End Point Nystagmus Differentiation

Do you ever struggle distinguishing between different types of nystagmus, such as end point nystagmus (EPN) and gaze evoked nystagmus (GEN)? If so, you're not alone. Working closely with therapists in vestibular therapy over the years we've found this to be a common issue. Given such, let's review three practical tips when assessing patients that we have found helpful.

Actually, before we get too far, we need to remember that end point nystagmus is a normal physiological response, while a true positive gaze evoked nystagmus finding is indicative of a central or peripheral vestibular disorder.

As we will see in this blog, end point nystagmus is a bit of a misnomer as it can be seen prior to the end range of ocular motion. Some have proposed using the terms physiological gaze evoked nystagmus and pathological gaze evoked nystagmus, which may be better descriptors, but for the purposes of this blog we will keep it to the traditional terms being end point nystagmus and gaze evoked nystagmus. 

Nystagmus Assessment Tips:

Tip #1: Consider Eyewear Influence

Acknowledge that external factors, like a patient's eyewear, can complicate nystagmus detection. Glasses, especially those with thicker rims, may obscure vision of the target and lead to false positive GEN results.

Tip #2: Restrict Eye Movement Range

When evaluating for GEN, limit lateral gaze movement to 30 degrees. This helps prevent triggering EPN, especially in patients prone to displaying it. (Ritter 2020)

Tip #3: Extend Observation Duration

During GEN assessment, maintain the eye position for at least 4 seconds. Prolonged observation allows a more comprehensive evaluation, especially for frequent blinkers.

With that in mind, Serra notes that EPN is typically unsustained, with low frequency and amplitude. If GEN is observed it is typically obvious, sustained, may demonstrate other ocular findings, and additional information is needed to determine its origin (central vs peripheral vestibular).

Additional Information

Consider that, as with any test, there can be "normal abnormalities". In some studies lateral nystagmus has been observed in healthy subjects, with prevalence ranging up to 21% at 10° of lateral gaze, increasing up to 93% at extreme lateral gaze (Abel 1978 & Whyte 2010). Again, worth noting, if the nystagmus is of a central or peripheral vestibular cause, the nystagmus will be sustained and obvious, with other possible ocular findings.

Conclusions

By paying attention to these nuances, healthcare providers can enhance diagnostic skills in preventing false positives as well as differentiating between end point nystagmus and gaze evoked nystagmus. These precautions contribute to more accurate assessments, leading to precise interventions in vestibular therapy.

If you would like to see examples of GEN, feel free to explore our vestibular ocular library.

We hope that this helps improves your confidence and skillset with oculomotor testing.