MRI Findings in Amyotrophic Lateral Sclerosis
Yes, there are specific MRI findings associated with ALS, though imaging is primarily used to exclude mimics rather than confirm the diagnosis, as ALS remains a clinical and electrophysiological diagnosis.
Brain MRI Findings
Most Common Finding: Corticospinal Tract Hyperintensity
The most common MRI finding in ALS is abnormal T2/FLAIR or proton density signal hyperintensity along the corticospinal tracts 1. This can appear anywhere from subcortical white matter to the pons, but is most frequently seen in:
- Posterior limb of the internal capsule (most common location)
- Cerebral peduncles (second most common)
This hyperintensity corresponds pathologically to demyelination, axonal degeneration, and gliosis 1.
Highly Specific Finding: Precentral Gyrus Hypointensity
T2-weighted or susceptibility-weighted imaging (SWI) showing abnormal hypointensity in the precentral gyrus and gray matter is highly sensitive and specific for ALS* 1. This finding is particularly valuable when present, as it has been noted to have both high sensitivity and specificity for the diagnosis 1.
Atrophy Patterns
Macroscopic atrophy is uncommon in ALS on routine visual inspection 1. However, advanced volumetric studies using voxel-based morphometry have demonstrated volume loss in:
- Precentral gyrus (motor cortex)
- White matter along corticospinal tracts
These changes are typically not apparent on standard clinical MRI review.
Spinal Cord MRI Findings
"Snake Eyes" Sign
The most common spinal MRI finding is abnormal T2/STIR (short tau inversion recovery) hyperintensity in the anterior horns, creating a "snake eyes" appearance 1.
Important caveats:
- This finding corresponds to lateral corticospinal tract involvement and reflects lower motor neuron disease
- Not specific for ALS - can occur in other conditions
- Often absent early in disease - may only appear later in the disease course 1
- Spinal MRI is often normal early in ALS 1
Recommended Imaging Approach
Brain Imaging
MRI head without IV contrast is the appropriate initial imaging modality 1. The protocol should include:
- T2/FLAIR sequences
- T2-weighted or susceptibility-weighted imaging* (critical for detecting precentral gyrus hypointensity)
- Proton density-weighted images
Spinal Imaging
MRI spine without IV contrast may be appropriate in certain clinical circumstances 1, primarily to exclude structural, infectious, or neoplastic mimics of motor neuron disease 1. Consider adding IV contrast if there is increased risk for infection or neoplastic processes.
Clinical Context and Limitations
Imaging in ALS serves primarily to exclude mimics, not to confirm the diagnosis 1. The diagnosis relies on:
- Clinical presentation (upper and lower motor neuron signs)
- Electromyography and nerve conduction studies (key diagnostic tests)
- Exclusion of other conditions
What NOT to Order
The following have no role in initial ALS evaluation 1:
- FDG-PET/CT brain
- MR spectroscopy
- Functional MRI (fMRI)
- CT (limited soft-tissue characterization compared to MRI)
Key Pitfalls to Avoid
- Do not rely on MRI to make the diagnosis - ALS is diagnosed clinically and electrophysiologically
- Absence of MRI findings does not exclude ALS - many patients have normal MRI, especially early in disease
- The "snake eyes" sign is not pathognomonic - it can occur in other conditions affecting the anterior horns
- Don't forget susceptibility-weighted sequences - standard T2/FLAIR alone may miss the highly specific precentral gyrus hypointensity