Skip to Content

How do they rule out ALS?

Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig’s disease, is a progressive neurodegenerative disease that affects nerve cells in the brain and spinal cord. Diagnosing ALS can be challenging as there is no single test that can definitively confirm the diagnosis. Doctors need to rule out other possible conditions before arriving at a diagnosis of ALS.

What are the symptoms of ALS?

The early symptoms of ALS often include:

  • Muscle weakness in one or more limbs
  • Muscle twitches (fasciculations)
  • Muscle cramping and stiffness
  • Difficulty speaking or swallowing

As the disease progresses, symptoms get worse and may include:

  • Loss of muscle mass
  • Difficulty breathing
  • Inability to move parts of the body

How is ALS diagnosed?

There is no single test that can conclusively diagnose ALS. Doctors diagnose ALS based on the person’s symptoms, a neurological exam, and by ruling out other possible causes. Tests that may help confirm ALS include:

  • Electromyography (EMG) – Records the electrical activity of muscles and can detect abnormal muscle activity due to ALS.
  • Nerve conduction study – Measures how quickly electrical signals move through a nerve. Can show nerve damage due to ALS.
  • MRI – Provides images of the brain and spinal cord to check for other possible conditions.
  • Blood and urine tests – Help rule out other diseases that may cause similar symptoms.

However, none of these tests can definitively diagnose ALS. The diagnosis is made based on the overall clinical presentation and by excluding other possible causes.

What other conditions need to be ruled out?

Because ALS symptoms are similar to many other conditions, doctors need to rule out a number of other possible diagnoses first. These include:

Peripheral neuropathy

Peripheral neuropathy is nerve damage that causes weakness, numbness and pain, usually in the hands and feet. It can be caused by diabetes, chemotherapy, autoimmune diseases, infections, toxins, and vitamin deficiencies. Electromyography can help distinguish ALS from peripheral neuropathies.

Muscular dystrophies

Muscular dystrophies are a group of diseases that cause progressive weakness and loss of muscle mass. They are genetic disorders often detected in childhood. Genetic testing and muscle biopsies can help differentiate muscular dystrophies from ALS.

Myasthenia gravis

Myasthenia gravis is an autoimmune disorder that causes muscle weakness and fatigue. It leads to fluctuating weakness that worsens with activity and improves with rest. Unlike ALS, myasthenia gravis often causes eye symptoms like drooping eyelids. Blood tests can detect antibodies characteristic of this disease.

Spinal muscular atrophy

Spinal muscular atrophy is an inherited motor neuron disease causing progressive muscle wasting and weakness. It typically begins in infancy or childhood. Genetic testing can help diagnose spinal muscular atrophy.

Multiple sclerosis

Multiple sclerosis (MS) is an autoimmune disease that affects the central nervous system and can cause muscle weakness, numbness, fatigue, and disability. However, MS tends to cause episodes of worsening symptoms followed by partial or full recovery. MRI and spinal fluid analysis can distinguish MS from ALS.

Spinal cord injury

Injury to the spinal cord can cause muscle weakness and paralysis below the level of injury. Unlike ALS, spinal cord injury does not cause progressive worsening. Imaging like MRI can detect damage or compression of the spinal cord.


Strokes can sometimes cause muscle weakness on one side of the body. However, the weakness has a sudden onset rather than being progressive over time. Imaging like MRI can identify strokes.


Radiculopathies are pinched nerves in the spine that can cause pain, numbness, or weakness in a limb. However, symptoms are typically limited to one area of the body. Electrodiagnostic tests can distinguish radiculopathies from ALS.

Inclusion body myositis

Inclusion body myositis is a rare inflammatory muscle disease leading to progressive muscle weakness. It may initially involve only the quadriceps and forearm muscles. Muscle biopsies show characteristic microscopic protein deposits.


Polymyositis causes muscle inflammation and weakness. Unlike ALS, it often causes swelling and tenderness of the muscles. Blood tests detect high muscle enzymes and autoantibodies.

What tests help rule out other conditions?

Some key tests used to exclude other diagnoses that may mimic ALS include:

Test Purpose
Electromyography (EMG) Detects abnormal electrical muscle activity indicating motor neuron damage. Helps differentiate ALS from conditions like peripheral neuropathies.
Nerve conduction studies Measures how quickly electrical signals move through a nerve. Can identify nerve damage distinct from ALS.
MRI of brain and spine Detects other central nervous system disorders like MS, tumors, injuries.
Genetic testing Identifies gene mutations of spinal muscular atrophy, muscular dystrophies.
Muscle biopsy Examines muscle tissue for signs of conditions like muscular dystrophies, inflammatory myopathies.
Blood tests Check for signs of diabetes, vitamin deficiencies, thyroid disorders. Also test for autoantibodies of myasthenia gravis or inflammatory muscle diseases.


ALS is a challenging condition to diagnose because there is no one definitive test. Doctors must rule out a number of other possible conditions before arriving at a diagnosis of ALS. Key steps include evaluating symptoms, performing a neurological exam, and utilizing tests like EMG, MRI, genetic tests, muscle biopsy and blood work. Ruling out other mimicking disorders like peripheral neuropathies, spinal cord injuries, multiple sclerosis and inflammatory myopathies can help confirm the diagnosis of ALS.