Aetiology

Etiologies are typically indicated by whether onset is sudden (e.g., stroke or herpes encephalitis) or progressive (e.g., primary progressive aphasia or some tumours).

Vascular

All vascular causes of stroke are characterised by sudden onset of impairment. This is subsequent to reduced blood flow (ischaemia) due to thrombosis or embolus, or rupture of a vessel leading to bleeding into the brain or subarachnoid space.

  • Ischaemic stroke: ensues when a focal area of the brain receives inadequate oxygen and glucose for function, and eventually too little to survive. Urgent treatment with thrombolysis can restore language and prevent brain damage if given to selected patients within 4 and a half hours after stroke, or within 3 hours in selected patients with diabetes who have had a previous stroke.[18][19][20]​ Mechanical thrombectomy can be beneficial if given within 6 hours of symptom onset in selected patients.[18][19][20]

  • Intracerebral haemorrhage: causes symptoms similar to that of ischaemic stroke.

  • Subdural haematoma: a distinctly unusual, but possible, aetiology of aphasia.

These 3 causes need to be distinguished rapidly, as they require different evaluation and treatment. Brain imaging should be completed within 1 hour of presentation, and earlier if possible.[21]​​[22]

Subarachnoid haemorrhage rarely causes immediate aphasia. It can cause delayed focal ischaemia due to vasospasm that can lead to aphasia caused by infarct or poor perfusion.

Migraine may present with focal neurological deficits, but rarely aphasia.

Infectious

Infections can cause aphasia if they occur in the language cortex (left posterior frontal, ventral parietal, or temporal cortex).

  • Herpes encephalitis: has a predilection for the mesial and inferior temporal cortex, and frequently causes a striking aphasia with severe impairments in word meaning.[23] The language profile may be very similar to Wernicke's aphasia, but herpes encephalitis is associated with fever, malaise, and other systemic symptoms.

  • West Nile virus: can cause encephalitis, and a primary locus of encephalitis in the left hemisphere can cause aphasia.

  • Bacterial infection or abscess: may cause aphasia if it occurs in the language cortex (left posterior frontal, ventral parietal, or temporal cortex).

  • Fungal abscess: follows a course similar to that of bacterial infection.

  • Lyme disease: may cause encephalitis and aphasia, but it typically causes a more diffuse cognitive impairment.[24]

Neurodegenerative

There are 3 variants of primary progressive aphasia (PPA): non-fluent/agrammatic-variant PPA, semantic-variant PPA (formerly known as semantic dementia), and logopenic-variant PPA.[25]​ All are considered primary progressive aphasias because language is impaired for at least 2 years before the onset of other cognitive or behavioural deficits.[26][27]

  • Alzheimer's disease: causes an isolated aphasia in rare forms where the pathology affects predominantly the left superior temporal and inferior parietal cortex. The usual form is logopenic progressive aphasia.

  • Non-fluent-variant PPA: characterised by non-fluent, agrammatical speech and impaired articulation.[28] As in Broca's aphasia:

    • the patient often has difficulty understanding syntactically complex sentences, although words and simple sentences are understood.

    • the area most affected is the posterior inferior frontal cortex and insula, and the most common pathology is a tauopathy (neurodegeneration resulting from tau protein neurofibrillary aggregates in glial cells as well as neurons) such as frontotemporal lobar degeneration, corticobasal degeneration, or progressive supranuclear palsy.

  • Semantic-variant PPA: similar to Wernicke's aphasia, with fluent, meaningless speech and prominent comprehension deficits, but with impaired recognition of objects (associative agnosia).[29][30]​ It generally affects the inferior and anterior temporal cortex, and the pathology is most often a ubiquitinopathy (one of a class of intracellular regulatory protein inclusions or accumulations) such as transactivator regulator DNA-binding protein (TDP-43).

  • Logopenic-variant PPA: characterised by phonological errors in speech and poor repetition, similar to conduction aphasia and typically caused by Alzheimer's disease.[28][31]

  • Aphasia dysarthria motor neuron disease: a ubiquitinopathy, also known as amyotrophic lateral sclerosis frontotemporal degeneration. Can cause any of the aphasias associated with frontotemporal lobar degeneration. However, the most common form is progressive non-fluent aphasia.

  • Prion disease (e.g., Creutzfeldt-Jakob disease): aphasia is an atypical presenting feature. Isolated aphasia may be of short duration (e.g., weeks) or prolonged, prior to the onset of other symptoms.[32]​​[33]​​[34]

Neoplastic

  • Primary brain tumours and brain metastases cause aphasia when both rapidly growing and occurring in the language cortex.[35]​

  • Slow-growing tumours (e.g., low-grade glioma and meningioma): rarely cause aphasia except as a result of focal seizure.

Autoimmune

Several entities can cause aphasia when lesions selectively undercut the language cortex, but an isolated aphasia syndrome is very rare because lesions are usually more widely distributed.

  • Multiple sclerosis

  • Sarcoidosis

  • Acute disseminated encephalomyelitis: can cause aphasia when localised to the left hemisphere. However, it typically causes a more diffuse cognitive impairment.

Other pathologies

A number of progressive and nonprogressive neurological pathologies can produce aphasia.

  • Seizure: can be associated with temporary aphasia in certain developmental epilepsy syndromes but is not a frequent cause of aphasia. The aphasia resolves gradually in the postictal period. Complex partial seizures with onset in the left temporal lobe (e.g., due to stroke or tumour) can cause ictal aphasia.

  • Conversion disorder: may manifest as sudden onset of bizarre speech or mutism in an individual with psychiatric or environmental vulnerabilities (e.g., recent major life event).

  • Head injury: usually easily distinguished from vascular causes by history or external signs of injury, such as bruises, bleeding, or skull fractures. Traumatic cases are often, but not always, accompanied by other cognitive deficits, including impairment of attention, memory, reasoning, and judgment.

  • Wernicke's encephalopathy (thiamine deficiency): causes relatively acute-onset confabulation that can sound much like Wernicke's aphasia.

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