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Have you ever been stopped by a police officer and made to go through a field sobriety test to determine whether you have been driving under the influence? If yes, you have already gone through the horizontal gaze nystagmus (HGN) test where you are asked to follow an object with your eyes to check for a particular eye movement reaction. Let's see what HGN is and find out what is it that the law enforcement officers are looking for.
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What is Nystagmus?
Nystagmus is a vision condition in which the eyes make repetitive and uncontrolled movements that can occur from side to side, up and down or in a circular pattern. These movements usually occur when you hold your head in the forward position and try to look in a particular direction at an angle of about 45 degrees. You may not be able to control these movements and, in fact, are usually unaware that it is occurring.
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What is Gaze Nystagmus?
Based on the type of action, nystagmus can be of the pendular type (the speed of movement is same in both directions) or the jerk type (the eye slowly moves in one direction and jerks to the other direction). Gaze nystagmus belongs to the jerk type of this vision condition.
In normal conditions, your eyes are able to gaze and follow an object smoothly. However, under certain conditions, they are not able to do so and instead will lag behind. In order to correct themselves, they have to make a saccadic movement in the direction in which the eye is moving. This is gaze nystagmus.
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What is HGN?
Gaze nystagmus occurs due to disruptions in the nervous system. It can occur due to alcohol, drugs or certain medical conditions. When the condition occurs as the eye moves up from looking straight ahead, it is called vertical gaze nystagmus or VGN. When it occurs while the eye moves to the side, it is called horizontal gaze nystagmus or HGN.
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What is the HGN Test?
The HGN test is one of the three field sobriety tests that are used by police officers to test if you are driving under the influence (DUI). High blood alcohol content reduces your coordination, balance and dexterity. These tests look at these three measures to find out if you are intoxicated.
The law enforcement officer will place an object such as a pen roughly twelve to fifteen inches from your face and slightly higher than eye level. You are then asked to follow it with your gaze. Any involuntary jerking movements of the eyes will indicate impairment including nystagmus and you may be charged with a DUI.
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What are the Other Causes of HGN?
Although most people have heard about horizontal gaze nystagmus as something related to DUI, the fact is that there can be other causes of this condition. These can include central nervous system diseases, inner ear diseases, congenital cataracts and even anti-epilepsy medications. If this is the case, there is no cure for this vision condition. However, prescription glasses or contact lenses and surgery may help to treat other eye-related conditions that may be exaggerating the nystagmus.
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NHTSA: Horizontal Gaze Nystagmus: The Science & The Law
DUIAnswer.com: The Horizontal Gaze Nystagmus Test Used in DUI Arrests
Horizontal Gaze Nystagmus: Horizontal Gaze Nystagmus (HGN) Tests and Testing Procedures
American Optometric Association: Nystagmus