What is the most likely finding on diagnostic magnetic resonance imaging (MRI) of the brain in a patient with a 2-year history of worsening personality changes, impaired judgment, and language disturbances, with intact memory, cranial nerves, strength and muscle tone, sensation, coordination, and gait?

Frontotemporal Dementia with Knife-Edge Atrophy of Frontal and Temporal Lobes

The MRI of this patient is most likely to demonstrate knife-edge atrophy of the frontal and temporal lobes, consistent with frontotemporal dementia (FTD). This clinical presentation—characterized by personality changes, impaired judgment, and language disturbances over two years, with notably preserved memory, motor function, and coordination—is the hallmark of behavioral variant FTD (bvFTD) 1.

Clinical Reasoning

The key diagnostic features that point away from other dementia syndromes include:

• Preserved memory: This essentially excludes Alzheimer's disease, which characteristically presents with early episodic memory impairment as the predominant feature 1
• Prominent personality and behavioral changes: These are the defining features of bvFTD, including disinhibition, apathy, loss of empathy, and impaired judgment 1, 2
• Language disturbances: These can occur in FTD variants, particularly when temporal lobes are involved 3, 4
• Intact motor examination: This excludes Parkinson's spectrum disorders (which would show substantia nigra atrophy) and other movement disorders 1
• Two-year progressive course: This timeline is consistent with neurodegenerative FTD rather than rapidly progressive dementia from prion disease 1, 5

Neuroimaging Patterns in FTD

Knife-edge atrophy refers to the characteristic severe, focal atrophy of the frontal and temporal lobes that creates sharp gyral edges on MRI, distinguishing FTD from the more generalized, symmetric atrophy seen in Alzheimer's disease 3, 4. Research demonstrates that:

• BvFTD shows prominent degeneration in frontal and medial temporal lobes, insula, cingulate cortex, and limbic system 4
• The anterior temporal lobes are particularly affected, which correlates with socio-emotional impairments 4
• Deformation-based morphometry reveals focal cortical and subcortical atrophy patterns specific to FTD subtypes 3

Why Other Options Are Incorrect

Generalized symmetric cerebral cortical atrophy is characteristic of Alzheimer's disease, which would present with prominent memory impairment as the earliest and most severe symptom—not personality changes with preserved memory 1.

Atrophy of the substantia nigra pars compacta occurs in Parkinson's disease and related syndromes, which would manifest with motor symptoms (rigidity, bradykinesia, tremor) and gait abnormalities—all explicitly absent in this patient 1.

Numerous scattered T2-hyperintense subcortical white matter lesions suggest vascular dementia, which typically presents with stepwise cognitive decline, focal neurological deficits, and vascular risk factors 1. The gradual progressive course without motor or sensory deficits makes this unlikely.

Remote bilateral anterior cerebral artery-territory infarcts would cause acute onset symptoms with motor deficits (particularly lower extremity weakness) and executive dysfunction, not the insidious two-year progression described here 6.

Critical Diagnostic Considerations

The ACR Appropriateness Criteria emphasize that imaging in dementia serves to identify the specific pattern of atrophy that correlates with clinical syndrome 1. In FTD:

• Clinical symptoms remain the most important diagnostic cues, even when imaging shows atypical patterns 7
• The combination of behavioral symptoms (personality changes, impaired judgment) with language disturbances and preserved memory strongly suggests FTD 1, 2
• Neuroimaging with MRI is superior to CT for detecting the focal atrophy patterns characteristic of FTD subtypes 1, 3

Common Pitfalls to Avoid

Do not mistake early FTD for primary psychiatric disorders such as bipolar disorder, depression, or schizophrenia, as behavioral and personality changes can initially mimic these conditions 8. The progressive nature over two years with cognitive decline distinguishes neurodegenerative disease from primary psychiatric illness 2, 8.

Do not assume memory must be impaired for dementia diagnosis—FTD characteristically spares memory until late stages, with behavioral and executive dysfunction predominating 1, 2. The diagnostic criteria for dementia require impairment in at least two cognitive domains, which can include personality/behavior changes and language without memory involvement 1.