Can HGN show Nystagmus if the defendant is intoxicated by an unknown substance other than alcohol?

Depressants, inhalants, and dissociative anesthetics, the so-called "DID drugs", may cause HGN. They also may cause VGN, if in a very high dose for the individual

Thank you for your response, it is very helpful. I have a public intoxication case, but the only evidence I have is the HGN. Is this enough evidence?

That's a question that just can't be answered with certainty, especially without more context. A jury believing in HGN comes down to two things: how well your officer testifies about the science behind it and whether you get the jury to believe that it can be accurately detected by that that officer under those conditions.

The Science: You should sit down with your officer and the NHTSA manual and be prepared to not only explain the science but the very few other causes of HGN and how they were eliminated as possibilities for this suspect. You should also explain other types of nystagmus and their causes and show how they couldn't be causing what the officer saw. Going out to the scene and taking some pictures might help with this.

Believing in HGN: I like to use the demonstrative exhibits under the DWI Resources tab on TDCAA.com to show people what the eye is going to look like with and without nystagmus. Remember that many jurors are older, and often can't see well, so get the judge to ask if anyone is having problems seeing the exhibit so they can get closer. Also, ask lots of questions about how accurate and proficient your officer had to become at using HGN to determine intoxication to become certified to use it in the first place. Finally, ask the officer how many times he/she's performed the HGN test and whether in their personal experience they've found it to be an accurate indicator of intoxication.

The Rest: There's got to be some reason the test was given in the first place, so examine every interaction the officer had with the suspect that night. Use the "Mom's Field Sobriety Tests" indicators of intoxication (watch the video under the DWI Resources Tab). Ask people in voir dire whether they ever knew a person was intoxicated without having to perform a sobriety test, and how they knew it; if they say whatever your officer saw, then you're in good shape. You've got a lot more tools in your toolbox to prove intoxication than just your HGN test...

Thank you very much for the help. I believe that I can use what you suggested in describing the circumstances to eliminate other causes for HGN. Here, the student was in class and another student (unnamed)reported it and the officer did the HGN. However, the officer saw no other signs of intoxication.

I had a PI arrest of mine hang once (5-1) because while we convinced the 6th juror that the defendant was intoxicated, they didn't feel that we had proven that he was a danger to himself or others.

I've heard from several courses that you shouldn't be doing the FST's for public intox cases, they were meant for DWI's (the divided attention and all that). The best tests suggested were the "Mom's Field Sobriety Tests" mentioned above.

I see a couple of hurdles with HGN as your only evidence, but since I don't have all the information in front of me you may already have these covered. First, was this person intoxicated to a degree that they may endanger themselves or another? If they were in class, what were they doing that created the situation for another student to notice them, e.g., were they mixing chemicals in a chem lab, or were they sleeping through a history class? Second, was the substance administered for therapeutic purposes by a licensed physician? For DWI, that doesn't matter, but in PI, it creates a defense. Let us know how it works out!

Sec. 49.02. PUBLIC INTOXICATION. (a) A person commits an offense if the person appears in a public place while intoxicated to the degree that the person may endanger the person or another.
...
(b) It is a defense to prosecution under this section that the alcohol or other substance was administered for therapeutic purposes and as a part of the person's professional medical treatment by a licensed physician.

I am not saying not to proceed if you don't know whether there will be a defense presented; it is the defendant's job to raise that issue. I'm just throwing that out there as a possible hurdle if it is raised, so you are not surprised by it. I am going to assume that if there was evidence of that defense, the attorney would have brought it to you by now to get rid of the case.

Thanks for all the information. It will be very helpful in future trials. I did a little detective work and went to the school and even though the officer said that there were no witnesses, I talked to the school nurse. She examined him and had great information to prove intoxication. The Defendant was found guilty, I believe based on this information.