Nystagmus refers to involuntary, rapid, and rhythmic eye movements. They can occur horizontally, vertically, or in a circular motion. While the brain usually coordinates eye movement with the inner ear to maintain stable vision, nystagmus indicates a disruption in this system. Positional alcohol nystagmus, often called PAN, is a specific type of this eye movement directly linked to alcohol consumption. This response indicates alcohol’s effect on the body’s balance system.
What is Positional Alcohol Nystagmus?
Positional alcohol nystagmus (PAN) describes the involuntary jerking of the eyes that occurs when an individual’s head is in certain positions while alcohol is present in the body. The eye movements often appear as a jerky motion, typically observed horizontally. This reflex cannot be controlled or stopped by the person experiencing it.
PAN appears when the head is tilted to the side, causing the eyes to oscillate. This reaction directly results from alcohol’s influence on the inner ear’s balance mechanisms. The presence and intensity of these eye movements can vary depending on the amount of alcohol consumed and the individual’s current state of intoxication.
How Alcohol Affects Eye Movement
Alcohol impacts the inner ear’s vestibular system, which is responsible for balance and spatial orientation. This system includes the semicircular canals, filled with endolymph, and tiny hair cells within the cupula. Normally, the cupula and endolymph have similar specific gravities, preventing gravitational influence on the cupula.
When alcohol enters the bloodstream, it diffuses into the endolymph and the cupula. Alcohol is less dense than water, changing the cupula’s specific gravity more rapidly than the surrounding endolymph. This makes the cupula lighter, causing it to float and become sensitive to gravity. The altered buoyancy sends erroneous signals to the brain, leading to involuntary eye movements.
PAN occurs in two distinct phases. PAN I occurs as blood alcohol content (BAC) rises and peaks. During this phase, the cupula is lighter than the endolymph, causing nystagmus to the left when the left ear is down and to the right when the right ear is down. PAN II occurs as alcohol is eliminated from the body and BAC falls. In this phase, alcohol leaves the cupula faster than the endolymph, making the cupula relatively heavier and leading to nystagmus in the opposite direction. Overstimulation of the semicircular canals during both PAN phases can contribute to unsteadiness, nausea, and vertigo, often called “the spins.”
Observing Nystagmus: The HGN Test
Positional alcohol nystagmus is frequently observed through the Horizontal Gaze Nystagmus (HGN) test, a component of the Standardized Field Sobriety Tests (SFSTs). During this test, an officer instructs an individual to follow a moving stimulus, such as a pen or finger, with their eyes while keeping their head still. The stimulus is held 12-15 inches from the face and moved horizontally.
Officers look for three specific clues in each eye, totaling six indicators. These clues include a lack of smooth pursuit, where the eye jerks noticeably rather than moving smoothly as it follows the stimulus. Another clue is sustained nystagmus at maximum deviation, where the eye jerks clearly when held as far as possible to the side for at least four seconds. The third clue is the onset of nystagmus prior to 45 degrees, indicating that the eye begins to jerk before reaching its full lateral gaze. The presence of these eye movements indicates central nervous system impairment, which can be caused by alcohol.
Other Causes of Nystagmus
While alcohol consumption is a common cause of nystagmus, other factors can also cause these eye movements. Certain medications, such as anti-seizure drugs and sedatives, can induce nystagmus as a side effect. Medical conditions affecting the inner ear, like Meniere’s disease or benign paroxysmal positional vertigo (BPPV), can disrupt balance signals and cause nystagmus.
Neurological disorders, including brain injuries, strokes, and multiple sclerosis, can also impair the brain’s control over eye movements, resulting in nystagmus. In some cases, nystagmus may be present from birth due to congenital conditions or developmental issues. Therefore, observing nystagmus alone does not definitively confirm alcohol impairment; it requires corroborating evidence and consideration of other potential causes.