Residency · Residency · Neurology

Frontotemporal Dementia Spectrum

Introduction

Frontotemporal dementia (FTD) is the second most common cause of dementia in individuals under age 65 and encompasses a clinically, pathologically, and genetically heterogeneous group of neurodegenerative disorders characterized by progressive degeneration of the frontal and temporal lobes. Recognizing FTD is critical because it is frequently misdiagnosed as a psychiatric disorder or Alzheimer disease, and its management differs substantially from other dementias.

Epidemiology

FTD has a prevalence of 15-22 per 100,000 in the 45-64 age group, with a mean age of onset between 50 and 60 years, though the range extends from the 20s to the 80s. The sex distribution is roughly equal overall, although behavioral variant FTD is more common in men. Genetic forms account for 30-50% of cases, giving FTD the highest heritability of any dementia. It is the leading cause of dementia in individuals under 60 years old.

Clinical Syndromes

Behavioral Variant FTD (bvFTD)

Behavioral variant FTD is the most common FTD subtype, representing approximately 50-60% of cases. The International bvFTD Criteria define six core diagnostic features. Disinhibition manifests as socially inappropriate behavior, loss of manners, and impulsive actions. Apathy and inertia present as loss of motivation, reduced initiative, and emotional blunting. Loss of empathy involves diminished emotional responsiveness and reduced social interest. Perseverative, compulsive, or ritualistic behavior includes simple repetitive movements, hoarding, and rigid routines. Hyperorality and dietary changes encompass food cravings (especially for sweets), binge eating, and oral exploration of inedible objects. Executive dysfunction with relative sparing of memory and visuospatial function is characteristic, especially early in the disease. Three or more of these six features are required for a diagnosis of "possible" bvFTD, and the presence of an MRI or PET abnormality elevates the diagnosis to "probable." BvFTD is frequently misdiagnosed as a psychiatric illness such as depression, bipolar disorder, OCD, or late-onset schizophrenia.

Primary Progressive Aphasia (PPA)

Semantic Variant PPA (svPPA)

Semantic variant PPA is characterized by progressive loss of word meaning and object knowledge. Speech remains fluent but becomes empty, with loss of single-word comprehension. Surface dyslexia produces regularization errors, such as reading "pint" as rhyming with "mined." Later stages bring prosopagnosia and associative agnosia. MRI shows asymmetric anterior temporal lobe atrophy, usually left-predominant. The underlying pathology is most commonly TDP-43 type C.

Nonfluent/Agrammatic Variant PPA (nfvPPA)

This variant presents with effortful, halting speech characterized by grammatical errors (agrammatism) and/or apraxia of speech. Comprehension of single words is preserved, but difficulty arises with syntactically complex sentences. MRI shows left posterior fronto-insular atrophy involving the inferior frontal gyrus, insula, and premotor cortex. The underlying pathology is most commonly tau, specifically 4-repeat tauopathy associated with corticobasal degeneration or progressive supranuclear palsy.

Logopenic Variant PPA (lvPPA)

Logopenic variant PPA is characterized by word-finding pauses and impaired sentence repetition, with preserved single-word comprehension and motor speech. MRI shows left temporoparietal junction atrophy. The underlying pathology is most commonly Alzheimer disease (discussed in Lecture 68).

Motor Neuron Disease Overlap (FTD-ALS)

Approximately 15% of FTD patients develop ALS, while 50% of ALS patients show some degree of cognitive or behavioral impairment. The C9orf72 repeat expansion is the most common shared genetic cause. Clinically, patients present with behavioral or language changes combined with upper and lower motor neuron signs. The prognosis is poor, with a median survival of 2-3 years from symptom onset. EMG and NCS should be performed in FTD patients who develop new weakness, fasciculations, or dysphagia.

FTD-Parkinsonism Overlap

Progressive Supranuclear Palsy (PSP)

PSP classically presents as Richardson syndrome, featuring vertical supranuclear gaze palsy (especially of downgaze), axial rigidity greater than appendicular rigidity, early falls (typically backward), and pseudobulbar affect. A PSP-parkinsonism phenotype may mimic Parkinson disease. The pathology is 4-repeat tau. MRI shows characteristic midbrain atrophy, producing the hummingbird sign on sagittal view and the morning glory sign on axial view.

