Ewald's Laws for BPPV & Nystagmus

Ever heard of Ewald's Laws?

If not, with assessing BPPV do you ever wonder why:

• With BPPV affecting the posterior canal, going into the Dix-Hallpike Test is more provocative than sitting up after?
• With BPPV affecting the lateral canal, going into the Supine Roll Test the side that is more provocative is not always the affected side?
• With cupulolithiasis affecting the lateral canal, the nystagmus beats towards the affected ear with the Lean Test?

Maybe you have not, but understanding the WHY behind these will help us treat the atypical forms of BPPV. It will also help us determine if there is another vestibular condition at play, mimicking BPPV.

What are Ewald’s Laws?

There are three laws that were discovered by a German physiologist named JR Ewald. From doing studies on the semicircular canals of pigeons, using  a pneumatic hammer, he came up with three consistent observations. These later became known as Ewald’s Laws.

Ewald’s First Law

The axis of the nystagmus parallels the anatomic axis of the affected semicircular canal.

Clinical relevance:

The direction of the nystagmus is in the same plane as the affected canal. So:

• If there is lateral nystagmus with going into a positional test it is indicative of BPPV affecting one of the lateral canals.
• If there is downbeat torsional nystagmus with going into a positional test it is indicative of BPPV affecting one of the anterior canals.
• If there is upbeat torsional nystagmus with going into a positional test it is indicative of BPPV affecting one of the posterior canals.
• If there is any other direction of nystagmus with positional testing (i.e. pure vertical or torsional), the nystagmus is not due to BPPV. 

Here is a summary of the nystagmus associated when each canal is stimulated:

 To see a video animation of the nystagmus for Ewald's First Law click HERE.

Ewald’s Second Law

With the lateral canal, deflection of the cupula away from the canal creates a stronger excitatory response that when it deflects towards the canal and creates an inhibitory response.

Clinical relevance:

Lateral canal BPPV and the Supine Roll Test

If the otoconia are:

• Free floating in the posterior arm, which is more common (Image A): Turning the head towards the affected side or ear (Image B), causes the otoconia to move closer to the cupula. This then pushes the endolymph into the cupula and deflect it away from the canal. This will cause a greater response (more symptoms and nystagmus) than when the head is turned away from the affected side, causing the cupula to deflect towards the canal. 

Image A

Image B

• Free floating in the anterior arm (Image C) or stuck to the cupula causing cupulolithiasis (Image D): Turning the head towards the affected side or ear causes the cupula to deflect towards the canal due to the movement of the loose otoconia moving away from the cupula or due to the weight of the otoconia being stuck to the cupula (heavy cupula). This will cause less of a response than when the head is turned away from the affected side and the cupula deflects away from the canal.

Image C

 Image D

Lateral canal BPPV and the Bow & Lean Test

If the otoconia are:

• Free floating in the posterior arm and the patient performs the Bow Test: The loose otoconia move towards the cupula causing it to deflect away from the canal. This creates an excitatory response, with the nystagmus beating towards the affected ear. With the Lean Test the otoconia move away from the cupula causing it to deflect towards the canal and an inhibitory response with nystagmus beating away from the affected ear. 
• Free floating in the anterior arm or stuck to the cupula and the patient performs the Bow Test: The loose otoconia or heavy cupula causes the cupula to deflect towards the canal. This then creates an inhibitory response and nystagmus beating away from the affected ear. With the Lean Test the cupula deflects away from the canal and an excitatory response with the nystagmus beating towards the affected ear.

Bottom line

With lateral canal BPPV affecting the posterior arm the direction of the Supine Roll Test that is most provocative is the affected side (assuming the test was performed at the same speed and amount of movement). So, if the right Supine Roll Test was more provocative than the left, the right lateral canal would likely be the affected canal. Also, the nystagmus with the Bow Test beats towards the affected ear and away from the affected ear with the Lean Test.

On the other hand with lateral canal BPPV affecting the anterior arm or cupulolithiasis, the direction of the Supine Roll Test that is less provocative is the affected side (assuming the test was performed at the same speed and amount of movement). Also, the nystagmus with the Bow Test beats away from the affected ear and towards the affected ear with the Lean Test.

Note: Do not worry if you have to read through this again as it is not light material. Below are some additional resources to help you understand these concepts. 

Ewald’s Third Law

With the posterior and anterior canal, deflection of the cupula towards the canal creates a stronger excitatory response than when it deflects away from the canal and creates an inhibitory response..

Clinical relevance:

This explains why going into the Dix-Hallpike is more provocative, symptom and nystagmus wise, than sitting up from the positional test. With going into this positional test the cupula deflects towards the canal as the loose otoconia move away from the cupula (canalithiasis) or due to the weight of the heavy cupula with cupulolithiasis. Then, when the patient sits up the reverse happens.

For us, sometimes using an anatomical model that can be purchased from groups like Vestibular First helps conceptualize what is going on.

Wrap-up On Ewald's Laws

We hope that this sheds some light on why BPPV responds in certain ways. Knowing these intricacies helps improve our outcomes by helping us figure out how to deal with the atypical presentations of BPPV. It also helps us determine when another condition may be mimicking BPPV.

Further Education Regarding Ewald's Laws 

To reinforce or understand better the above information we recommend checking out these resources.

• Further details about Ewald’s Three Laws can be found in our A Field Guide to BPPV 6.5 hour online course.
• Further information about assessing and treating atypical BPPV affecting the lateral canal can be found in our Getting Crystal Clear with Lateral Canal BPPV 30 min online course.
• Supine Roll test for BPPV - 3.5 min YouTube video from North 49.
• Best Tip to Tease Out Torsional Nystagmus with Dix-Hallpike Testing - 2.5 min YouTube video from North 49.
• Quick Tip: Ewald's Laws and Horizontal Canal BPPV - 6 min video from Balancing Act Rehab.
• Video animations of Ewald’s First Law for various semicircular canals - from Dr. Teixido.
• Further information about determining the affected ear with lateral canal BPPV and other interesting facts about lateral canal BPPV can be found on Dr. Hain’s website.

If you have any further questions, please do not hesitate to reach out to us at [email protected].