MND 2008 Motor neurone disease is a progressive, degenerative disease of the motor system, causing muscle wasting, weakness and eventually death. It affects upper and lower motor neurones in the anterior horn cells (LMN signs) and the corticospinal tract & motor nuclei of the brain stem (UMN signs). The disease is of unknown aetiology.
MND has a prevalence of 5 per 100,000 and usually develops in late middle age with a male:female ratio of 2:1.
It most commonly presents with upper or lower motor neurone signs, or a mixture of the two. The disease affects limb muscles or those supplied by cranial nerves. It does not affect the sphincters, eye movements or sensory nerves. Characteristics of lower motor neurone disease include: weakness and wasting, hypotonicity, decreased reflexes, fasciculation.
Characteristics of upper motor neurone disease include: weakness, spasticity, increased tendon reflexes with clonus, extensor plantar response.
It is important to note that in this condition both upper and lower motor neurone signs can be found in the same limb.
Three types of MND are recognised but the distinction is somewhat artificial since most patients eventually develop features of all three:
This is the most common form of MND and accounts for 65% to 85% of all cases of MND. It results from lesions to the corticospinal tract and the anterior horn cells.
There are different clinical features in amyotrophic lateral sclerosis for the upper and lower limbs:
Lower motor neurone lesions develop at a late stage so that for much of the time, the presentation is that of an uncomplicated picture of corticospinal tract degeneration. Weakness and spasticity are the major features but they are rarely severe.
Upper and lower motor neurone signs are evident causing weakness much greater than suggested by the degree of wasting.
Reflexes may be impaired or exaggerated depending upon whether the LMN or UMN degeneration is greatest. Consequently, exaggerated reflexes may occur in the presence of severe wasting - a unique picture
Characteristically, the clinical features of bulbar palsy are:
Atrophy and fasciculations in bulbar muscles: tongue appears wasted and folded with prominent fasciculations which are prominent and produce a writhing appearance. Orbicularis oris is often affected at the same time as the tongue; orbicularis oculi and other facial muscles tend to be affected later and less severely. Note the muscles of ocular movement are NOT affected. The palate and the extrinsic muscles of the pharynx and larynx are affected after the tongue causing dysarthria and dysphonia - from paresis of the lips, tongue and palate and nasal speech is typical. Also dysphagia is a problem as food and fluids may be difficult to swallow. Jaw jerk and gag reflex are absent.
The characteristic features of pseudobulbar palsy are:
Apparent weakness of the muscles of mastication and facial expression presenting with difficulty in chewing and an expressionless face. The jaw jerk is exaggerated.
They usually have a husky or gravelly voice. Palatal and pharyngeal weakness cause difficulty in swallowing. There may be a brisk gag reflex and the tongue is immobile, pointed and cannot protrude. This should help discriminate MND from the slow moving tongue seen in Parkinsonian patients. Emotional lability is a feature with increased emotions with unprovoked outbursts of laughing or crying.
The clinical features of progressive muscular atrophy are:
Atrophy, weakness and pronounced muscle wasting. Fasciculations are a noticeable feature - if not evident, can usually be evoked by tapping the muscle sharply.
Muscle cramps are common.
The hand is often affected first. The patient may become aware of stiffness and difficulty in performing fine movement. Gradually, a "guttered" or skeletal appearance develops as a result of prominence of the 1st dorsal interosseous muscle and tendons. One hand may be affected before the other or both may deteriorate together. The forearms tend to be affected next, the flexors before the extensors.
The lower limbs are rarely affected early. Bilateral foot drop may occur. Weakness of the muscles of the trunk and of respiration are a late feature.
Other diseases which lead to muscular wasting, especially of the upper limbs, must be considered:
Syringomyelia - fasciculations are rare; dissociated sensory loss from an early stage.
Intramedullary tumour - sensory loss usually prominent.
Cervical spondylosis - sensory loss is usually present but the upper limb weakness and lower limb spasticity may be remarkably similar to MND. MND has a more rapid myelopathy and cervical disc protrusion will be absent on MRI. Occasionally, MND may co-exist with cervical spondylosis.
Peripheral nerve lesions - localised wasting, usually accompanied by sensory loss.
Peroneal muscular atrophy - sensory loss.
Chronic polymyositis - differentiate by electromyography and muscle biopsy.
Myasthenia gravis - bulbar signs but rarely muscular wasting; responds rapidly to anticholinesterase.
Diagnosis – Often difficult to make due to the wide differential and that there is no specific test. EMG, NCS, Brain CT & MRI etc may all be required before the diagnosis is confirmed.
Motor neurone disease shows a typical pattern on EMG of severe chronic denervation: spontaneous fibrillation potentials and reduced number of spikes on activity due to the reduced number of motor neurones and thus, motor units present.
increased duration and amplitude of action potentials of surviving units
There is no cure for motor neurone disease. Professional counselling (MND association) and support (Primary Care MDT & Palliative Care team) are essential.
NICE has recommended that riluzole is used in the treatment of the amyotrophic lateral sclerosis form of MND, as it extends life by a few months.
Symptomatic treatment includes:
Dysarthria & dysphonia - speech therapy and communication aids.
Dysphagia – speech therapy, dietetic advice, feeding gastrostomy; cricopharyngeal myotomy; pay caution to food consistency - that of porridge is usually swallowed better than solids or liquids.
Drooling - anticholinergics, e.g. benztropine or scopoderm TTS patches.