Motor neuron disease

Published on 05/05/2015 by admin

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Last modified 05/05/2015

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69 Motor neuron disease

Instruction

Examine this patient’s cranial nerves.

Examine this patient’s upper limbs.

Examine this patient’s lower limbs.

Salient features

History

• Fasciculations and cramps: these may precede other symptoms by months

• Painless, asymmetrical weakness of muscles of the upper limb or lower limb

• Dysarthria and dysphagia

• Emotional lability if there is bulbar involvement.

Examination

• Fasciculations, absent reflexes and weakness in the upper limbs (see Fig. 62.1)

• Spasticity, exaggerated reflexes and upgoing plantars in lower limbs

• Sluggish palatal movements, absent gag reflex, brisk jaw jerk

• Check the tongue for fasciculations (Fig. 69.1)

• Check neck muscles; head droop is seen when there is weakness of the thoracic and cervical paraspinal muscles

• A combination of the above signs may be seen.

image

Fig. 69.1 Atrophy of the tongue in amyotrophic lateral sclerosis, a form of motor neuron disease.

(With permission from Bradley et al. 2008.)

Remember: The therapy is generally palliative.

Diagnosis

This patient has a combination of upper and lower motor neuron signs (lesions) caused by motor neuron disease (aetiology), although I would like to exclude cervical cord compression. The patient is wheelchair bound as a result of the disease (functional status).

Questions

What important cause should be ruled out before making a firm diagnosis of motor neuron disease?

Cord compression may produce a similar clinical picture and hence it is important to do an MRI scan of the spine and/or a myelogram to exclude it.

Advanced-level questions

What are the symptoms attributable to motor neuron disease?

• Direct (owing to motor neuronal degeneration):

• Weakness and atrophy
• Fasciculations and muscle cramps
• Spasticity
• Dysarthria
• Dysphagia
• Dyspnoea
• Emotional lability.

• Indirect (as a result of primary symptoms):

• Psychological disturbances
• Sleep disturbances
• Constipation
• Drooling
• Thick mucous secretions
• Symptoms of chronic hypoventilation
• Pain.

What are the characteristic features of this disease?

• It rarely begins before the age of 40 years

• Presence of upper and lower motor neuron involvement of a single spinal segment, and motor dysfunction involving at least two limbs or one limb and bulbar muscles

• Sensory symptoms or signs are not seen

• Ocular movements are not affected

• There are never cerebellar or extrapyramidal signs

• Sphincters are involved late, if at all

• Remission is unknown and the disease is fatal within 5 to 7 years (caused by bronchopneumonia).

What are the clinical patterns of motor neuron disease?

• Bulbar: bulbar or pseudobulbar palsy (in 25%)

• Amyotrophic lateral sclerosis (ALS; in 50%): flaccid arms and spastic legs

• Progressive muscular atrophy (in 25%): a lesion in the anterior horn cells affecting distal muscles. Characteristically there is retention of deep tendon reflexes in the presence of severe muscular atrophy

• Primary lateral sclerosis (rare): signs progress from an upper motor neuron to a lower motor neuron type

• Others conditions affect lower motor neurons:

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