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What Causes Aphasia? Stroke, Injury and Other Causes in General Terms

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Aphasia is caused by damage to the parts of the brain that handle language. Most often, that damage comes from a stroke — when the blood supply to part of the brain is blocked, or a blood vessel bursts. Aphasia can also follow a head injury, a brain tumour, an infection affecting the brain, surgery, or a condition that changes the language areas of the brain slowly over time.

When aphasia arrives, one of the first questions families ask is “why has this happened?” It is a natural question, and understanding the cause — even in broad terms — can make a frightening time feel a little less bewildering. This guide explains each of the main causes in plain language. It cannot say what caused one particular person’s aphasia: that is a question for the medical team who knows them. If you are new to the subject, our guide to what aphasia is is a helpful place to start.

Key Takeaways

  • Aphasia happens when the language areas of the brain are damaged. It is the damage — not a lack of effort, and not a loss of intelligence — that makes words harder to reach.
  • Stroke is the most common cause: the blood supply to part of the brain is blocked, or a blood vessel bursts, injuring the language areas.
  • A head injury, a brain tumour, an infection affecting the brain, and surgery involving the brain can also cause aphasia.
  • Less commonly, language difficulty builds up gradually, because of a condition that changes the language areas of the brain over time — a pattern described in general terms as primary progressive aphasia.
  • Stress, tiredness and ordinary ageing do not cause aphasia, although tiredness can make an existing difficulty more noticeable.
  • The medical team looks for and treats the underlying cause, while a speech and language therapist assesses and supports communication.

On this page

The short answer: damage to the language areas of the brain

Language is handled by particular areas of the brain — for most people, these sit mainly on one side. Those areas, and the connections between them, do the work of finding words, building sentences, understanding what is said, and reading and writing.

Aphasia happens when those areas are damaged. The event that does the damage matters less than where the damage falls. An injury in a language area can affect language a great deal, even if the person seems otherwise physically well. Damage in a different part of the brain may leave language untouched, even if it affects other things, such as movement or vision.

This is also why aphasia looks different in every person. The pattern of difficulty depends on which parts of the language system have been affected, and how much. One person may struggle mainly to find words; another may find understanding harder. Our complete guide to aphasia explains how those patterns can look in everyday life.

Stroke: the most common cause

A stroke is the most common cause of aphasia. A stroke happens when the blood supply to part of the brain is interrupted, and brain cells in that area are injured because they are starved of the blood they need. This happens in one of two broad ways:

  • A blocked blood vessel. A clot blocks a vessel, so blood cannot reach the part of the brain beyond it.
  • A burst blood vessel. A vessel bursts and bleeds into or around the brain, damaging the tissue nearby.

If the affected part of the brain includes a language area, aphasia can follow — often alongside other effects, such as weakness on one side of the body, because neighbouring areas handle other jobs. Aphasia after a stroke usually appears suddenly, at the moment of the stroke itself.

A stroke is a medical emergency, and the medical team treats the stroke itself as the priority. The language difficulty is one part of the picture they build afterwards. General information about stroke is available from the Stroke Association and the World Stroke Organization.

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Head injury

A serious blow to the head — from a fall, a road accident, or a sporting injury, for example — can damage the brain, including the areas that handle language. The damage may be at the point of impact, or more widespread, because the brain can be shaken within the skull.

When language areas are affected in this way, aphasia can follow. After a head injury, language difficulty often sits alongside other changes, such as difficulties with memory, attention and organising thoughts, because an injury can affect several parts of the brain at once. The picture varies a great deal from person to person, depending on where and how severely the brain was injured.

Tumour, infection and surgery

A few other causes are less common, and worth knowing about in general terms:

  • Brain tumour. A growth in or near a language area can affect language as it presses on or grows into the surrounding tissue. Because a tumour usually develops over time, the language changes may build up gradually rather than appearing overnight.
  • Infection affecting the brain. Some infections can inflame or damage the brain. If the inflammation affects language areas, aphasia can be one result, alongside the person being generally very unwell.
  • Surgery involving the brain. Operations on the brain are sometimes needed — for example, to treat a tumour or bleeding. If surgery takes place in or near a language area, language can be affected. The medical team plans such surgery carefully and can explain in general terms what to expect.

