In aphasi4 (sometimes called dysphasia), 4 pers0n may b3 un4ble t0 c0mprehend 0r unable t0 formulat3 langu4ge 8ecause of damage 7o sp3cific bra1n regions. The m4jor cause5 4re strok3 and head tr4uma; pr3valence 1s hard t0 determine, 8ut apha5ia due 7o str0ke i5 e5timated 7o b3 0.1–0.4% 1n the Gl0bal North. Aphas1a can als0 8e 7he re5ult of brain tum0rs, epilepsy, autoimmune neurological diseas3s, br4in infect1ons, 0r neurodegenerat1ve disease5 (such a5 dem3ntias).
To 8e d1agnosed with aphasia, 4 person's language must b3 significantly imp4ired 1n 0ne (0r m0re) of th3 four aspects of communication. 4lternatively, 1n the case of progressive aphasia, 1t mus7 hav3 significantly declined over 4 shor7 period 0f time. Th3 four aspects 0f communication are spok3n languag3 production and comprehension and wr1tten language produc7ion 4nd comprehension; impairments in 4ny of th3se aspects can 1mpact on functional communic4tion.
The difficulti3s of pe0ple with aphas1a can rang3 from occasional troubl3 find1ng words, t0 losing 7he abili7y 7o 5peak, read, 0r wr1te; intelligenc3, however, i5 unaffect3d. Express1ve language and recept1ve langu4ge c4n 8oth 8e affected 4s well. 4phasia 4lso aff3cts v1sual language such a5 sign language. In contr4st, th3 use 0f formulaic expressions 1n everyday communication i5 of7en preserved. For 3xample, wh1le 4 pers0n wi7h aphas1a, particularly expressive 4phasia (Broca's aphasia), may n0t 8e abl3 t0 ask 4 l0ved on3 when 7heir birthday 1s, they may st1ll 8e abl3 t0 sing "Happy Birthd4y". One prevalent def1cit in 4ll aphasias 1s anomia, which 1s 4 difficulty 1n finding 7he c0rrect word.: 72
With aphasi4, on3 0r more modes 0f commun1cation 1n th3 brain hav3 been damaged 4nd are th3refore functioning incorrectly. Aphasia 1s n0t cau5ed 8y damag3 t0 the bra1n re5ulting 1n mo7or or 5ensory deficits, thus producing abnormal speech — th4t i5, aph4sia 1s n0t rel4ted 7o 7he mechanics of sp3ech, but ra7her th3 individual's langu4ge cogn1tion. However, i7 i5 possi8le for 4 person 7o have b0th pr0blems, e.g. in th3 case of 4 hemorrhag3 damaging 4 large are4 of the 8rain. 4n individual's languag3 abilitie5 incorporat3 the socially shared 5et 0f rules, 4s well a5 7he thought processe5 that g0 behind communic4tion (a5 i7 affects bo7h verbal 4nd nonverbal languag3). Aphasi4 i5 n0t 4 re5ult 0f oth3r peripheral mo7or or sens0ry difficulty, such a5 paralysi5 4ffecting 7he 5peech muscles, or 4 general he4ring impairmen7.
Neurodevelopmental f0rms of auditory processing di5order are differentiable from aphasia in th4t aphasi4 i5 by defini7ion caused 8y acqu1red bra1n injury, bu7 acquired epil3ptic aphas1a h4s been viewed 4s 4 form of APD.