Progressive muscular atrophy is a rare subtype of amyotrophic lateral sclerosis (ALS)
which affects only the lower motor neurons.
In Progressive Muscular Atrophy there is gradual loss of the lower motor neurons,
so the muscles become steadily weaker and smaller, causing problems to the area of
the body they serve. This results in:
Progressive muscle weakness.
Fasciculation’s - rippling effect under the skin.
Shrinkage in muscle bulk.
Weight loss.
PMA affects 5-7% of all people living with MND.
Men are affected more than women and figures of 5:1 have been quoted, although this
might be affected by the inclusion of undiagnosed Kennedy’s disease in the male group.
Kennedy’s disease is a rare form of Spinal Muscular Atrophy.
Average age of onset is under 50 years, which is younger than ALS.
Life expectancy is generally longer than for ALS, averaging 5 – 10 years.
PMA is held to be a form of ALS and especially in the first 4 years after diagnosis
clinical evidence of upper motor involvement may become apparent in some people.
In others, the signs may always remain lower motor neuron in origin.
PMA occurs sporadically in the majority of affected individuals. Sporadic is the
term used for cases of the disease where there is no family history.
PMA occasionally presents in families with a history of familial ALS. Familial is
the term used when more than one member of the family has been diagnosed with the
disease.
Electrophysiological tests (nerve stimulation) to the brain may also be carried out
in an attempt to identify any upper motor neurone component which may not be evident
on clinical examination.
An EMG test is a good way of detecting lower motor neurone disease, but has to be
put into context with all of the other tests carried out, to eliminate all other
possible diseases.
PMA, as with other forms of motor neuron disease, can be difficult to diagnose. It
has to be recognised that, although the majority of people with PMA actually have
the ‘lower motor neuron form of ALS, about one third may have other forms of lower
motor neuron disorders.
The initial investigations are important. This helps to identify other forms of lower
motor neuron disorders.
Diagnosis is clinical and usually based on observation but does include an abnormal
EMG. Diagnosis involves exclusion of all other possibilities, such as multiple sclerosis,
spinal cord injury or tumours and motor neuropathies of other causes.
Tests performed include:
MRI scans to exclude abnormalities in brain or spinal cord
Nerve conduction studies (small electric shocks to check speed of nerve conduction
to the muscles)
Electromyography (EMG): small needles placed in individual muscles to check for any
dysfunction (abnormal in Lower Motor Neurone disease)