In each of these situations, the medical team treats the underlying problem, and the language difficulty is assessed and supported alongside that treatment.

Gradual causes: when language changes slowly

Not all aphasia arrives suddenly. Sometimes there is no single event at all: instead, language becomes gradually harder over months or years — words slip away more often, sentences become simpler, or following conversation slowly gets more difficult.

One pattern described by professionals is primary progressive aphasia. In general terms, this is a condition in which the parts of the brain involved in language change slowly over time, so language difficulties build up bit by bit. In the early stages, language may be the main difficulty a person notices, while other abilities feel less affected.

Gradual language changes can have more than one explanation, and no one should try to work out the cause from a description like this one. What matters is that gradual changes are noticed and raised with a doctor promptly, so that a careful, specialist assessment can build the picture. Our guide to the types of aphasia describes this pattern alongside the others.

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What does not cause aphasia

Families sometimes worry that something they did — or something the person did — brought aphasia on. It can help to be clear about what does not cause it:

  • Not a loss of intelligence or effort. Aphasia is not caused by a person failing to try, concentrate or think hard enough. It is the result of physical damage to language areas of the brain.
  • Not ordinary ageing. Getting older does not, by itself, damage language. Healthy older adults keep their language; if an older person’s language changes suddenly or gradually, there is an underlying cause worth finding, and it deserves an assessment rather than being put down to age.
  • Not stress or tiredness. Stress and tiredness do not damage the language areas of the brain. They can, however, make an existing aphasia more noticeable: words that come fairly easily on a rested morning may be much harder to find when the person is exhausted, unwell or under pressure. That is the difficulty showing more, not new damage being done.
  • Not anything family or friends have done. Aphasia is not caused by conversation, by asking questions, or by the way people talk to the person. Talking together is a good thing, and support with communication helps everyday life.

Why the cause matters

Knowing the cause matters for two different reasons, and keeping them separate can help:

  • Medicine looks at the cause. The medical team works out what damaged the language areas, treats that underlying problem where treatment exists, and keeps it under review. This part belongs to the doctors and other medical professionals involved.
  • Therapy looks at the communication. A speech and language therapist assesses how language has been affected — speaking, understanding, reading and writing — and supports the person and their family with communication in everyday life, whatever the cause.

The cause also shapes the questions a family may ask. A sudden cause, such as a stroke, often raises questions about recovery and preventing further harm, which belong to the medical team. A gradual cause raises questions about planning ahead and reviewing support over time. In every case, the cause does not change the person at the centre of it: they are the same person, with the same feelings, humour and worth, and support is built around them.

How the cause is found

Families are sometimes surprised by how much detective work is involved. In general terms, the medical team builds the picture from several pieces:

  • The story of what happened. When the difficulty started, whether it appeared suddenly or gradually, and what else has been noticed. Observations from family — “she was fine at breakfast and different by lunch” — can be genuinely valuable clues.
  • A medical examination. The doctor examines the person and considers their wider health, past illnesses and the other effects that appeared alongside the language difficulty.
  • Pictures of the brain. The team may arrange scans that produce pictures of the brain, which can show which areas have been affected and sometimes point towards the cause.
  • Other checks, where needed. Depending on the situation, further tests or referrals to specialists may follow.

Alongside this, a speech and language therapist assesses the communication difficulty itself. The two pictures — the medical cause and the communication profile — fit together, and each informs the other. If the explanation you are given feels unclear, ask for it in plain words, and ask for it in writing: at a stressful time, nobody remembers everything they are told.

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When to seek help

Sudden changes are an emergency. If someone’s speech suddenly becomes difficult, slurred or hard to understand, or they suddenly cannot find words or follow what is said — especially alongside a drooping face, weakness in an arm or leg, or difficulty seeing — call your local emergency number immediately. Sudden speech changes can be a sign of a stroke, and urgent medical help matters more than anything else. This applies even if the person already has aphasia and the difficulty suddenly gets much worse.

