Frontotemporal Dementia
The most likely diagnosis is frontotemporal dementia (FTD), specifically the behavioral variant (bvFTD), given the prominent personality changes, impulsivity, socially inappropriate sexual behavior, absence of memory impairment, and characteristic frontal/temporal lobe atrophy on neuroimaging. 1
Clinical Reasoning
Key Diagnostic Features Present
The patient's presentation is classic for behavioral variant frontotemporal dementia based on several critical features:
- Progressive behavioral changes with disinhibition: The sexual advances toward his relative in public settings represent classic disinhibition, one of the six core clinical features of bvFTD 1, 2
- Personality changes over 2 years: The insidious onset with gradual progression over two years is typical for bvFTD 1
- Absence of memory impairment: Unlike Alzheimer disease, episodic memory is characteristically preserved early in bvFTD 1
- No psychiatric symptoms: The absence of depression, hallucinations, or other psychiatric features helps distinguish this from primary psychiatric disorders 1
- Frontal/temporal atrophy on MRI: The coronal MRI showing frontal and/or anterior temporal atrophy is the hallmark neuroimaging finding in bvFTD 3, 4
Why Other Diagnoses Are Less Likely
Alzheimer disease typically presents with progressive amnestic syndrome—difficulty learning and remembering new information as the predominant early feature 1. This patient has no memory impairments, making AD unlikely despite it being the most common dementia overall.
Dementia with Lewy bodies requires fluctuating cognition, recurrent visual hallucinations, spontaneous parkinsonism, and/or REM sleep behavior disorder 1. None of these features are described in this case.
Vascular dementia would typically show stepwise progression, focal neurological deficits, and vascular risk factors with corresponding imaging findings 1. The gradual progression and behavioral predominance argue against this.
Creutzfeldt-Jakob disease presents as rapidly progressive dementia (weeks to months, not years) with myoclonus, ataxia, and characteristic EEG/MRI findings 1. The 2-year course is far too slow for CJD.
Syndromic Classification Framework
According to the 2025 Alzheimer's Association guidelines, this patient fits the progressive dysexecutive and/or behavioral syndrome (exemplified by bvFTD), characterized by changes in executive function, judgment, problem-solving, reasoning, with or without apathy or changes in personality or social/emotional behavior 1.
The differential diagnosis for this syndrome includes:
- Frequently FTLD (FTLD-tau or FTLD-TDP43) - most likely in this case 1
- Frequently AD or AD + vascular cognitive impairment and dementia (VCID)
- Sometimes FTLD-PSP, FTLD-CBD, or VCID
- Rarely Lewy body dementia
Diagnostic Criteria for bvFTD
The revised international consensus criteria require three of six core features for "possible" bvFTD 1, 2:
- Disinhibition - present (inappropriate sexual advances in public)
- Apathy/inertia
- Loss of sympathy/empathy
- Perseverative/compulsive behaviors
- Hyperorality
- Dysexecutive neuropsychological profile
This patient clearly meets criteria for disinhibition and likely has additional features that would be revealed on detailed examination 1, 2. The revised criteria have 86% sensitivity for "possible" bvFTD and 76% for "probable" bvFTD in autopsy-confirmed cases 2.
Critical Clinical Pitfalls
Misdiagnosis as primary psychiatric disorder: A positive psychiatric history or family psychiatric history can bias clinicians toward missing bvFTD diagnoses 1. However, this patient explicitly lacks psychiatric symptoms, reducing this risk.
Age considerations: While bvFTD typically presents between ages 40-70, the specific age would help refine the differential 1, 3. Behavioral changes with middle-to-late adult onset warrant thorough dementia evaluation 1.
Memory impairment can occur: Although episodic memory is typically preserved, approximately 10% of pathologically-confirmed bvFTD cases show marked memory deficits at initial presentation 1, 5. One autopsy case demonstrated devastating hippocampal cell loss with profound anterograde amnesia in rapidly progressive FTD 5.
Pathological Correlation
The histological findings on autopsy would show frontotemporal lobar degeneration (FTLD), most commonly with either tau pathology (FTLD-tau) or TDP-43 pathology (FTLD-TDP43) 1, 3, 4. The specific protein abnormality cannot be determined from clinical features alone but would be confirmed on the hematoxylin-eosin staining and immunohistochemistry 3, 4.