Positional alcohol nystagmus (PAN) is nystagmus is produced when the head is placed in a sideways position.
Nystagmus is the visible jerkiness in eye movement.
PAN occurs when the specific gravity of the membrane space of the semicircular canals in the ear differs from the specific gravity of the fluid in the canals because of the presence of alcohol.
PAN type I:
With the consumption of alcohol, the alcohol is carried by the bloodstream and diffused into the water compartments of the body.
The specific gravity of a semicircular canal membrane is the same as the specific gravity of the surrounding fluid.
Though the Earth’s gravity is a constant force of acceleration, the semicircular canals do not respond to it.
Since alcohol has a lighter specific gravity than water when it enters the canal membrane via capillaries, the specific gravity of the membrane is lower than that of the surrounding fluid.
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 and PAN 1 occurs.
The alcohol diffuses very slowly from the membrane to the extracellular fluid.
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 (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.
Positional alcohol nystagmys1 is typically present during a rising and peak blood alcohol concentration.
PAN II
The rate of elimination of alcohol is fairly constant.
Initially, the absorption rate of alcohol exceeds the rate of elimination, which results in a rising blood alcohol concentration (BAC).
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 resulting 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.
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 of 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, and neither PAN I nor PAN II is present.
Horizontal Gaze Nystagmus (HGN) testing is used in the identification of persons who are intoxicated or under the influence of a controlled substance.
PAN is identified when the head is tilted to one side or the other.
For HGN to be properly identified, the head must be positioned in line with the spine.