Chapter 4

Motor neuron disease

DEFINITION

Amyotrophic lateral sclerosis (ALS)

Progressive muscular atrophy (PMA)

Primary lateral sclerosis (PLS)

Progressive bulbar palsy (PBP)

EPIDEMIOLOGY

Prevalence

Age

Gender

AETIOLOGY

Familial MND

SOD1 (superoxide dismutase1)

Other genes

Sporadic MND

PATHOLOGY

Cellular mechanisms

Aggregation (clumping)

Oxidative stress

Glutamate toxicity

Mitochondria failure

Transport disruption

Abnormal glial cells

OUTCOME AND PROGNOSIS

SIGNS AND SYMPTOMS

Motor

Amyotrophic lateral sclerosis (ALS)

Progressive muscular atrophy (PMA)

Primary lateral sclerosis (PLS)

Progressive bulbar palsy (PBP)

Secondary complications

Definition

Motor neuron disease (MND) is a group of progressive neurodegenerative diseases that attack the upper and lower motor neurons (S2.13). Although four distinct types of MND are described, there is a great deal of overlap between them.

Amyotrophic lateral sclerosis (ALS)

This is the most common form of MND affecting both upper and lower motor neurons.

Progressive muscular atrophy (PMA)

This form is rare and only affects the lower motor neuron.

Primary lateral sclerosis (PLS)

This is a rare form of MND involving only the upper motor neurons and is usually of late onset (>50 years of age).

Progressive bulbar palsy (PBP)

This form of MND affects about one-quarter of people diagnosed and involves both the upper and lower motor neuron related to the motor cranial nerves (IX, X, XII) (S2.10) and is usually of late onset (50–70 years of age).

Epidemiology

Prevalence

Approximately two people in every 100 000 develop MND per year.

Age

MND can affect adults of any age but the highest incidence occurs between the ages of 50 and 70, with a peak between 55 and 60 years.

Gender

Men are affected approximately twice as often as women.

Aetiology

In 5–10% of MND cases, there is a strong family history evident, when it is termed ‘familial MND’. However, in 90–95% of cases, the cause is unknown and termed ‘sporadic MND’. More specifically, this is the term used for MND cases where there is no family history of the disease.

Familial MND

In this case MND is directly caused by a gene mutation passed down from parent to child. Unfortunately, researchers have been unable to identify a single gene as causal but several genes have been implicated:

SOD1 (superoxide dismutase1) (20% of all familial cases)

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