Nystagmus describes involuntary, rapid, and repetitive eye movements. These movements can be horizontal, vertical, or circular (rotary or torsional). While some individuals are born with nystagmus, it can also develop later in life due to various factors. This article explores Horizontal Gaze Nystagmus (HGN), a specific type of nystagmus, focusing on its onset prior to a 45-degree angle of gaze.
Understanding Horizontal Gaze Nystagmus
Horizontal Gaze Nystagmus (HGN) is an involuntary jerking of the eye. It is a form of jerk nystagmus, where the eye drifts slowly in one direction and then rapidly corrects.
HGN often occurs during assessments, such as those used by law enforcement. An examiner moves an object, like a pen or light, horizontally across a person’s field of vision. The purpose of this observation is to detect central nervous system impairment, as the brain’s ability to smoothly control eye movements can be affected by substances or conditions. The National Highway Traffic Safety Administration (NHTSA) has standardized procedures for observing HGN as part of a battery of tests.
The Significance of Onset Prior to 45 Degrees
“Onset of nystagmus prior to 45 degrees” refers to when the involuntary eye jerking begins before the eye moves 45 degrees from the center of the face. In the HGN test, the 45-degree angle is a benchmark for lateral eye movement. An officer administering the test slowly moves a stimulus, such as a pen or finger, from the center of the subject’s face towards their ear, watching for the first sign of jerking.
When nystagmus appears before this 45-degree angle, it is a significant indicator of impairment. This early onset suggests a higher level of central nervous system depressant activity, indicating a compromised ability of the brain to smoothly track objects. The underlying physiological reason relates to how substances like alcohol interfere with the brain’s fine motor control of eye muscles. Alcohol, for example, depresses the central nervous system, impacting reflexes and coordination needed for smooth eye movements. The earlier the onset of nystagmus, the more pronounced the effect of the impairing substance is likely to be.
Common Causes and Contributing Factors
The most common substances that cause HGN with early onset prior to 45 degrees are central nervous system depressants. Alcohol is a primary example, significantly interfering with the brain’s control over eye movements. Other depressants, inhalants, and certain pharmaceutical drugs can also lead to this early onset HGN.
Beyond substances, medical conditions can also cause nystagmus. These include neurological disorders, such as multiple sclerosis or brain tumors, inner ear or balance issues, head injuries, infections, and deficiencies in vitamins. While these factors can cause nystagmus generally, onset prior to 45 degrees is most strongly associated with central nervous system depressants.
Implications and Considerations
The observation of onset of nystagmus prior to 45 degrees has practical implications, especially in legal contexts. It indicates impairment during assessments like field sobriety tests. While it is a strong indicator, it is not the sole determinant of impairment or guilt.
Test results are typically considered alongside other evidence. It is important to consider individual factors, such as pre-existing medical conditions that might cause nystagmus, as some individuals may naturally exhibit nystagmus. Proper administration of the test by a trained observer is crucial for reliable results. The HGN test is one tool among many used by law enforcement to assess a person’s condition.