Illusory palinopsia

Illusory palinopsia is often worse with high stimulus intensity and contrast ratio in a dark adapted state. Multiple types of illusory palinopsia often co-exist in a patient and occur with other diffuse, persistent illusory symptoms such as halos around objects, dysmetropsia (micropsia, macropsia, pelopsia, or teleopsia), Alice in Wonderland Syndrome, visual snow, and oscillopsia. Illusory palinopsia consists of the following four symptom categories. Prolonged indistinct afterimage Prolonged indistinct afterimages are unformed and occur at the same location in the visual field as the original stimulus. Stimulus intensity, contrast, and fixation length affects the generation and severity of these perseverated images. For example, after seeing a bright light such as a car headlight or a camera flash, a persistent afterimage remains in the visual field for several minutes. Afterimages from lights tend to last longer than the indistinct afterimages from other brightly colored objects. Palinoptic prolonged light afterimages of the complementary color are differentiated from physiological afterimages based on afterimage intensity and duration. Light streaking Light streaking describes a comet-like tail which is seen due to motion between a person or a light. These motion-induced afterimages may be discontinuous such as in a film reel or may be blurred together such as in a long-exposure photograph. If discontinuous, the patient also usually reports akinetopsia. The perseverated images last a few seconds and are usually identical in color and shape to the original stimulus. Most cases describe visual trails during movement of an object, although there are also reports from the movement of the observer's head or eyes. or may be black or translucent. These variants usually lack the realistic clarity of hallucinatory palinopsia, and the generation of the palinoptic images is affected by fixation time, motion, stimulus intensity, or contrast. These variants probably represent an overlap in hallucinatory and illusory palinopsia but are included in illusory palinopsia since they often co-exist with the other illusory symptoms. ==Cause==

Of the published cases of palinopsia that are idiopathic or attributed to migraines, HPPD, prescription drugs, or head trauma, 94% described illusory palinopsia. mirtazapine, topiramate, clomiphene, oral contraceptives, and risperidone have been reported to cause illusory palinopsia. Clomiphene and oral contraceptives are the only prescription drugs reported to cause permanent symptoms. ==Pathophysiology==

Illusory palinopsia is a dysfunction in visual perception, presumably related to diffuse neuronal excitability alterations in the anterior and posterior visual pathways. Because of the drugs that cause illusory palinopsia, 5-HT2a receptor excitotoxicity or a disruption of GABAergic transmission have been proposed as possible mechanisms. However, the neuropharmacology of the visual system is probably too complex to pinpoint the visual disturbances to a single neurotransmitter or neurotransmitter receptor. The generation of illusory palinopsia is often dependent on ambient light or motion, and the symptoms could be a pathological exaggeration of normal light perception and motion perception mechanisms. Palinopsia in migraineurs Illusory palinopsia may occur during a migraine aura, as do other diffuse illusory symptoms such as halos around objects, visual snow, dysmetropsia, and oscillopsia. In a rare migraine subtype known as persistent visual aura without infarction, illusory palinopsia symptoms (prolonged indistinct afterimages, light streaking, and visual trailing) persist after the migraine has abated. Alternatively, up to 10% of all migraineurs report of formed afterimages that only last a couple seconds and do not occur with other illusory symptoms. These momentary afterimages appear at a different location in the visual field than the original stimulus, occur a few times per month, and are affected by external light and motion. (variant image perseveration). Migraineurs with these momentary afterimages report significantly fewer migraine headaches than migraineurs without these afterimages (4.3 vs. 14.4 attacks/year). These afterimages probably represent an overlap in hallucinatory and illusory palinopsia. Studying these momentary formed afterimages, in relation to alterations in cortical excitability, could advance our understanding of migraine pathogenesis and mechanisms associated with encoding visual memory. ==Diagnosis==

Palinopsia necessitates a full ophthalmologic and neurologic history and physical exam. There are no clear guidelines on the work-up for illusory palinopsia, but it is not unreasonable to order automated visual field testing and neuroimaging since migraine aura can sometimes mimic seizures or cortical lesions. However, in a young patient without risk factors or other worrisome symptoms or signs (vasculopathy, history of cancer, etc.), neuroimaging for illusory palinopsia is low-yield but may grant the patient peace of mind. ==Treatment==

There is limited data on treating the visual disturbances associated with HPPD, persistent visual aura, or post-head trauma visual disturbances, and pharmaceutical treatment is empirically based. It is not clear if the etiology or type of illusory symptom influences treatment efficacy. Since the symptoms are usually benign, treatment is based on the patient's zeal and willingness to try many different drugs. There are cases which report successful treatment with clonidine, clonazepam, lamotrigine, nimodipine, topiramate, verapamil, divalproex sodium, gabapentin, furosemide, and acetazolamide, as these drugs have mechanisms that decrease neuronal excitability. However, other patients report treatment failure from the same drugs. ==References==