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. ALS causes loss of muscle control, eventually making it difficult or impossible to speak, eat, move and breathe. There is currently no cure for ALS and no effective treatments to halt or reverse the progression of the disease.
Diagnosing ALS can be challenging, as there are no definitive tests. Doctors typically make a diagnosis based on a person’s medical history, a physical exam, and a series of tests to rule out other possible causes. One test that is sometimes used as part of the diagnostic process is an MRI (magnetic resonance imaging) scan. But can an MRI actually detect or diagnose ALS?
How an MRI Works
An MRI scan uses magnets and radio waves to create detailed cross-sectional images of organs and tissues inside the body. It allows doctors to see the structure and function of the brain and spinal cord in great detail.
During an MRI, the patient lies inside a large cylinder-shaped scanner containing a strong magnetic field. Radio waves are then pulsed through the body, which stimulates the protons in cells to align themselves with the magnetic field. When the radio waves are turned off, the protons realign and emit energy signals that are picked up by the scanner. These signals create images based on the specific characteristics of different tissues.
MRIs are useful for detecting structural abnormalities in the brain and spinal cord that may be associated with certain conditions. But MRIs have limitations in diagnosing conditions like ALS that cause progressive cell damage over time.
ALS Symptoms and Diagnosis
ALS causes motor neurons in the brain and spinal cord to gradually break down and die. As these neurons degenerate, they can no longer send signals to muscles, leading to muscle weakness, twitching, cramping and eventual paralysis.
Some early symptoms of ALS may include:
- Tripping or stumbling
- Weakness in the hands, arms, legs or mouth
- Slurred speech
- Muscle cramps and twitches
As ALS advances, more severe symptoms emerge:
- Loss of muscle strength in legs, arms, torso
- Difficulty breathing
- Difficulty swallowing (choking)
- Cognitive and behavioral changes
There are no definitive laboratory or imaging tests that can diagnose ALS. Doctors typically look for the following criteria:
- Evidence of progressive muscle weakness and wasting in multiple body regions
- Signs of upper and lower motor neuron damage through clinical exam
- Absence of findings that suggest alternative diagnoses
They may use tests like EMGs, electrical studies, blood tests, and imaging scans to help rule out other disorders. But the diagnosis is still largely based on the physician’s clinical expertise and observation of disease progression.
Limitations of Using MRI to Diagnose ALS
While MRI scans are sometimes used in trying to diagnose ALS, there are a few key reasons why MRIs have significant limitations:
1. MRIs show structure, not function
An MRI provides static images of the brain and spinal cord structure at one point in time. But it doesn’t show how well the neurons are actually functioning. In ALS, neurons may appear normal on an MRI early in disease progression, but they are functionally impaired. So a “normal” MRI does not rule out an ALS diagnosis.
2. ALS-related changes occur at a microscopic level
The loss of motor neurons in ALS occurs over time at a cellular level. But MRI scans may not be detailed enough to detect these microscopic cellular changes early on. The changes may not be obvious on MRI until significant neuron death has already occurred.
3. Lack of large-scale changes early in disease
In early ALS, there is selective loss of motor neurons, but not widespread loss of large areas of brain tissue. These smaller areas of cell loss are difficult to visualize on MRI. Widespread changes don’t usually occur until later stages.
4. ALS is progressive
ALS is a progressive disease. Cell damage occurs over months and years. An MRI provides just a single snapshot in time. It can’t demonstrate the progressive pattern of increasing damage that points to a diagnosis of ALS.
5. Better tools for functional assessment
Other neuroimaging techniques may be better at detecting early functional neuronal changes in ALS. For example, fMRI (functional MRI) can measure activity in the motor cortex. And PET (positron emission tomography) scans assess metabolic activity that indicates functioning.
What Does the Research Say?
Numerous research studies have investigated the utility of MRI for diagnosing ALS. Overall, the research suggests MRI has significant limitations:
Low sensitivity in early disease stages
A 2008 study evaluated spine MRIs in 408 ALS patients. 71% of patients had a normal MRI. The study concluded, “MRI of the spine has a low sensitivity in diagnosing ALS patients, particularly in clinically early disease stages.”
Limited value in early diagnosis
A 2019 review stated, “Conventional MRI sequences are of limited value for the early diagnosis of ALS.” Advanced techniques like diffusion tensor imaging were noted to have slightly higher sensitivity.
Better for excluding other conditions
A 2016 study suggested spinal cord MRI is useful to exclude spinal cord pathologies that can mimic ALS. But MRI has “insufficient sensitivity in early disease stages to establish or exclude the diagnosis of ALS.”
Changes seen in late stages
A 2017 study found that spinal cord atrophy and signal changes were detectable in 79% of ALS patients – but only in advanced disease stages. Such changes were uncommon early on.
| Study | Main Conclusions on MRI for ALS Diagnosis |
|---|---|
| Ellis et al. 2008 | Low sensitivity, especially in early disease |
| Sharma et al. 2019 | Limited value for early diagnosis |
| Kassubek et al. 2016 | Useful to exclude other conditions |
| Parry et al. 2017 | Changes detectable only in late stages |
Role of MRI in ALS Diagnosis
Based on the current evidence, here is the role MRI scans play in diagnosing ALS:
- MRIs are not definitive for diagnosing ALS, especially early on
- MRIs may be used to rule out spinal cord lesions or other clinical mimics of ALS
- MRIs may demonstrate findings supportive of ALS in late or advanced disease
- Normal MRIs do not exclude an ALS diagnosis, even late on
- MRIs are just one piece of the clinical diagnostic process
Experts emphasize that the diagnosis of ALS is a clinical one based on examination over time. Neuroimaging like MRI can be helpful but is not sufficient on its own to diagnose or exclude ALS.
Promising Advanced MRI Techniques
While conventional MRI has limitations for ALS diagnosis, some evolving techniques show promise:
Diffusion Tensor Imaging (DTI)
DTI detects microscopic changes in the integrity of white matter tracts. Small studies show it may detect early ALS-related damage.
Functional MRI (fMRI)
fMRI measures brain activity. It may identify functional changes in ALS patients’ motor neurons.
Spinal Cord Texture Analysis
Computer algorithms can detect unique textural properties associated with spinal cord damage in ALS patients.
Muscle MRI
Imaging muscles may reveal early ALS-related denervation changes.
Larger studies are still needed to demonstrate whether these techniques can improve early ALS diagnosis.
Conclusion
While MRI is sometimes used as part of an ALS diagnostic workup, current research demonstrates significant limitations of conventional MRI for definitively detecting or diagnosing ALS. The gradual, microscopic progression of neuronal damage, especially early on, is difficult to visualize on standard MRI sequences. Advanced techniques like functional MRI show promise to improve sensitivity, but more research is still needed. For now, MRI remains just one piece of the comprehensive clinical assessment required to arrive at an ALS diagnosis. A normal MRI result does not rule out ALS, particularly in early stages. But comparing MRI changes over time may provide supportive information in later stages of the disease.