|Progressive muscular atrophy|
|Classification and external resources|
Progressive muscular atrophy (PMA) is a rare subtype of amyotrophic lateral sclerosis (ALS) or motor neurone disease (MND) which affects only the lower motor neurones. PMA is thought to account for around 4% of all ALS/MND cases. This is in contrast to the most common form of ALS/MND, amyotrophic lateral sclerosis, which affects both the upper and lower motor neurones, or another rare form of ALS/MND, primary lateral sclerosis, which affects only the upper motor neurones. The distinction is important because PMA is associated with a better prognosis than classical ALS/MND.
Despite being rarer than ALS/MND, PMA was actually described earlier, when in 1850 French Neurologist Francois Aran described 11 cases which he termed atrophie musculaire progressive. Contemporary neurologist Duchenne also claimed to have described the condition one year earlier, but the written report was never found; an archaic term for the disease was once "Aran-Duchenne disease" or "Duchenne-Aran disease". 
As a result of lower motor neurone degeneration, the symptoms of PMA include:
Some patients have symptoms restricted only to the arms or legs (or in some cases just one of either). These cases are referred to as "Flail Arm" (FA) or "Flail Leg" (FL) and are associated with a better prognosis. 
PMA is a diagnosis of exclusion, there is no specific test which can conclusively establish whether a patient has the condition. Instead, a number of other possibilities have to be ruled out, such as multifocal motor neuropathy or spinal muscular atrophy. Tests used in the diagnostic process include MRI, clinical examination, and EMG. EMG tests in patients who do have PMA usually show denervation (neurone death) in most affected body parts, and in some unaffected parts too.
It typically takes longer to be diagnosed with PMA than ALS/MND, an average of 20 months for PMA vs 15 months in ALS/MND.
The importance of correctly recognizing progressive muscular atrophy as opposed to ALS/MND is important for several reasons.
An initial diagnosis of PMA could turn out to be slowly progressive ALS/MND many years later, sometimes even decades after the initial diagnosis. The occurrence of upper motor neurone symptoms such as brisk reflexes, spasticity, or a Babinski sign would indicate a progression to ALS/MND; the correct diagnosis is also occasionally made on autopsy. 
Since its initial description in 1850, there has been debate in the scientific literature over whether PMA is a distinct disease with its own characteristics, or if lies somewhere on a spectrum with ALS/MND, PLS, and PBP. Charcot, who first described ALS/MND in 1870, felt the PMA was a separate condition, with degeneration of the lower motor neurones the most important lesion, whereas in ALS it was the upper motor neurone degeneration that was primary, with lower motor neurone degeneration being secondary. Such views still exist in archaic terms for PMA such as "Primary progressive spinal muscular atrophy". Throughout the course of the late 19th century, other conditions were discovered which had previously been thought to be PMA, such as pseudo-hypertrophic paralysis, hereditary muscular atrophy, progressive myopathy, progressive muscular dystrophy, peripheral neuritis, and syringomyelia. 
The neurologists Dejerine and Gowers were among those who felt PMA was part of a spectrum of "motor neurone disease" including ALS, PMA, and PBP, in part because it was almost impossible to distinguish the conditions at autopsy. Other researchers have suggested that PMA is just ALS/MND in an earlier stage of progression, because although the upper motor neurones appear unaffected on clinical examination there are in fact detectable pathological signs of upper motor neurone damage on autopsy. 
In favour of considering PMA a separate disease, some patients with PMA live for decades after diagnosis, which would be unusual in typical ALS/MND. 
To this day, terminology around these diseases remains confusing because in the UK "((motor neurone disease))" refers to both ALS specifically and to the spectrum of ALS, PMA, PLS, and PBP. In the United States the most common terms are ALS (both specifically for ALS and as a blanket term) or "Lou Gehrig's disease".