A priori there is nothing to tell us if a patient telling us about a positional vertigo has developed canalolithiasis of the external canal. Only the appearance of a horizontal nystagmus during provocation manoeuvres tells is that the lithiasis is located in the horizontal canal.
Unfortunately, the direction of the nystagmus does not indicate its location. In fact, the nystagmus could be either geotropic or ageotropic.
Explanation: It should be remembered that the horizontal canals work complementarily to each other. Similarly, priority information is that stemming from the movement of the cupula towards the utricle.
In a position with the head turned to the side at a 45° angle or more, which can be produced with a DIX and HALLPIKE provocation manoeuvre, there are a number of possibilities:
- If the nystagmus is geotropic, this means that the amalgam is in the posterior part of the canal. For an amalgam located in the lowest canal, it descends into the lowest part of the arch, and as such pushes the cupula towards the utricle. BUT, the same nystagmus will be produced by an amalgam located in the posterior part of the highest canal, migrating towards the exit of the canal and producing a deflection of the cupula towards the branch of the canal.
- If the nystagmus is ageotropic, with the same head position, in other words, the amalgam is in the anterior half of the canal. This positions it close to the ampulla of the lowest canal, and migrating towards the lower portion of the arch will cause the cupula to move towards the canal. Similarly, if the amalgam is in the anterior part of the canal of the highest ear; the amalgam, as it migrates towards the ampulla, is going to push the cupula towards the utricle producing a nystagmus beating towards the top ear.
- In the latter scenario, sometimes the migration of debris is such that it is deposited onto the cupula, causing a nystagmus that will not fade away while this position is maintained. The movements of both debris and cupula result in cupulolithiasis.
Once we have observed the direction of the nystagmus, it is a good idea, in order to determine which ear is causing it, to bring the subject back to a sitting position, to bend their head forward until the horizontal canals are on a vertical plain. In this case, and only in this position, the debris is going to migrate, pushing the cupula towards the utricle and provoking a nystagmus beating towards the side of the affected ear. We wait for the nystagmus to fade away.
Armed with the knowledge that the utricular exit from the horizontal canal is smaller that that of the crus commune: the effect of gravity on the debris will not be enough to ensure its migration and to empty the canal. An endolymphatic flux must be produced to expel the debris from the canal. To do this we are going to carry out a “barbecue spit” manoeuvre that requires the participation and the coordination of both the practitioner and the patient.
Technique: to better understand the therapeutic manoeuvre, let us take the example of a VP of the left horizontal CSC. We have been able to determine the side by the observation of a left horizontal nystagmus with the subject seated, facing upwards so that the horizontal canal is in the vertical position.
Explanation: In this position, the migration of the debris, under the effect of acceleration due to gravity, will be in the direction of the ampulla. During said migration, the cupula will lean towards the utricle. The movement of the cupula towards the utricle corresponds to the physiological direction and will cause a nystagmus on the same side as the solicited cupula.
Starting off from the sitting position, with the head facing down, we are going to ask the subject to turn towards the right before lying down on their right hand side. Throughout this movement, the head is maintained in an upward-facing position. Once lying down, they are asked to lie down on their front while the practitioner ensures that the head is always in the same position (this time downward facing).
Explanation: to be able to carry out the “barbecue spit” manoeuvre, the subject must be lying down. To be able to use the force of inertia of the debris and to expel them from the canal, we must add an additional movement of the endolymph. The movement of endolymph makes the ampulla move towards the opening at the exit of the canal. During the transition from the sitting position to the lying down position, the debris must remain grouped together and in close proximity to the ampulla.
Now we come to the delicate stage of the therapeutic manoeuvre: we are going to ask the subject to continue with their “barbecue spit” movement until they are lying down on their left hand side. At the moment when the body movement causes the head to turn, we carry out a swift movement of the head in the same direction as body movement. A movement that is going to bring the head from a downward facing position to a 45° position towards the left and upwards.
Explanation: the rotation of the head is going to cause the required endolymphatic flow. Once in the final position, the migration of the debris will take place in sync with the endolymphatic flow causing a nystagmus beating in the opposite direction to the initial nystagmus.
We are now going to focus exclusively on the nystagmus that is about to appear: it is going to beat towards the right. As soon as the frequency of the nystagmus begins to fade, we are going to turn the head at a 15° angle towards the right. As this is carried out, we are going to see a rise and then a fall in nystagmic frequency. Having made this new observation, the head is then turned an additional 15°, and then again, until it has been moved 30° to the right, but no more.
Explanation: the SCC, as the name indicates is semicircular. Depending on the position of the head, the slope of the canal changes. Upon reaching the end position following rotation of the head, the debris is in the vertical part of the canal, at the end of which the slope falls off. The speed of migration of the debris must be maintained along the whole length of the canal. A fall in speed of the movement of the debris is reflected by a fall in the frequency of the nystagmus. Each small additional 15° movement increases the slope of the canal and allows us to maintain a constant speed since the debris is located in a part of the canal which is still vertical, all the way to the exit. The reason why the 30° angle should not be surpassed is simply a safety measure to ensure that the debris that is "lagging behind" do not head of towards the ampulla, aided by the favourable direction of the slope of the canal.
The subject is left in this final position for ten minutes.
The subject is then put back in the sitting position and asked not to allow their head to lean forward. They can raise their head as much as they like but full flexion is forbidden. They are asked to sleep flat on their backs with a pillow on either side of the head so as not to turn to one side or the other.
As with VP of the posterior canal, the patient is asked to come back in a week for a check-up.
The success rate of the manoeuvre is comparable to that of the posterior vertical canal.
One might wonder about the existence of VPs of the horizontal canal while the natural position to empty the horizontal canal is decubitus, lying down. The answer is simple: the vast majority of subjects who develop a VP of the horizontal canal sleep procubitus, lying down on their front. The remaining minority are subjects who have had a VP of the posterior canal and who were not treated properly or who did not follow the instructions they were provided following the manoeuvre.
There are a number of causes behind the increase in the subject population presenting with VPs of the horizontal canal:
- lack of knowledge about the existence of this disorder and above all lack of knowledge about its treatment. Or even the rumour that is generally taken as the truth that there is no effective lifesaving manoeuvre. For many years, an atypical positional nystagmus was believed to be caused centrally. It was best to simply leave it alone.
- the other reason, which we believe to be even more serious is the upsurge in manoeuvres carried out by unskilled individuals, without any former training. We also hear stories about patients who ask for a consultation only after having tried to treat themselves. The consequences of such irresponsible behaviour can sometimes be quite dramatic. Instead of a manoeuvre whose effectiveness is widely acknowledged, it can take days or even weeks of immobility or vestibular reeducation sessions that should never have been needed if everything had been carried out according to the rules. Ever tried asking a patient to try to put their own dislocated shoulder back in place?