PRIMARY LATERAL SCLEROSIS
Patients newly diagnosed with motor neuron diseases quickly become familiar with different varieties of motor neuron-related illness. At the Packard Center, most queries we get about other diseases center on primary lateral sclerosis (PLS), probably because its early stages and those of ALS in some patients can resemble each other. Here we’ve pooled information on PLS from several online sources and medical journals and verified it with Packard Center physicians as a service to those who visit this site.

What it is
Like ALS, PLS is a disease of motor neurons. It’s progressive and causes nerve degeneration. But unlike ALS, PLS affects primarily upper motor neurons—those whose nerve cell bodies are in the brain and which deliver impulses to, and thus control, the activity of lower motor neurons. The latter innervate the muscles of the face, throat, larynx and limbs, the trunk and respiratory muscles. With ALS, there’s degeneration of both upper and lower motor neurons.

This diagram shows the different nerve tracts affected in PLS and ALS.
(P. Shaw, Nat Genet 2001;29 (2):103-4.)

How Does PLS Differ From ALS in Appearance?
PLS brings about the onset of stiffness and spasms (spasticity) as well as progressive weakness in various voluntary muscles, frequently beginning with the legs. Typically, symptoms extend to the arms, hands and both speech and swallowing muscles.

The disease progresses gradually—usually over 20 years—and, though life-changing and disabling, it isn’t fatal. This contrasts with ALS, which typically causes death three to five years after diagnosis.

Also, unlike ALS, PLS does not result in muscle-wasting. People with PLS do not experience the small local contractions of muscles beneath the skin, or fasciculations, often seen in ALS patients.

Questions in diagnosis can arise, though, because early on, PLS and ALS may have a number of signs and symptoms in common. That’s because, in some patients, ALS begins as an upper motor disease; its complete extension to lower neurons doesn’t come until later. So it looks like it could be PLS.

Because the average age of onset is similar, thorough testing is necessary. But even then, diagnosis may be uncertain for some patients and it’s only after waiting several years that the matter becomes clear. How many years is a matter of debate. Most physicians say at least three because the quicker course of ALS will become obvious by then.

“We sometimes think of PLS as an early stage of ALS,” says Jeffrey Rothstein, who heads the Packard Center. “That’s because it can evolve into the more serious disease. Even though it may be the last thing patients who have no obvious signs of lower motor disease want to do, we advise them to get checked every six months or so by their neurologist. They’ll have an electromyelogram (see below) to tell if muscles are changing in a characteristic way. If you know what’s ahead, you can do things to postpone disability longer or change your surroundings so you cope better.”

PLS may also resemble one or two other diseases early on, such as progressive multifocal leukoencephalopathy, but they’re fairly quickly crossed out because of faster progression.

How common is PLS?
According to information on one reputable web site (see list below), PLS is only about one half of one percent as common as ALS. That’s about one person in 10 million getting newly-diagnosed each year. The figures, though, are uncertain, in contrast to those for ALS, which affects two or three in 100,000 people in the United States.

Diagnosing PLS
A diagnosis of PLS basically comes by eliminating it from other neurological diseases. Neurologists first take a detailed patient history. They’ll ask about hoarseness, chronic muscle cramping and stiffness, for example, and if neurological disease runs in the family. They’ll also perform a thorough neurological exam, observing reflexes, balance, muscle strength and coordination, among other things. Blood tests are called for, to rule out conditions like long-standing syphilis. There’s typically an MRI of the brain and spine, motor and sensory nerve conduction studies and an electromyogram (EMG) to check on muscle response to stimulation. EMGs—especially repeated periodically at intervals—can reveal if upper or lower motor nerves are involved, or both. Spinal fluid is often checked to look for antibodies typical in multiple sclerosis.

For More Information
You may find these web sites useful for PLS questions: