The knowledge of semicircular canal spatial posture and otolith theory are the basis for our design of BPPV diagnostic test.
In fact, we don't know enough about the spatial posture of semicircular canal, especially the spatial posture of membranous semicircular canal and crista ampullaris.
BPPV simulation model is an important tool to study BPPV. Its premise is to establish the membrane labyrinth model in standard spatial coordinate system.
We established the BPPV simulation model and further measured and obtained the spatial attitude information of semicircular canal and crista ampullaris, which makes it possible to design the posterior semicircular BPPV diagnostic maneuver according to the predetermined goal.
Firstly, the existing diagnostic tests for PSC BPPV are reviewed and analyzed.
Dix and Hallpike described in detail all clinical manifestations of BPPV, introduced that specific manipulation can stimulate vertigo symptoms and nystagmus, which is still used today and named Dix Hallpike maneuver.
When Dix and Hallpike designed the Dix Hallpike maneuver, they did not fully understand the pathophysiology of BPPV, so they did not realize that the Dix Hallpike maneuver induced otolith in PSC move away from the ampulla.
Although the Dix Hallpike maneuver is still the standard maneuver for the diagnosis of PSC BPPV, the scientificity of its design is worth discussing.
There are some modified Dix Hallpike maneuver, such as mini Dix Hallpike maneuver , side-lying maneuver and half Dix-Hallpike maneuver.
Compared with the classic Dix Hallpike maneuver, these methods mainly have a smaller head tilt angle, which is not much different in essence.
It is generally believed that Dix Hallpike maneuver is used to diagnose PSC-BPPV and ASC-BPPV, but in fact, Dix Hallpike maneuver can also induce vertigo in 39% of LSC-BPPV patients, which makes it difficult to analyze the positive results of Dix Hallpike maneuver, and correct judgment can be made in combination with the characteristics of nystagmus.
The nystagmus characteristics of BPPV include latency, direction of nystagmus, duration, reversal, and fatigability .
Usually, the upbeating rotatory nystagmus induced by Dix Hallpike test is judged as ipsilateral posterior semicircular canal excitatory stimulation, which is recognized as ipsilateral posterior semicircular canal BPPV. However, in theory, it may also be inhibitory stimulation of the contralateral superior semicircular canal. Moreover, the physical simulation experiment shows that during the Dix Hallpike test, the otoconia in the contralateral utricle will fall into the superior semicircular canal, causing inhibitory stimulation.
By optimizing the diagnostic test and controlling the angle range of head tilt back, so that the free otoconia in the contralateral utricle will not fall into the superior semicircular canal, the interpretation of upbeating rotatory nystagmus can be simplified as the excitatory stimulation of the ipsilateral posterior semicircular canal.
Although fatigue is considered an important feature of BPPV, it is not recommended to repeat Dix Hallpike action to demonstrate fatigability, because it unnecessarily subjects patients to repeated vertigo symptoms, which is discomforting.
More importantly, the nystagmus induced by repeated diagnostic tests may be inconsistent due to the fatigability, it will interfere with the diagnosis and treatment of BPPV.
The intuitive explanation for the fatigability is that repeated operation turns large otoliths into small otoliths, but this is inconsistent with the research results of otolith hydrodynamics.
Roselli's research shows that the lower arm of the posterior semicircular canal is straight, repeated diagnostic tests will move the otoconia away from the ampulla and reduce the nystagmus.
Theoretically, if the starting position of otoconia movement is consistent, the nystagmus induced by repeated diagnostic test should also be consistent.
This inspires us that if the diagnosis test is improved to keep the starting position of otolith movement consistent, fatigue can be eliminated.
By bending forward, the otoconia scattered in the lower arm of the posterior semicircular canal slide down to the opening of the ampulla under the action of gravity, so that the starting position of the otolith movement in the repeated diagnostic test can be consistent.
In Dix Hallpike test, nystagmus occurs after lying flat for a moment, which is called latency period, and it is an important feature of BPPV nystagmus.
The cause of delayed nystagmus is that the otoconia moves in the ampulla with weak hydrodynamic effect, and the obvious hydrodynamic effect appears only after entering the narrow semicircular canal.
