ConditionBrain & nerves
Primary Lateral Sclerosis (PLS)
Primary lateral sclerosis (PLS) is a rare disease in which the upper motor neurons, the nerve cells in the brain that control voluntary movement, slowly degenerate. It causes gradually worsening stiffness (spasticity), slowness and weakness, usually starting in the legs, and later affecting the arms, speech and swallowing. Unlike ALS, the lower motor neurons are spared, so muscles do not waste away and progression is much slower over years to decades. Life expectancy is often near normal. Treatment focuses on easing stiffness and maintaining function.
Also called: PLS, upper motor neuron disease
At a glance
- What it is
- Slow degeneration of upper motor neurons, causing stiffness and weakness without muscle wasting.
- Onset
- Usually age 40 to 60. Slightly more common in men.
- Symptoms
- Leg stiffness, slow clumsy walking, falls, then arm stiffness, slurred speech, swallowing difficulty, emotional lability.
- Diagnosis
- Exclusion of other causes, MRI, EMG, and time: definite PLS needs symptoms for at least 4 years without lower motor neuron signs.
- Treatment
- Antispasticity medicines, physical therapy, speech therapy and mobility aids.
When to get help
Call 911 or go to the ER
- Choking episodes or inability to swallow saliva
- Sudden weakness on one side, facial drooping or slurred speech that comes on suddenly (possible stroke)
See a doctor within a day or two
- Rapidly worsening weakness over weeks, or new muscle wasting or twitching (rule out ALS or another cause)
- Frequent falls with injury
- Breathing difficulty
Book a routine appointment
- Gradually increasing stiffness or clumsiness in the legs over months
- Slurred speech or difficulty swallowing developing slowly
- Unexpected laughing or crying that is hard to control
What happens in PLS
Voluntary movement depends on two sets of nerves:
- upper motor neurons in the brain's motor cortex, which send signals down the spinal cord
- lower motor neurons in the spinal cord and brainstem, which connect directly to muscles
In PLS, only the upper motor neurons degenerate. Muscles still receive basic signals, but control is lost, causing stiffness, exaggerated reflexes and slowed movement. The cause is unknown. Most cases are sporadic, and rare juvenile forms are linked to genes such as ALS2.
PLS vs ALS
| Feature | Primary lateral sclerosis | Amyotrophic lateral sclerosis |
|---|---|---|
| Neurons affected | Upper only | Upper and lower |
| Muscle wasting and twitching | No | Yes |
| Progression | Slow, over years to decades | Faster, usually over a few years |
| Life expectancy | Often near normal | Typically 2 to 5 years from symptom onset |
| Breathing failure | Uncommon | Common |
Some people first diagnosed with PLS develop lower motor neuron signs within the first few years and are then diagnosed with upper motor neuron-predominant ALS. This is why doctors wait and repeat tests before confirming PLS.
Symptoms
- stiffness and tightness in the legs, often starting on one side
- slow, effortful, stiff walking and tripping
- poor balance and falls
- muscle spasms and cramps
- clumsiness and stiffness in the hands later
- slurred, slow or strained speech (spastic dysarthria)
- difficulty swallowing
- pseudobulbar affect: uncontrollable laughing or crying out of proportion to feelings
- urinary urgency in some people
Sensation and thinking are usually unaffected.
Diagnosis
There is no single test. Diagnosis involves:
- neurological examination: spasticity, brisk reflexes, upgoing toes (Babinski sign) without muscle wasting
- MRI of the brain and spine: to exclude multiple sclerosis, spinal cord compression, tumors and other causes
- EMG (electromyography): to look for lower motor neuron involvement that would suggest ALS
- blood tests: vitamin B12, copper, HIV, HTLV-1, syphilis and others that can mimic PLS
- genetic tests: for hereditary spastic paraplegia, which looks similar and often runs in families
- lumbar puncture in some cases
Consensus criteria (2020) classify probable PLS after 2 to 4 years and definite PLS after 4 years of upper motor neuron symptoms without lower motor neuron signs.
Treatment
There is no cure or treatment to slow PLS. Care focuses on symptoms and function:
Spasticity
See spasticity for general management.
- baclofen and tizanidine tablets
- intrathecal baclofen pump for severe spasticity
- botulinum toxin injections for focal stiffness
- stretching and physical therapy
Mobility and daily living
- physical therapy for strength, flexibility and balance
- canes, walkers, ankle-foot orthoses and later wheelchairs
- occupational therapy and home adaptations
Speech and swallowing
- speech therapy and communication devices
- swallowing assessment and diet changes
Emotional and other symptoms
- dextromethorphan-quinidine or antidepressants for pseudobulbar affect
- medicines for bladder urgency
- support for mood and caregivers
Outlook
PLS progresses slowly, and many people remain walking for years, though most eventually need mobility aids. Life expectancy is often near normal. A multidisciplinary neuromuscular clinic helps people maintain independence.
Frequently asked questions
What is primary lateral sclerosis?
PLS is a rare neurological disease in which nerve cells in the brain that control voluntary movement slowly degenerate. It causes progressive stiffness and weakness, usually starting in the legs, without the muscle wasting seen in ALS.
How is PLS different from ALS?
PLS affects only upper motor neurons, progresses slowly over years to decades, and usually does not shorten life much. ALS affects both upper and lower motor neurons, causes muscle wasting and twitching, and progresses faster.
What is the life expectancy with primary lateral sclerosis?
Life expectancy is often near normal. PLS progresses slowly, and serious breathing problems are uncommon. Quality of life depends on managing stiffness, mobility, speech and swallowing.
How is PLS diagnosed?
Diagnosis relies on a neurological exam, MRI to rule out other conditions, EMG to check for lower motor neuron involvement, blood and genetic tests, and time. Definite PLS is diagnosed after 4 years of symptoms without lower motor neuron signs.
Is there treatment for primary lateral sclerosis?
There is no cure, but treatments help symptoms: baclofen, tizanidine and botulinum toxin for stiffness, physical and speech therapy, mobility aids and medicines for emotional lability.
Sources
This guide was written from current patient and clinical guidance, including:
- National Institute of Neurological Disorders and Stroke (NINDS): Primary lateral sclerosis
- National Organization for Rare Disorders (NORD): Primary lateral sclerosis
- Spastic Paraplegia Foundation: PLS
- Consensus diagnostic criteria for primary lateral sclerosis (Turner et al., Neurology 2020)
- MedlinePlus: Primary lateral sclerosis