e-pearls newsletter, February 2009:

I chose this month’s topic because of a strong personal interest in schizophrenia (in which hallucinations are most often of anauditory nature; however, visual hallucinations may occur in a significant percentage of patients over the course of their illness). In addition, therapists will likely come across clients suffering from hallucinations from time to time, and I believe it may be helpful, as mental health providers, to know something about their medical basis. I chose to focus on visual hallucinations because of the different varieties, which are important to distinguish.

Background

Hallucinations have many physical causes including disturbances of the brain structure itself (such as a brain tumor), delirium (which itself has many causes including substance withdrawal, infections, substance intoxication, among others), migraine, and even epilepsy. In fact, up to 75% of delirious patients have hallucinations. They are also commonly seen in psychiatric disorders.

The key fact to keep in mind is to always be suspicious that a client’s hallucinations may be due to a so-called “organic” cause (this is a bit of a misnomer, since even psychotic disorders such as schizophrenia have as their basis some physical mechanism). However, for purposes of discussion, organic illness here refers to non-psychiatric disease.

Formed vs. Unformed Visual Hallucinations

There are basically two main types of visual hallucinations: formed and unformed. Unformed hallucinations consist of blotches of color, geometric shapes, flashing lights, or some other ill-defined image. Formed hallucinations, on the other hand, consist of well-defined objects, people, animals, or anything else. It is easy to figure out what type is being experienced by a client, simply from their description. As one might guess, formed and unformed visual hallucinations have very different sites of origin in the nervous system.

The presence of formed visual hallucinations is generally consistent with psychiatric disease; they are believed to originate in the temporal lobe. If they occur in the presence of auditory hallucinations in a young adult, it goes along with a diagnosis of a primary psychiatric disorder (organic causes must still be excluded; this is an important point). On the other hand, unformed visual hallucinations are believed to originate outside of the temporal lobe. They may be the result of a brain disturbance in the occipital lobe (which is in the back of the brain, and is the area that receives visual information from the eyes), retina (the light-sensitive part of the eyes), optic nerves (which convey the visual signal from the retina back into the brain), or other areas of the brain. The point here is not to necessarily know all the details, but to keep in mind that unformed visual hallucinations should always make one very concerned about an underlying organic, nonpsychiatric process.

References 

Adams, R. D., & Victor, M. (1993). Principles of Neurology (5th ed.). New York: McGraw-Hill, Inc.

Cummings, J. L., & Miller, B. L. (1987). Visual hallucinations: Clinical occurrence and use in

differential diagnosis. Western Journal of Medicine, 146, 46-51.