PLS vs ALS — The Conversion Question

When a neurologist first suggests PLS, or when you search for what it is, you will almost immediately encounter ALS alongside it. The two diseases are related, and the early clinical picture of PLS can look like the early clinical picture of ALS. This page explains why the distinction takes time, what the 23% conversion figure actually means, and what the data says about your likely trajectory.

The first question after a PLS suggestion: could it be ALS?

Yes — it could be, during the early phase of the disease. This is not a failure of your doctor to diagnose you correctly. It is how the biology works. Both PLS and ALS can begin with identical symptoms: leg stiffness, progressive weakness, brisk reflexes, difficulty walking. At onset, some people who will eventually be diagnosed with ALS present with a purely upper motor neuron picture — the same picture that characterises PLS throughout its course.

The difference between them emerges over time. In ALS, lower motor neuron signs eventually appear — muscle wasting, fasciculations, EMG changes. In PLS, they do not. But "eventually" and "do not" are statements about what happens over months to years, not something that can be established from a single clinic visit.

This is why a new PLS diagnosis often comes with a degree of uncertainty attached: "this looks like PLS, we will monitor." That monitoring is not hedging or incompetence. It is the appropriate clinical approach given what the biology makes possible to know at a given point in time.

Why the clinical distinction is hard early on

The difference between PLS and ALS is, at its core, a difference in which motor neurons are affected. In PLS, only upper motor neurons degenerate — those in the motor cortex projecting down the corticospinal tracts. In ALS, both upper and lower motor neurons degenerate.

The upper motor neuron signs — spasticity, hyperreflexia, the Babinski sign, slowness of movement — are present in both diseases. They are the symptoms that bring people to a neurologist in the first place. What distinguishes ALS is the additional presence of lower motor neuron signs: muscle wasting, visible fasciculations, and electrophysiological evidence of active denervation on EMG.

The challenge: in early ALS, lower motor neuron signs may not yet be clinically obvious. The disease may have started its lower motor neuron damage without it being detectable on standard examination. EMG can detect early lower motor neuron degeneration before it becomes clinically visible — and this is why EMG is a central part of PLS workup — but even EMG has limits in very early disease.

The diagnostic process is therefore a longitudinal one. You are not just being evaluated at a single point in time. You are being followed over months and years, with periodic clinical reassessment and repeat testing, to see whether the picture remains purely upper motor neuron or whether lower motor neuron signs emerge.

The 2020 criteria: probable PLS and definite PLS

In 2020, an international group of PLS experts published consensus diagnostic criteria that for the first time provided a standardised, widely applicable definition of PLS. The criteria established two levels of diagnostic confidence:

• Probable PLS: Progressive, pure upper motor neuron syndrome — affecting at least two body regions — for 2 to 4 years from symptom onset, with no significant lower motor neuron signs on examination or EMG.
• Definite PLS: The same picture, sustained for 4 or more years from symptom onset.

The four-year threshold for "definite PLS" is not arbitrary. It reflects what the natural history data shows about when conversion to ALS occurs: if it is going to happen, it almost always happens within the first four years. After four years of pure upper motor neuron symptoms, you are almost certainly looking at PLS rather than ALS that has not yet declared its lower motor neuron component.

If you are in the first two years of symptoms, neither label may yet apply — you may be at the stage where "upper motor neuron syndrome under investigation" is the most honest description. That is uncomfortable, but it is accurate.

The 23% conversion figure — what the research actually shows

The figure you are most likely to encounter: approximately 23% of people who present with a pure upper motor neuron syndrome convert to an ALS diagnosis over time.

Several things are important to understand about this number:

It applies to the entire observation period, not to your current moment

The 23% figure reflects the cumulative conversion rate across all patients in cohort studies, followed from the time of their initial presentation. It does not mean that at any given moment you have a 23% chance of converting. The risk is highest in the first two to four years and drops substantially after that. If you have had pure upper motor neuron symptoms for three years without lower motor neuron signs appearing, your probability of conversion is lower than 23% and is actively decreasing as time passes.

Conversion is front-loaded

Conversion to ALS almost always occurs within the first four years of symptom onset. This is the basis for the "definite PLS" threshold in the 2020 criteria. Data from natural history cohorts shows that after four years of pure upper motor neuron signs, further conversion becomes rare.

The 77% who do not convert

Most people who present with a pure upper motor neuron syndrome do not convert to ALS. The 77% who do not develop lower motor neuron signs within four years are, by the data, extremely unlikely to do so later. This is the group for whom the PLS label and its associated near- normal life expectancy applies.

