Aphasia is caused by damage to the parts of the brain that control language. Stroke is the most common cause, responsible for roughly one-third of the 795,000 strokes that occur in the United States each year. But strokes are far from the only trigger. Brain tumors, head injuries, infections, and neurodegenerative diseases can all disrupt the brain’s language network, leading to problems with speaking, understanding, reading, or writing.
Why the Left Side of the Brain Matters
Language processing is concentrated in one hemisphere of the brain, and for most people that’s the left side. Among strong right-handers, only about 4% have language centered in the right hemisphere. That number rises with left-handedness, reaching roughly 27% in strongly left-handed people. This means any condition that damages the left hemisphere, particularly the regions around the temples and the front of the brain, carries a high risk of disrupting language.
The brain doesn’t handle language in just one or two spots, though. A network of nerve fiber bundles connects different language regions. One major bundle, the arcuate fasciculus, links areas involved in understanding speech with those that plan speech movements. Damage to this pathway impairs the ability to repeat words and produce complex sentences. Other bundles running along the underside of the brain handle comprehension. Because language depends on this entire network, the specific type of aphasia a person develops depends on where the damage occurs and which connections are severed.
Stroke: The Leading Cause
Stroke causes more cases of aphasia than anything else. When a blood vessel supplying the left hemisphere is blocked or bursts, brain tissue in the language network loses oxygen and dies within minutes. The location of the stroke determines what kind of language problems follow. A stroke affecting the left frontal lobe, near the region known as Broca’s area, typically makes speech effortful and halting while leaving comprehension relatively intact. A stroke in the left temporal lobe tends to do the opposite, preserving the ability to produce fluent-sounding speech while severely impairing the ability to understand words or retrieve the right ones.
Most people with stroke-related aphasia experience some degree of spontaneous improvement in the weeks and months afterward. Research tracking stroke patients who were still aphasic at four weeks found continued language gains through at least 34 weeks. A person’s language ability at six months could be reasonably predicted from testing done at four weeks. This early recovery window is driven by the brain reducing swelling and rerouting some functions to undamaged tissue, but it doesn’t mean improvement stops there. Speech therapy can produce meaningful gains well beyond that initial period.
Brain Tumors
Both benign and malignant brain tumors can cause aphasia when they grow in or press against language areas. Tumors in the left temporal lobe are the most common culprits, followed by those in the left parietal and temporoparietal regions. The type of tumor matters, too. Slow-growing tumors often allow the brain time to compensate, so language problems may be subtle or absent for years. Fast-growing, high-grade tumors in the left hemisphere tend to cause more severe and sudden language deficits.
The pattern of aphasia maps closely to tumor location. A tumor in the left temporal lobe commonly leads to difficulty finding the right words during conversation. Tumors in the left frontal lobe, near Broca’s area, can impair speech production and sometimes cause temporary mutism. Because tumors can affect language networks through direct tissue destruction, compression of surrounding brain tissue, or swelling, some language problems may partially reverse if the tumor is successfully treated and swelling decreases.
Traumatic Brain Injury
A severe blow to the head can cause aphasia in two ways. A penetrating injury or a focused impact to the left side of the skull can destroy specific language areas, producing symptoms similar to stroke-related aphasia. More diffuse injuries, like those from car accidents or blast waves, damage the long nerve fiber bundles that connect language regions across the brain. This kind of widespread damage can impair both production and comprehension simultaneously, creating a more complex pattern of language difficulty that doesn’t fit neatly into a single aphasia category.
Recovery from traumatic brain injury tends to be less predictable than recovery from stroke. Younger patients and those with less extensive damage generally fare better, but the combination of language problems with other cognitive issues like memory loss and attention deficits can make rehabilitation more challenging.
Brain Infections
Certain infections that reach the brain can damage language areas quickly and severely. Herpes simplex encephalitis is the most well-known example. The herpes simplex virus has a particular affinity for the temporal lobes and surrounding structures, which is exactly where much of the brain’s language and memory processing takes place. About 40% of people with herpes simplex encephalitis develop aphasia or altered speech.
The illness typically begins with a prodromal phase of fever, malaise, headache, and nausea before progressing to more severe neurological symptoms, often within less than a week. Seizures, confusion, and focal deficits like language impairment follow. Even with treatment, long-term consequences frequently include lasting aphasia alongside memory problems and other cognitive changes. Brain abscesses and other forms of encephalitis can produce similar damage depending on their location.
Neurodegenerative Diseases
Unlike stroke or injury, neurodegenerative diseases cause aphasia that begins gradually and worsens over time. The condition is called primary progressive aphasia, and it results from the slow destruction of the brain’s language network by abnormal protein buildup. The specific protein involved determines which variant a person develops.
There are three main variants. The nonfluent/agrammatic type is most often caused by the accumulation of tau protein. People with this form struggle to produce grammatically correct sentences and speak with increasing effort. The semantic variant is linked to a different protein called TDP-43. It erodes the meaning of words, so a person might speak fluently but gradually lose the ability to understand what common words mean or recognize familiar objects. The logopenic variant is most commonly associated with Alzheimer’s disease pathology. It primarily affects word retrieval and the ability to repeat phrases, and it often progresses into broader Alzheimer’s symptoms over time.
Primary progressive aphasia differs from aphasia caused by stroke in a critical way: there is no single event that triggers it. Language difficulties emerge slowly, sometimes over years, before other cognitive problems appear. A person might first notice persistent trouble finding words in conversation or following complex sentences, and these difficulties gradually expand rather than improving with time.
Less Common Causes
A transient ischemic attack, sometimes called a mini-stroke, can cause temporary aphasia lasting minutes to hours. Because blood flow is only briefly interrupted, language typically returns to normal, but the episode is a warning sign that a full stroke may follow. Seizures affecting the left hemisphere can also produce brief periods of aphasia during or immediately after the seizure, a phenomenon called postictal aphasia.
Autoimmune conditions that cause brain inflammation, surgical complications involving the left hemisphere, and radiation therapy to the head are additional causes. In rare cases, migraine aura can temporarily disrupt language processing, though this resolves completely and doesn’t reflect permanent brain damage.
How Aphasia Is Identified
Clinicians use standardized tests to determine the type and severity of aphasia. One of the most widely used is the Western Aphasia Battery, which evaluates fluency, comprehension, repetition, and naming ability. The results produce an overall severity score and a classification that helps guide treatment. Other tools focus on specific aspects of language, such as the ability to construct sentences or understand individual words, which can be especially useful for identifying the subtler deficits seen in progressive forms of aphasia.
Because the cause of aphasia directly shapes both treatment and outlook, identifying the underlying condition is just as important as measuring the language deficit itself. Brain imaging, blood tests, and neurological exams are used alongside language testing to determine whether the cause is vascular, degenerative, infectious, or structural.