Hallucinogen persisting perception disorder
Hallucinogen persisting perception disorder (abbreviated as HPPD) is a psychiatric disorder characterized by the persisting presence of sensory disturbances, most commonly visual, that resemble those that are produced by the use of hallucinogenic substances. Previous use of hallucinogens is necessary, but not sufficient, for diagnosis of HPPD, and the symptoms cannot be due to another medical condition. HPPD is distinct from flashbacks by reason of its relative permanence and the degree to which it interfere's with one's social and occupational functioning; while flashbacks are transient, condition-specific manifestations of a hallucinogenic state, HPPD is characterized by its persistence.
According to a 2016 review, there are two subtypes of the condition:
Type 1 HPPD
This is where people experience HPPD in the form of random, brief flashbacks.
Type 2 HPPD
People with this kind of HPPD experience ongoing changes to their vision, which may come and go.
There are a number of perceptual changes that can accompany HPPD. Typical symptoms of the disorder include: halos or auras surrounding objects, trails following objects in motion, difficulty distinguishing between colors, apparent shifts in the hue of a given item, the illusion of movement in a static setting, air assuming a grainy or textured quality (visual snow or static, by popular description, not to be confused with normal "blue field entoptic phenomenon"), distortions in the dimensions of a perceived object, intensified hypnagogic & hypnopompic hallucinations, and a heightened awareness of floaters. The visual alterations experienced by those with HPPD are not homogeneous and there appear to be individual differences in both the number and intensity of symptoms.
Visual aberrations can occur periodically in healthy individuals – e.g. after images after staring at a light, noticing floaters inside the eye, or seeing specks of light in a darkened room. However, in people with HPPD, symptoms are typically persistent enough that the individual cannot ignore them.
There is some uncertainty about to what degree visual snow constitutes a true HPPD symptom. There are many individuals who have never used a drug which could have caused the onset, but yet experience the same grainy vision reported by HPPD sufferers. There are a few potential reasons for this, the most obvious of which being the theory that the drug usage may exaggerate the intensity of visual snow. Another theory is that instead, there may be no change in the severity or magnitude of the visual snow, but perhaps the drug usage opens sensory pathways that result in the individual becoming more aware of any visual disturbances that may have simply not been noticed before the incidence of substance use. As for root cause of visual snow, some theories suggest that it is the result of thermal noise in the visual cortex or in the 'Optic Pathway' (encompassing photoreceptor cells on the retina, the optic nerve, and the optic chiasm) as eye tests for individuals who experience visual snow often reveal that physically, the eye is perfectly normal, and in many cases the individual still maintains 20/20 vision.
HPPD usually has a visual manifestation. Drugs affecting the auditory sense, like diisopropyltryptamine (DiPT), may produce auditory disturbances, though there are few known cases. Some hallucinogenic substances can produce temporary tinnitus-like symptoms as a side effect.
It also should be noted that the visuals do not constitute true hallucinations in the clinical sense of the word; people with HPPD recognize the visuals to be illusory, or pseudohallucinations, and thus maintain the ability to distinguish what is real (in contrast to some mental illnesses such as schizophrenia).
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