Corticobasal Syndrome (CBS)

CBS presents with asymmetric rigidity, apraxia (limb or orobuccal), cortical sensory loss, alien limb phenomenon, myoclonus, and dystonia. The underlying pathology is heterogeneous and may include corticobasal degeneration, AD, PSP, or TDP-43. MRI shows asymmetric frontoparietal atrophy contralateral to the more affected side.

Genetics of FTD

The C9orf72 hexanucleotide repeat expansion is the most common genetic cause of both FTD and ALS. It is autosomal dominant with a highly variable phenotype spanning bvFTD, ALS, FTD-ALS, psychiatric presentations, and parkinsonism. MAPT (microtubule-associated protein tau) mutations cause tau-positive FTD with behavioral and parkinsonian presentations and are linked to chromosome 17. GRN (progranulin) mutations cause haploinsufficiency leading to TDP-43 pathology, often with asymmetric atrophy; plasma progranulin levels serve as a useful screening biomarker. Genetic counseling is essential for all familial cases and should be offered to apparently sporadic cases given the high rate of genetic forms.

Neuropathology

FTD pathology is classified by the predominant aggregated protein. FTLD-tau (40-50%) includes Pick bodies (3R tau), corticobasal degeneration (4R tau), PSP (4R tau), and MAPT mutations. FTLD-TDP (40-50%) involves TDP-43 inclusions classified into subtypes A through E, and is associated with GRN mutations and C9orf72 expansion. FTLD-FUS (5-10%) involves FUS protein inclusions and typically presents as young-onset bvFTD. The clinical syndrome does not reliably predict the underlying pathology, though svPPA is usually TDP-43 and nfvPPA is usually tau.

Diagnosis

The diagnostic evaluation begins with clinical assessment including a detailed behavioral history from an informant. Neuropsychological testing characteristically reveals executive and social cognition deficits with relative sparing of episodic memory early in the disease. MRI shows frontal and/or temporal atrophy, with the specific pattern depending on the clinical variant. FDG-PET demonstrates frontal and anterior temporal hypometabolism and is more sensitive than MRI early in the disease. Amyloid PET is negative in FTD, making it a useful tool for excluding AD. Genetic testing with a gene panel or whole exome sequencing should be pursued in familial cases. Plasma progranulin levels serve as screening for GRN mutations. CSF AD biomarkers (normal Abeta42 and p-tau ratios in FTD) help exclude Alzheimer disease.

Management

No FDA-approved disease-modifying therapy exists for FTD. Symptomatic treatment relies on SSRIs (sertraline, citalopram, fluvoxamine) for behavioral symptoms including disinhibition, compulsions, and dietary changes. Trazodone is useful for agitation and sleep disturbances. Cholinesterase inhibitors should be avoided, as they are not effective and may worsen behavioral symptoms. Antipsychotics should also be avoided when possible because of increased sensitivity in FTD. Speech-language therapy benefits patients with PPA variants. Caregiver support and education are essential, as FTD places an extraordinary burden on caregivers. Emerging therapies include antisense oligonucleotides for C9orf72 and GRN-targeted therapies currently in development.

Clinical Pearls

FTD is frequently misdiagnosed as a psychiatric disorder; early behavioral changes with preserved memory should raise suspicion for bvFTD, particularly in patients under 65. A negative amyloid PET in a young patient with progressive cognitive or behavioral decline strongly favors FTD over Alzheimer disease. C9orf72 expansion should be tested in all patients with FTD, ALS, or FTD-ALS, even without a clear family history, as reduced penetrance and phenotypic variability can mask the familial pattern. Cholinesterase inhibitors should not be used in FTD, and SSRIs are the first-line pharmacological approach for behavioral symptoms. The clinical syndrome does not reliably predict the underlying pathology, making genetic and biomarker data essential for accurate pathological prediction and future disease-modifying trial enrollment.

References

1. Rascovsky K, Hodges JR, Knopman D, et al. Sensitivity of revised diagnostic criteria for the behavioural variant of frontotemporal dementia. Brain. 2011;134(Pt 9):2456-2477.
2. Gorno-Tempini ML, Hillis AE, Weintraub S, et al. Classification of primary progressive aphasia and its variants. Neurology. 2011;76(11):1006-1014.
3. Greaves CV, Rohrer JD. An update on genetic frontotemporal dementia. J Neurol. 2019;266(8):2075-2086.
4. Tsai RM, Boxer AL. Diagnosis and management of frontotemporal dementia and its overlap with other neurological and psychiatric conditions. Continuum (Minneap Minn). 2019;25(1):187-210.

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