<unk>  is most often caused by stroke, where about a quarter of patients who experience an acute stroke develop<unk> . However, any disease or damage to the parts of the brain that control language can cause<unk> . Some of these can include brain tumors, traumatic brain injury, and progressive neurological disorders. In rare cases,<unk>  may also result from herpesviral encephalitis. The herpes simplex virus affects the frontal and temporal lobes, subcortical structures, and the hippocampal tissue, which can trigger<unk> . In acute disorders, such as head injury or stroke,<unk>  usually develops quickly. When caused by brain tumor, infection, or dementia, it develops more slowly. Substantial damage to tissue anywhere within the region shown in blue (on the figure in the infobox above) can potentially result in<unk> .<unk>  can also sometimes be caused by damage to subcortical structures deep within the left hemisphere, including the thalamus, the internal and external capsules, and the caudate nucleus of the basal ganglia. The area and extent of brain damage or atrophy will determine the type of<unk>  and its symptoms. A very small number of people can experience<unk>  after damage to the right hemisphere only. It has been suggested that these individuals may have had an unusual brain organization prior to their illness or injury, with perhaps greater overall reliance on the right hemisphere for language skills than in the general population. Primary progressive<unk>  (PPA), while its name can be misleading, is actually a form of dementia that has some symptoms closely related to several forms of<unk> . It is characterized by a gradual loss in language functioning while other cognitive domains are mostly preserved, such as memory and personality. PPA usually initiates with sudden word-finding difficulties in an individual and progresses to a reduced ability to formulate grammatically correct sentences (syntax) and impaired comprehension. The etiology of PPA is not due to a stroke, traumatic brain injury (TBI), or infectious disease; it is still uncertain what initiates the onset of PPA in those affected by it. Epilepsy can also include transient<unk>  as a prodromal or episodic symptom.<unk>  is also listed as a rare side-effect of the fentanyl patch, an opioid used to control chronic pain.
Aphasia
Visual impairment
Aphantasia
Ectopia 
Ophthalmia
Color blindness
Cortical blindness
Amblyopia