What conversion would look like

If the picture were to change — if ALS were to emerge from what initially looked like PLS — the change would be detectable. Conversion is not a switch that flips invisibly. It produces clinical and electrophysiological signs that your care team will be watching for:

• Fasciculations: Visible, spontaneous twitching of muscle fibers — particularly in the tongue, hands, or other limb muscles. In PLS, fasciculations are absent. Their appearance is a meaningful warning sign.
• Muscle atrophy: Visible wasting of muscles, particularly in the hands, forearms, or tongue. PLS does not cause significant muscle wasting; its appearance would indicate lower motor neuron involvement.
• EMG changes: Active denervation on electromyography — fibrillations and positive sharp waves in muscles — is a sensitive indicator of lower motor neuron degeneration before it becomes clinically visible. Your neurologist may repeat EMG at intervals to watch for these changes.
• A faster rate of decline: PLS progresses slowly — the functional rating scale declines at roughly 1.6 to 1.9 points per year. A significantly faster rate of decline than expected might prompt reassessment.
• Unexpectedly early respiratory changes: Respiratory function is largely preserved in PLS; only about 6% ever require non-invasive ventilation. Early or significant respiratory changes would warrant re-evaluation.

If any of these signs appear, tell your neurologist promptly. A repeat clinic visit and repeat EMG would be indicated. Most of the time, they will not appear — but monitoring for them is the reason for regular neurology follow-up in PLS.

The reassuring data: pure UMN at 4+ years

The most reassuring thing the research shows: if you have had pure upper motor neuron symptoms for four or more years with normal or near-normal EMG, conversion to ALS becomes very rare.

A 2025 analysis of PLS natural history published in the Journal of Neurology, Neurosurgery and Psychiatry found that median survival from symptom onset in PLS is 23.1 years, with a median age at death of 79.5 years — close to the general population reference of 81.9 years. Only 10% of patients in that cohort required a gastrostomy (feeding tube), and only 6% required non-invasive ventilation. These are the outcomes for people with definite PLS — people who have been at this for long enough that the diagnosis is established.

The functional scale (ALSFRS-R) declines at approximately 1.6 points per year in PLS — about three times slower than in ALS. Over a 23-year median disease course, that adds up to significant disability, primarily in mobility. But it is not the relentless, fatal trajectory of ALS. Life expectancy is anticipated to be near-normal.

Side-by-side comparison

Why the distinction matters for prognosis

The difference between PLS and ALS is not a matter of naming. It is the difference between a prognosis of near-normal life expectancy with progressive disability and a prognosis of 2–5 years of rapidly escalating decline. That difference has profound implications for how you plan your life, your care, your finances, and your relationships.

It is also why the diagnostic process takes the time it takes. A neurologist who told you definitively that you have PLS after three months would be telling you something they cannot yet know with confidence. A neurologist who follows you for two to four years and watches for lower motor neuron signs before applying the "definite PLS" label is doing the job correctly, even if the waiting is hard.

The treatment implications also differ: ALS has four FDA-approved disease-modifying drugs that are not appropriate for PLS. Knowing which disease you have determines which treatment conversations make sense. For detail on what treatment looks like for PLS, see current treatments.

How to live with diagnostic uncertainty

For most people, the hardest part of early PLS is not the physical symptoms — which may be manageable at this stage — but the uncertainty. The question that has no clear answer yet: is this going to stay PLS, or is it going to become something else?

There is no honest way to make that uncertainty disappear before time resolves it. What can be said honestly:

• The majority of people who present with a pure upper motor neuron syndrome — 77% in the available cohort data — do not convert to ALS. The probability is on your side.
• Conversion, if it occurs, produces detectable signs. You are not going to convert without your neurologist being able to see it in the clinic or on EMG. The monitoring that feels like waiting is also watching.
• The longer you maintain a pure upper motor neuron picture, the lower your probability of conversion. Time is informative, even when it is difficult.
• Living well with a condition requires some degree of acceptance of what is not yet known. Psychological support — therapy, peer community with other PLS patients, honest conversation with people you trust — is a legitimate and important part of managing the early phase of PLS.

The community of people who have been living with PLS for years and decades exists and is accessible. Connecting with them — through the ALS Association's PLS support group, PLS Friends, or patient stories from those who have walked this path — provides a kind of evidence that research papers cannot: lived proof of what the longer arc looks like. See community and support groups.