Aphasia is a communication disorder that affects someone's ability to speak or understand speech. It also impacts how they understand written words and their ability to read and to write.

It is important to note that aphasia can take different forms. Some people with aphasia only have difficulty understanding language, a result of damage to the temporal lobe, which governs how sound and language are processed in the brain. Others have only difficulty with speaking, indicating damage to the frontal lobe. A loss of comprehension of language would suggest damage to both the large temporal lobe and frontal lobe.

In most cases, aphasia results from a stroke or haemorrhage in the brain. It can also be caused by damage to the brain from impact injury such as a car accident. Brain tumours can also result in aphasia.

There is also a separate form of the condition called primary progressive aphasia. This starts off with mild symptoms but gets worse over time. The medical community doesn't know what causes primary progressive aphasia. We know that it affects the same brain regions as in cases where aphasia results from a stroke or haemorrhage, but the onset of symptoms follows a different trajectory.

Aphasia is unfortunately common. Approximately one-third of all stroke survivors have it. In the US, around 2 million people have aphasia and around 225,000 Americans are diagnosed every year. Right now, we don't know what proportion of people with aphasia have the primary progressive form of the condition.