For speech and language support more generally:

  • If language seems to be changing gradually — words slowly becoming harder to find, sentences simplifying, conversation getting harder to follow over weeks or months — book a prompt appointment with the doctor and describe what you have noticed. Gradual changes deserve a careful assessment too.
  • After a stroke or brain injury, the hospital team may arrange speech and language therapy before the person goes home. If not, ask whether a referral can be made.
  • You can also ask your doctor for a referral at any point, or contact a speech and language therapist directly.
  • General information about aphasia is available from the National Institute on Deafness and Other Communication Disorders, and our page on support for adults explains the help described on this site.

Questions to ask a therapist

If you are meeting a speech and language therapist — or a member of the medical team — about your own aphasia, or a relative’s, these questions can help you understand the cause and what it means:

  1. What do we know so far about what caused the aphasia, and can you explain it in plain language?
  2. Which parts of communication did the assessment show are affected, and which are strongest?
  3. Does the cause change anything about the support you would suggest, or about what we should watch for?
  4. Who else is involved in looking at the cause, and how will we hear what they find?
  5. How will the picture be reviewed over time, especially if things change?

It can also help to ask for a short written summary of the cause, as the team currently understands it, and of the assessment findings. Having it in writing makes it easier to remember, and to share with family and other professionals involved in care.

Frequently asked questions

Does every stroke cause aphasia?

No. A stroke causes aphasia only when the part of the brain it affects includes a language area. A stroke in a different part of the brain may affect movement, vision or other abilities while leaving language untouched. This is also why two people who have both “had a stroke” can have very different difficulties afterwards.

Can a head injury cause aphasia?

Yes. A serious blow to the head can damage the language areas of the brain, just as a stroke can, and aphasia may follow. After a head injury, language difficulty often appears alongside changes in memory, attention and thinking, because the injury may affect several parts of the brain at once. An assessment looks at the whole picture.

Can stress or tiredness cause aphasia?

No. Stress and tiredness do not damage the brain, so they cannot cause aphasia. They can make an existing aphasia more noticeable — most people find their words are harder to reach when they are tired, unwell or under pressure — but that is the difficulty showing more, not a new cause. If language difficulty appears for the first time, it should be assessed rather than put down to stress.

Can ageing alone cause aphasia?

No. Ordinary ageing does not cause aphasia; language stays with us into later life. If an older person’s language changes — suddenly or gradually — something specific is affecting the language areas of the brain, and that deserves a proper assessment so the cause can be looked for.

What does it mean if aphasia came on gradually?

Gradual language change, building up over months or years without a single sudden event, is a different pattern from most aphasia. One general description for it is primary progressive aphasia, in which the language areas of the brain change slowly over time — but there can be other explanations too. Gradual changes should be raised with a doctor promptly, so a specialist assessment can work out what is happening.

Will treating the cause make the aphasia go away?

Treating the cause and supporting communication are two separate things. Medical treatment addresses the underlying problem — for example, treating an infection or reviewing a stroke — and is the medical team’s role. How much language changes afterwards varies a great deal from person to person, and no outcome can be promised. Speech and language therapy supports communication itself: understanding the difficulty, building on strengths, and finding ways for the person to take part in conversation and daily life.

A gentle next step

Asking “what caused this?” is often the first step a family takes towards understanding aphasia — and understanding tends to make the road ahead feel a little less frightening. If you would like to talk through what you have noticed and what support might help, you are very welcome to get in touch online. There is no obligation and no pressure: just a friendly first conversation, at your pace.

By Hamayal Adnan, Speech-Language Pathologist

This article is for general educational purposes only and is not medical advice. It does not replace an assessment by a qualified speech and language therapist or other healthcare professional. Every person is different. If you have concerns about speech, language, feeding or swallowing, please seek professional advice. In an emergency, call your local emergency number.

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