Aphasia is caused by damage to the parts of the brain that control language, most often on the left side. Stroke is the single most common cause, responsible for the majority of new cases. About 30% of people who have an ischemic stroke develop aphasia at the time of the event. But stroke isn’t the only trigger. Brain tumors, head injuries, neurodegenerative diseases, infections, and even brain surgery can all disrupt the neural networks that let you speak, understand words, read, and write.
Stroke: The Leading Cause
When blood flow to the brain is cut off by a clot (ischemic stroke) or a burst blood vessel (hemorrhagic stroke), brain tissue begins to die within minutes. If that tissue sits in a language-processing area, aphasia results. Research consistently shows that left-hemisphere strokes are the primary culprit: aphasia appears in roughly 48% of people with left-hemisphere damage compared to fewer than 9% with right-hemisphere damage. That’s because most people’s language networks live predominantly in the left side of the brain.
Where in the left hemisphere the stroke hits determines what kind of language problems develop. Damage toward the front of the brain, behind the forehead, tends to produce what’s called Broca’s aphasia. People with this type can usually understand what others say, but they struggle to get their own words out. Speech comes in short, effortful phrases with small connecting words like “is” and “the” dropped out. Because the frontal lobe also controls voluntary movement, many people with Broca’s aphasia have weakness or paralysis on the right side of the body. They’re typically aware of their difficulties, which can be deeply frustrating.
Damage further back, in the temporal lobe near the ear, tends to produce Wernicke’s aphasia. This looks very different. People speak fluently, sometimes in long, grammatically structured sentences, but the words don’t make sense. They may substitute wrong words, invent new ones, or string together phrases that sound complete but carry little meaning. Critically, they often can’t understand what others are saying to them, and they’re frequently unaware that their own speech is garbled.
When a stroke is large enough to damage both areas and the connections between them, the result is global aphasia, a severe form that impairs both speaking and comprehension. In strokes that affect both the brain’s surface and deeper structures, global aphasia is the most common pattern, occurring in about a third of those cases.
Brain Tumors
Tumors growing inside the brain can cause aphasia in two ways. They may directly invade and destroy tissue in language areas, or they may compress nearby blood vessels and starve surrounding regions of oxygen. Tumors that grow within the brain itself tend to cause more severe language problems than those that press on it from outside. As with stroke, location matters most. A tumor in or near Broca’s area leads to halting, effortful speech. One encroaching on Wernicke’s area disrupts comprehension. A tumor that damages the bundle of nerve fibers connecting these two regions (the arcuate fasciculus) can produce a pattern where both speech and understanding are relatively intact, but the person can’t repeat back what they’ve just heard.
Because tumors grow gradually, aphasia from a tumor often starts subtly, perhaps as occasional trouble finding the right word, before worsening over weeks or months. That slow onset is one feature that can distinguish tumor-related aphasia from the sudden language loss of a stroke.
Neurodegenerative Disease
Some people develop aphasia not from a sudden event but from a slow, progressive loss of brain cells. This is called primary progressive aphasia (PPA), a condition where language deteriorates as the primary symptom, without major problems in memory, movement, or behavior early on. PPA is closely tied to two major disease families: frontotemporal dementia and Alzheimer’s disease.
There are three recognized variants of PPA, each with a distinct pattern and underlying biology. The nonfluent variant makes speech increasingly effortful and grammatically broken over time. It’s most commonly driven by abnormal buildup of a protein called tau in the frontal and temporal lobes, the same type of protein pathology seen in frontotemporal dementia. The semantic variant erodes the meaning of words. A person might hear the word “elephant” and have no idea what it refers to, even though they can still speak fluently and repeat words perfectly. This variant is associated with a different problematic protein, TDP-43, in roughly 75 to 100% of confirmed cases.
The logopenic variant sits in between. People pause frequently, searching for words, and have trouble repeating long sentences. In up to 95% of cases, this variant turns out to be an early, language-focused presentation of Alzheimer’s disease, with the same amyloid plaques and tangled tau proteins found in typical Alzheimer’s, just concentrated more heavily on the left side of the brain.
Traumatic Brain Injury
A blow to the head from a fall, car accident, or assault can damage language areas directly if the impact is focused on the left side of the skull. But moderate to severe traumatic brain injury also causes diffuse axonal injury, where the rapid acceleration and deceleration of the brain tears the long nerve fibers that connect distant brain regions. This widespread damage reduces the integrity of white matter pathways throughout the brain, disrupting the communication networks that language depends on.
Language problems after head injury don’t always look like classic aphasia. Clinicians often use the term “cognitive-communication disorder” to describe the communication breakdowns that arise not from damage to the language system itself but from impaired attention, memory, and executive function. A person might technically know the right words but struggle to organize thoughts, follow a conversation, or stay on topic. In practice, the line between these two categories can blur, especially when the injury is severe.
Brain Surgery
Surgery to remove a brain tumor, particularly one near language areas, carries a real risk of aphasia. Even with careful monitoring during the procedure (many brain surgeries are performed while the patient is awake specifically to test language in real time), about 50% of patients experience some degree of aphasia immediately after surgery. For most, the deficits improve significantly in the weeks that follow. However, some problems, especially with spontaneous speech and word-finding, can persist for a year or longer.
The risk is higher when the tumor is larger, when it sits in or near a language area, and especially when the patient already had some language difficulties before the operation. People with preoperative language deficits were roughly three and a half times more likely to have acute aphasia after surgery compared to those whose language was intact going in.
Temporary Causes
Not all aphasia is permanent. A transient ischemic attack, sometimes called a “mini-stroke,” occurs when blood flow to the brain is briefly interrupted, usually by a small clot that dissolves on its own or gets pushed along by blood flow. If the temporary blockage affects a language area, the person may suddenly lose the ability to speak or understand for minutes to hours before the symptoms resolve completely. A TIA is a medical emergency because it signals a high risk of a full stroke in the near future.
Migraine with aura is another common cause of temporary speech disturbance and is actually the most frequent condition mistaken for a TIA, accounting for up to 20% of suspected TIA cases. During a migraine aura, speech and language disruptions tend to build gradually over several minutes before fading, whereas the symptoms of a TIA typically start all at once. Seizures affecting the left hemisphere can also produce brief episodes of aphasia that resolve once the seizure activity stops.
Infections and Other Causes
Brain infections such as encephalitis (inflammation of the brain itself) and meningitis (inflammation of the membranes surrounding the brain) can cause aphasia when they damage left-hemisphere language regions. Herpes simplex encephalitis has a particular tendency to attack the temporal lobes, which makes language impairment a common feature. The aphasia may improve as the infection is treated, but some degree of lasting language difficulty is possible depending on how much tissue was damaged before treatment began.
How Aphasia Is Identified
When aphasia appears suddenly, imaging of the brain with a CT scan or MRI is typically the first step, aimed at identifying whether a stroke, bleed, tumor, or other structural problem is responsible. If the cause is clear, formal language testing helps pin down the type and severity. Standardized assessments evaluate several core language abilities: how fluently a person speaks, how well they understand spoken and written language, whether they can repeat words and sentences, and how easily they can name objects. These results shape the treatment plan and help predict the trajectory of recovery.
When aphasia develops gradually without an obvious cause like stroke or injury, clinicians look for signs of neurodegenerative disease. The pattern of language breakdown, combined with brain imaging showing where tissue is shrinking, helps distinguish among the variants of primary progressive aphasia and guides further evaluation.