What is the most appropriate imaging investigation for a 60-year-old woman suspected to have Motor Neuron Disease (MND) with symptoms of progressive proximal muscle weakness, atrophy, numbness, spasticity, and abnormal gait?

MRI Brain is the Most Appropriate Imaging Investigation for Suspected Motor Neuron Disease

MRI of the brain without IV contrast is the most appropriate imaging investigation for a 60-year-old woman with suspected motor neuron disease (MND) presenting with progressive proximal muscle weakness, atrophy, numbness, spasticity, and abnormal gait. 1

Rationale for MRI Brain as First-Line Imaging

MRI brain without contrast is recommended by the American College of Radiology (ACR) Appropriateness Criteria for the initial imaging evaluation of patients with suspected motor neuron disease for several key reasons:

1. Exclusion of mimics: MRI is primarily used to exclude other conditions with similar clinical presentations rather than to confirm MND 1

2. Visualization of characteristic findings: MRI can detect abnormal signal on T2/FLAIR or proton density-weighted images within the corticospinal tracts, particularly in:

   • Posterior limb of the internal capsule
   • Cerebral peduncles
   • Subcortical white matter to the pons 1

3. Detection of specific MND markers: T2*-weighted or susceptibility-weighted imaging can show abnormal hypointensity in the precentral gyrus and gray matter, which has been noted to be highly sensitive and specific for ALS 1

Why Other Options Are Less Appropriate

MRI Cervical Spine

While MRI spine without IV contrast may be appropriate for certain patients with suspected motor neuron disease, it is not the first-line recommendation. Spinal MRI is often normal early in the disease course and is primarily used to exclude structural, infectious, or neoplastic etiologies that can mimic MND 1. The characteristic "snake eyes" appearance (abnormal T2 signal in the anterior horns) may only be present later in the disease course and is not specific for ALS 1.

CT Cervical Spine

CT of the spine is not useful in making the diagnosis of ALS because of its limited soft-tissue characterization 1. It cannot adequately visualize the spinal cord parenchyma where pathological changes of MND occur.

Electromyography (EMG)

While EMG is a key diagnostic test for MND, it is not an imaging investigation. The ACR guidelines specifically note that "electromyography and nerve conduction velocity are key tests in diagnosing ALS" 1, but they are complementary to imaging rather than alternatives.

CSF Analysis

CSF analysis is not recommended as an initial investigation for suspected MND. It may be useful to rule out inflammatory or infectious conditions but is not part of the standard diagnostic algorithm for MND.

Clinical Application

For this 60-year-old woman with progressive proximal muscle weakness, atrophy, numbness, spasticity, and abnormal gait:

1. Start with MRI brain without contrast to:

   • Rule out structural lesions mimicking MND
   • Look for characteristic signal changes in the corticospinal tracts
   • Assess for precentral gyrus hypointensity on T2* or susceptibility-weighted imaging

2. Consider adding MRI cervical spine without contrast only if:

   • There are atypical features suggesting alternative diagnoses
   • Focal cervical symptoms predominate
   • Clinical suspicion for structural cervical pathology exists

Important Caveats

• MRI findings may be subtle or absent in early MND
• Isolated hyperintensity in the posterior limbs of the internal capsules can be seen in normal controls and is not pathological in isolation 2
• Diagnosis of MND should not rely solely on imaging but should incorporate clinical features and electrophysiological studies
• The combination of upper and lower motor neuron signs across multiple body regions is the clinical hallmark of MND

Diagnostic Algorithm

1. Clinical suspicion of MND based on progressive upper and lower motor neuron signs
2. MRI brain without contrast to exclude mimics and look for characteristic findings
3. Electrophysiological studies (EMG and nerve conduction studies) to confirm lower motor neuron involvement
4. Consider MRI spine only if needed to exclude structural causes or if focal symptoms predominate
5. Laboratory tests to exclude metabolic, inflammatory, or infectious mimics

By following this evidence-based approach, clinicians can efficiently evaluate patients with suspected MND while minimizing unnecessary testing.