Pulsatile nystagmus secondary to semicircular canal dehiscence

Abstract

We report the case of a 69-year-old man with a history of mastoidectomy for cholesteatoma, who developed spontaneous spinning vertigo on debridement of his cavity. Subsequent CT confirmed a lateral semicircular canal fistula, which was surgically closed with mastoid cavity obliteration. Following surgery, he developed a spontaneous, pulse-synchronous horizontal pendular nystagmus. We discuss the pathophysiology of this rare clinical sign.

Background

Nystagmus is an involuntary oscillation of the eyes. It can be unilateral or bilateral, with the direction of movement vertical, horizontal or torsional. It can be a jerk nystagmus during which slow pursuit eye movement is followed by a faster corrective phase, pendular nystagmus, in which both phases are of constant speed, or mixed nystagmus which is a variation between the two.¹

Cholesteatoma/mastoid cavity keratin can result in erosion of the otic capsule and dehiscence of the semicircular canals (SCC).² We report an unusual pattern of nystagmus following mastoid surgery in the presence of lateral SCC fistula.

Case presentation

A 69-year-old man presented with hearing loss. He had a history of previous right modified radical mastoidectomy for cholesteatoma, 15 years prior to presentation. On examination, he had dry keratinous debris in his right mastoid cavity. On manipulation of the keratin in his cavity, he experienced nystagmus. The audiogram revealed air conduction thresholds of 60 dB with a 35 dB air-bone gap. No further attempt was made to clean the mastoid cavity. A CT scan was requested which confirmed the presence of a right lateral semicircular canal fistula (figure 1). Revision mastoidectomy was carried out. Operatively, cholesteatoma/keratin was carefully elevated from the lateral SCC fistula and repaired with bone dust, tragal cartilage and temporalis fascia. Reconstruction was carried out with obliteration of the mastoid cavity with bone dust and a pedicled temporalis fascia flap. The ear canal was packed with BIPP impregnated ribbon gauze.

Figure 1.

Axial CT of the right temporal bone showing the right lateral semicircular canal fistula prior to surgery.

Two days postoperatively, the patient complained of spontaneous spinning vertigo. Examination revealed a spontaneous, horizontal, pendular nystagmus that was synchronous with his pulse (Video 1). The rest of the examination revealed a conductive hearing loss due to fluid and packing occupying the middle ear. No further testing of his vestibular system was performed due to the early postoperative nature of his symptoms.

Video 1.

Download video file ^{(1.8MB, mp4)}

DOI: 10.1136/bcr-2018-225963.video01

Spontaneous horizontal pendular nystagmus following surgery.

Outcome and follow-up

The patient was treated with prochlorperazine to reduce his symptoms. At the 10-day postoperative review, the patient’s symptoms of dizziness had significantly improved and there was no further nystagmus evident. The packing was removed from his ear. The ear canal was healing satisfactorily. Subsequent review at 12 months after surgery revealed a small volume self-cleaning cavity with hearing at similar levels to preoperative levels. The patient’s head impulse test revealed reduced function of the right lateral SCC.

Discussion

This patient presented with an unusual form of nystagmus following closure of his lateral SCC fistula.

The vestibulo-ocular reflex helps maintain the gaze in space on movement of the head. It compensates movements of the head by equal movement of the eyes in the opposite direction. During rotational movement, the relative inertia of the endolymph within the SCCs of the inner ear causes relative motion of the endolymph in the opposite direction to the movement. This in turn moves the cupula within the ampulla of the SCCs causing the deformation of stereocilia of the hair cells. The deformation of the stereocilia generates altered activity of the vestibular nerves. Impulses via the vestibular nerves are transmitted to the vestibular nuclei and from there to the extraocular muscles via the oculomotor nuclei, stimulating compensatory movement of the globe.³ Dehiscence to the bony structure of the SCCs can allow pressure from the middle ear, brain or, in this case, the mastoid cavity to be transmitted directly to the endolymph of the SCCs. This causes movement of the cupula and bending of the stereocilia of the hair cells. Ewald’s first law states that eyes move in the plane of the stimulated canal. Pressure transferred via a lateral canal dehiscence would therefore cause a horizontal nystagmus. In this case, we hypothesise that pulsating vascular healing tissue in the obliterated mastoid cavity resulted in transfer of pressure to the endolymph causing a horizontal pendular nystagmus synchronous with the patient’s pulse. An alternative explanation is that venous pulsations were transferred from the subarachnoid space in the posterior cranial fossa to inner ear fluids via the cochlea aqueduct. This is less likely, in view of the fact that the nystagmus resolved after a few weeks, presumably as the granulation tissue underwent fibrosis. Rambold et al report a similar case of a patient with cholesteatoma who developed spontaneous horizontal pulsatile nystagmus following mastoidectomy and was subsequently found to have a lateral SCC fistula. The authors performed 3D search coil recordings of the nystagmus and confirmed that the velocity vectors of the nystagmus aligned with the axis of the affected canal, consistent with Ewald’s first law. The study focused on analysing the jerk movements with no follow-up on patient improvement.⁴ They also noted granulation tissue overlying the area of the fistula on postoperative CT scans, supporting their theory of transfer of vascular pulsations to the inner ear fluids. There have been additional reports of a pulsatile torsional nystagmus in patients with superior canal dehiscence.^{3 5 6} In addition, experimental animal models have demonstrated that micromechanical indentation of the lateral SCC of the oyster toadfish causes cupular deformation and movement of hair bundle stereocilia, leading to an increased firing rate within the canal.⁷

In our case, the nystagmus resolved after 10 days. This may be explained by the fibrosis of the mastoid cavity and reduced vascularity as the healing process progressed. The fibrous tissue will be less vascular with reduced pressure transfer to the SCC. In addition, possible fibrosis within the lateral SCC (as noted with reduced head impulse test) would reduce its function.

Learning points.

• Pulsatile nystagmus can occur in the early postoperative period in patient undergoing mastoid surgery in the presence of lateral semicircular canal (SCC) fistula.

• The nystagmus resolves spontaneously.

• It is evident that there may be a reduction in the function of SCC following the resolution of nystagmus.