Ever heard of Ewald's Laws?
If not, with assessing BPPV do you ever wonder why:
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.
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.
The axis of the nystagmus parallels the anatomic axis of the affected semicircular canal.
The direction of the nystagmus is in the same plane as the affected canal. So:
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.
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.
If the otoconia are:
If the otoconia are:
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.
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..
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.
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.
To reinforce or understand better the above information we recommend checking out these resources.
If you have any further questions, please do not hesitate to reach out to us at [email protected].