Positional alcohol nystagmus (PAN) is
nystagmus
Nystagmus is a condition of involuntary (or voluntary, in some cases) eye movement. Infants can be born with it but more commonly acquire it in infancy or later in life. In many cases it may result in reduced or limited vision. Due to the invol ...
(visible jerkiness in
eye movement
Eye movement includes the voluntary or involuntary movement of the eyes. Eye movements are used by a number of organisms (e.g. primates, rodents, flies, birds, fish, cats, crabs, octopus) to fixate, inspect and track visual objects of interest ...
) produced when the head is placed in a sideways position. PAN occurs when the
specific gravity
Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water (molecule), wa ...
of the membrane space of the
semicircular canals
The semicircular canals or semicircular ducts are three semicircular, interconnected tubes located in the innermost part of each ear, the inner ear. The three canals are the horizontal, superior and posterior semicircular canals.
Structure
The ...
in the ear differs from the specific gravity of the fluid in the canals because of the presence of alcohol.
PAN I
When a person consumes alcohol, the alcohol is carried by the bloodstream and diffused into the water compartments of the body. Normally, the specific gravity of a canal membrane is the same as the specific gravity of the surrounding fluid. Because of this, even though the Earth's gravity is a constant force of acceleration, the semicircular canals do not respond to it. Alcohol has a lighter specific gravity than water. When alcohol enters the canal membrane via
capillaries
A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...
, the specific gravity of the membrane is lower than that of the surrounding fluid. The alcohol does diffuse from the membrane to the fluid, but it does so very slowly. While the specific gravity of the membrane is lower than the specific gravity of the extracellular fluid, the hair cells on the membrane become responsive to the Earth's gravity. This is the condition of PAN I.
PAN I is characterized by a nystagmus to the left when the left side of the head is down/right side when the right side of the head is down. It is typically present during a rising and peak
Blood Alcohol Concentration
Blood alcohol content (BAC), also called blood alcohol concentration or blood alcohol level, is a measurement of alcohol intoxication used for legal or medical purposes; it is expressed as mass of alcohol per volume or mass of blood. For exampl ...
(BAC).
PAN II
As soon as a person starts drinking, the body begins to process and eliminate the alcohol. The rate of elimination is fairly constant. Initially, the rate of absorption exceeds the rate of elimination, which results in a rising BAC. Some time after a person stops drinking, the rate of absorption drops below the rate of elimination, and the BAC begins falling. As alcohol is eliminated from the body, it is removed from the membrane of the semicircular canal faster than the surrounding fluid. This creates the reverse situation of PAN I, as the specific gravity of the fluid is now lower than that of the membrane. This results in PAN II.
PAN II is characterized by a nystagmus to the right when the left side of the head is down/left when the right side of the head is down.
Relationship between PAN and the effects of intoxication
The overstimulation of the semicircular canals during PAN I and PAN II is associated with the unsteadiness, nausea, and vertigo felt by intoxicated people. PAN I is more associated with postural problems (e.g. standing and walking) while PAN II has been more associated with the feelings of a hangover.
There is a brief period between PAN I and PAN II when the alcohol concentrations in the canal membrane and extracellular fluid are in equilibrium. During this time, neither PAN I nor PAN II is present.
PAN versus testing for HGN in intoxicated individuals
Horizontal Gaze Nystagmus (HGN) testing is a common practice used by law enforcement in the United States in the identification of persons who are intoxicated or under the influence of a controlled substance. The key difference between recognizing PAN and horizontal gaze nystagmus is the position of the subject's head in relation to the body. PAN is identified when the head is tilted to one side or the other. In order for HGN to be properly identified, the head must be positioned in line with the spine. Because of this, if the head is tilted towards the side when an evaluation for HGN is given, PAN may be induced and give a "false positive" for HGN. Some defendants may claim or argue that the nystagmus observed by an officer was positional and not horizontal gaze.
[Advanced DUI Investigation: A Training and Reference Manual, Daniel J. Haggin, Charles C Thomas (Publisher), 2005]
pg. 144
See also
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Optokinetic reflex
The optokinetic response is a combination of a slow-phase and fast-phase eye movements. It is seen when an individual tracks (pursuit movement) a moving object with their eyes, which then moves out of the field of vision, a point at which their ...
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Nystagmus
Nystagmus is a condition of involuntary (or voluntary, in some cases) eye movement. Infants can be born with it but more commonly acquire it in infancy or later in life. In many cases it may result in reduced or limited vision. Due to the invol ...
References
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Intoxication
Vision
Auditory system