respirator - is an option which few can afford, but may extend life for several years.

 
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