Introduction

Early onset dementias

Dementia is an umbrella term for a group of progressive neurodegenerative conditions involving brain atrophy, which encapsulates synapse and neuronal loss, gradually worsening through the years and eventually leading to a fatal outcome.¹ Dementia usually affects the elderly population (70 years or older); however, 15% of dementia cases present before 65 years of age. These cases are classified as early onset dementias, and the two syndromes mainly associated with these cases are Alzheimer's disease (AD) and frontotemporal dementia (FTD).^1,2

Frontotemporal dementia

Frontotemporal Dementia (FTD) is a condition characterised by the selective loss of neurons, specifically in the frontal and temporal lobes of the brain in the initial stages of the disease. It is a relatively rare form of dementia; however, it constitutes a significant proportion of early-onset dementia cases, as the majority of the patients affected by FTD are between 45 and 64 years old.^2,3

Types of FTD 

The degeneration of neurons in the frontotemporal lobe leads to deficits in behaviour, language, and executive functions. FTD encapsulates multiple clinical manifestations, and they are differentiated into the following types:

• Behavioural variant frontotemporal dementia (bvFTD)
• FTD associated with motor neuron disease
• Right temporal lobe variant
• Semantic and nonfluent primary progressive aphasias³ 

Understanding the semantic variant of FTD

This article focuses on the semantic subtype of frontotemporal dementia (sFTD or SD), also called semantic-variant primary progressive aphasia. 

Semantic dementia (SD) is a neurodegenerative disorder characterised by a progressive and relatively circumscribed (well-defined and limited) loss of semantic (factual) knowledge. SD typically presents with impaired word comprehension and naming, alongside preserved fluency and repetition. Patients show a profound loss of conceptual knowledge across multiple modalities, while their episodic memory remains relatively intact.^3,4 

Neuroimaging reveals asymmetrical atrophy of the anterior temporal pole and anterior fusiform gyrus, usually in the left hemisphere. As the disease progresses, patients may exhibit behavioural changes, including mental rigidity, obsessive tendencies, and alterations in social cognition. SD is most commonly associated with TDP43 type C pathology and usually occurs sporadically.⁴ 

The role of semantic memory

Memory can be broadly categorised into semantic and episodic memory. Semantic memory refers to general knowledge and facts about the world, while episodic memory involves the recollection of personal experiences and events. These distinct memory systems were initially identified through lesion studies of patients with brain damage, which revealed that different types of memory could be selectively impaired. Neuroimaging studies have further supported this distinction by showing that semantic and episodic memory processes activate different brain regions.⁵

Semantic memory encompasses our knowledge of concepts, facts, and the meanings of words. It allows us to understand and interact with the world around us. Therefore, patients affected by SD have greater difficulty carrying out everyday tasks and thinking ahead. However, patients often remember past events, especially the ones that happened more recently.^4,5 

Neuroimaging studies have consistently implicated the anterior temporal lobes in semantic processing. Patients with semantic dementia typically show atrophy in these regions. Functional neuroimaging has revealed that semantic tasks activate a distributed network of brain areas, including the left inferior frontal gyrus, middle temporal gyrus, and fusiform gyrus, in addition to the anterior temporal lobes. These findings suggest that semantic memory relies on a complex interplay of multiple brain regions working together to store and retrieve conceptual knowledge. In SD, these signalling pathways are affected by the loss of synapses, preventing patients from retrieving semantic information.^4,5

How is semantic memory affected in semantic dementia?

In SD, the deterioration of semantic memory is typically gradual and affects all modalities of input and output. Patients with SD show a pattern of impairment that begins with more specific and less familiar concepts, gradually progressing to affect more general, and common knowledge. For instance, a patient might initially struggle to name specific types of birds, then lose the ability to recognise birds as a category, and eventually fail to distinguish animals from other objects. This loss of semantic knowledge is reflected in various cognitive tasks, including naming, word comprehension, object recognition, and even non-verbal semantic matching tasks.^3,4,5

The deficit in SD is not limited to verbal information, but extends to all domains of conceptual knowledge, highlighting the fundamental role of the affected brain regions, particularly the anterior temporal lobes, in storing and processing semantic information across multiple modalities. As the disease progresses, patients may lose the ability to recognise even common objects or understand basic words, severely impacting their ability to interact with their environment and communicate effectively.⁴

Diagnosis and management of sFTD

Semantic frontotemporal dementia presents unique challenges in both diagnosis and management. As a progressive neurodegenerative disorder primarily affecting language and behaviour, early and accurate diagnosis is crucial for appropriate care and support.^2,4,6

Diagnostic approaches

The diagnosis of sFTD involves a multifaceted approach. It typically begins with a comprehensive clinical assessment, including a detailed medical history and evaluation of symptoms, particularly language difficulties and behavioural changes.^2,6

Neuropsychological testing is crucial, focusing on tests of confrontation naming, single-word comprehension, category fluency, and word-picture matching to identify the characteristic semantic knowledge loss.^3,4,5,6 

Neuroimaging plays a vital role, with structural MRI often revealing asymmetric anterior temporal lobe atrophy, typically left-lateralised. Functional neuroimaging, such as PET or SPECT, can identify hypometabolism (low metabolic activity) or hypoperfusion (poor blood flow) in anterior temporal regions. While genetic testing is rarely informative in sporadic cases, it may be considered in cases with a strong family history. ^2,4,6

Emerging research into biomarkers, particularly neurofilament light chain levels in cerebrospinal fluid, shows promise for future diagnostic applications. Most importantly, differential diagnosis is crucial to rule out other conditions that might mimic sFTD, such as Alzheimer's disease or mood disorders.^2,4,6

Treatment options

Currently, there is no cure for sFTD, and treatment focuses on symptom management and quality of life improvement. A multidisciplinary approach is typically employed. 

Non-pharmacological interventions form the cornerstone of management, with speech and language therapy playing a crucial role in maintaining existing language skills and developing alternative communication strategies. Behavioural management techniques, including environmental modifications and the establishment of routines, can help address compulsive behaviours and reduce confusion. Occupational therapy aids in maintaining independence in daily activities.^2,4,6

While no medications specifically treat sFTD, some pharmacological interventions may help manage behavioural symptoms. Selective serotonin reuptake inhibitors (SSRIs) may be used for social disinhibition or compulsive behaviours, while low doses of antipsychotics might be cautiously considered for severe behavioural disturbances.² 

Supportive care, including regular medical follow-ups and psychosocial support for patients and caregivers, is essential. As research progresses, clinical trials offer hope for potential disease-modifying treatments, and patients may have opportunities to participate in these studies.^2,4

Summary

In conclusion, semantic frontotemporal dementia (sFTD) is a complex neurodegenerative disorder primarily affecting semantic memory and language skills. It is characterised by progressive loss of conceptual knowledge, typically beginning with specific concepts and gradually affecting more general knowledge. Diagnosis involves a multifaceted approach, including clinical assessment, neuropsychological testing, and neuroimaging, with promising biomarker research ongoing. While there is currently no cure, management strategies focus on symptom control and quality of life improvement through a multidisciplinary approach. 

Non-pharmacological interventions, particularly speech and language therapy, play a crucial role in managing symptoms, supplemented by behavioural management techniques and, in some cases, medication for symptom relief. As research progresses, there is hope for more effective treatments in the future. Early diagnosis, comprehensive care, and support for both patients and caregivers remain essential in addressing the challenges posed by this disorder. Understanding sFTD not only aids in its management but also contributes to our broader knowledge of brain function and semantic memory processes.