Because the position of the otoconia in the ampulla is not fixed, the latency period is unstable and has a large range of variation, some of them even have no obvious latency period, which will weaken the diagnostic significance of the latency period for BPPV.
Through the forward bending motion, the scattered otoconia in the lower arm of the posterior semicircular canal slide down to the bottom of the crista ampullaris under the action of gravity, which can not only make the starting position of otolith movement in the repeated diagnostic test consistent, but also keep the time of the latency period consistent, so as to strengthen the diagnostic significance of the latency period for BPPV.
There is no effective method to distinguish the BPPV subtypes such as cupulolithiasis, short arm type canalolithiasis and long arm side canalolithiasis. All those types induced excitatory stimulation while performing the Dix-Hallpike maneuver.
The differential diagnosis can be made through the following steps:
Continue with the above operation,after sitting up, bend 45 degrees to make the crista ampullaris parallel to the gravity vector, and the otoconia in the long arm side moves towards the ampulla for a long distance, and reverse nystagmus occurs; while the otoconia in the short arm side or adherent to cupula have no obvious hydrodynamic effect.
Furthermore, if you increase the bend forward angle to more than 120 degrees, the otoconia in the short arm side will fall into the utricle, and the nystagmus disappears after repeated diagnostic test, and the short arm type canalolithiasis can be distinguished.
Traditionally, cupulolithiasis are distinguished by the duration of nystagmus. It is considered that the duration of nystagmus more than 1 minute is cupulolithiasis, but it may also be short arm type canalolithiasis.
Previous studies have analyzed nystagmus through deep learning method to locate otoliths, but the instability of nystagmus characteristics induced by Dix Hallpike test and the inaccuracy of interpretation label will seriously affect the effectiveness of deep learning model.
There is an urgent need to design a new posterior semicircular canal BPPV diagnostic test to overcome the above-mentioned shortcomings.
The shape of the semicircular canal is complex, and its rotation in three-dimensional space is particularly complicated. Although turning the plane's rotation problem into the plane's normal vector rotation problem is conducive to calculation and understanding, its rotation schemes are diverse, and it is difficult to accurately implement the angle in freehand operation.
It can be found that when the posterior semicircular canal is rotated to the sagittal plane, the otolith movement of the diagnostic test mainly occurs in the lower arm of the posterior semicircular canal. The most important rotational movement is rotation around the Z axis, and subsequent movements around the Y axis will not cause too much influence on the sliding of the otoconia.
According to the traditional custom, in order to facilitate operation and control, it is feasible to simplify the operation as rotation 45 ° to the right to rotate the right posterior semicircular canal parallel to the sagittal plane.
Forward bending makes the starting position of otoconia consistent, and the fault tolerance is very good. 30 ° forward bend is enough for the otoconia to slide to the opening of the ampulla, and 70 ° forward bend is enough for the otoconia to slide to the bottom of the crista ampullaris.
For easy operation and control, it can be set to reach the prone position. If conditions restrict the operation, it can be set to bend forward 45°.
Leaning back to induce the otoconia move away from the ampulla is the most critical step of the diagnostic test, but it is required that the free otoliths in the contralateral utricle will not fall into the common crus to enhance the specificity of the diagnostic test.
In order to facilitate operation and control, it can be set to return to the sitting position and then lean back 45°,but not more than 70° is effective. At this time, the otoconia slides down the lower arm of the posterior semicircular canal, which still has obvious hydrodynamic effects.
The backward angle has a large range of variation, and the fault tolerance is strong.
After returning to the sitting position, continue to bend forward to make the otoliths in the ipsilateral posterior semicircular canal move toward the ampulla, but the crista ampullaris is at the null point.
At this time, there was reverse nystagmus for otoconia in the long arm side, there was no obvious nystagmus for otoconia in short arm side or otoconia adherent to cupula.
Since the crista ampullaris is more sensitive to angle, the forward angle must be accurately calculated. It is necessary to bend forward 50.5° after returning to the sitting position, which can be simplified as bend forward 45°.
A new posterior semicircular BPPV diagnostic test is designed to make the induced nystagmus have the characteristics of long latency, reversal and repeatability, will not cause the inhibitory stimulation of the contralateral superior semicircular canal, and has good operation fault tolerance, which is of great value for clinical and scientific research.