Visual Hallucinations

What are they?

Visual sensations that are not based on viewed objects, but are generated by endogenous neural activity (“seeing things that are not there”)
Mechanisms
- Irritation of the outer retina, optic nerve, or vision-related cerebral cortex
- Deafferentation of the vision-related cerebral cortex (“release hallucinations”)
Common causes of irritation of the outer retina: vitreous or retinal detachment, photoreceptor dysfunction
Common causes of irritation of the optic nerve: optic neuritis, ischemic optic neuropathy
Common causes of irritation of the vision-related cerebral cortex: migraine, ischemia, tumor, medications, or recreational drugs
Common cause of deafferentation of the vision-related cerebral cortex: blindness of any cause (Charles Bonnet Syndrome)

Brief flashes = aging, liquefying vitreous tugging on retina and deforming photoreceptors
Stationary white flickers = irritation of photoreceptors or vision-related cortex

Stationary, often multicolored, flickers = irritation of vision-related cortex by seizure

Episodic superimposition of previously viewed object on a newly viewed scene [“visual perseveration,” “palinopsia”]

= irritation or deafferentation of vision-related cortex by various lesions
Geometric shapes that traverse a hemifield = migraine

Stationary complex patterns or figures

= irritation of vision-related cortex by infarction, seizure, medications, recreational drugs; deafferentation of vision-related cortex by blindness (Charles Bonnet syndrome), sleepiness, schizophrenia, dementia

Flickering black and white dots or lines that interfere with visual clarity (“visual snow”)

= visual hypersensitivity of psychogenic origin; very rarely irritation of photoreceptors by an outer retinal lesion

Retinal or vitreous detachment needs prompt surgical repair
Migraine needs reassurance and sometimes prophylaxis
Suspected vision-related cerebral cortex lesion needs brain imaging, electroencephalography, or other appropriate evaluation and treatment
Medications, recreational drugs need appropriate identification and withdrawal
Psychosis needs treatment
Charles Bonnet syndrome needs explanation, reassurance, and very rarely anxiolytic medication
Visual snow needs explanation, ruling out of an outer retinal lesion, and management of an underlying anxiety state