|ORIGINAL CONTRIBUTION: BREAKING FRONTIERS IN OTORHINOLARYNGOLOGY
|Year : 2015 | Volume
| Issue : 2 | Page : 60-63
Significance of facial canal labyrinthine segment diameter in the pathophysiology of Bell's palsy
AK Rai1, P Lal1, G Motwani1, BB Thukral2, S Goel1
1 Department of Otolaryngology, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi, India
2 Department of Radiodiagnosis, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi, India
|Date of Web Publication||28-Dec-2015|
Department of Otolaryngology, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi
Source of Support: None, Conflict of Interest: None
Aims and Objectives: To analyze whether there is a significant difference in the width of the labyrinthine segment of the facial canal between the affected and the unaffected sides in cases of unilateral Bell's palsy, and also to ascertain whether there is any correlation between the degree of facial nerve degeneration and the width of the facial canal.Study Design: Prospective observational study. Setting: Tertiary care university hospital. Material and Methods: A total of 30 patients with unilateral Bell's palsy underwent high-resolution computed tomography with 1-mm-thick contiguous axial sections. The width of the labyrinthine portion of the facial canal at its proximal end (meatal foramen) and at the mid-labyrinthine portion was assessed on both the affected and the unaffected sides and was compared using the paired t tests. Nerve conduction studies were done to assess the degree of facial nerve degeneration. The percentage of facial nerve degeneration was compared with the width of the labyrinthine portion of the facial canal at both the aforementioned sites using the t tests of significance. Results: The width of the labyrinthine segment, both at the meatal foramen and at the mid-labyrinthine part, was significantly different on the affected side as compared with the unaffected side. The degree of facial degeneration was assessed using nerve conduction studies. Majority of the patients with less than 50% degeneration recovered within 1 month, and by 2 months, 100% of patients recovered, including those with more than 75% degeneration. The compound action potentials did not correlate significantly with the width of the facial canal. Conclusion: The width of the labyrinthine segment of the facial canal may be a potential predisposing factor in the etiopathogenesis of Bell's palsy. The degree of facial nerve degeneration, however, did not correlate with the narrowing of the labyrinthine segment of the facial canal on the affected side in Bell's palsy.
Keywords: Bell's palsy, high resolution computed tomography, nerve conduction studies
|How to cite this article:|
Rai A, Lal P, Motwani G, Thukral B, Goel S. Significance of facial canal labyrinthine segment diameter in the pathophysiology of Bell's palsy. Astrocyte 2015;2:60-3
|How to cite this URL:|
Rai A, Lal P, Motwani G, Thukral B, Goel S. Significance of facial canal labyrinthine segment diameter in the pathophysiology of Bell's palsy. Astrocyte [serial online] 2015 [cited 2023 Sep 22];2:60-3. Available from: http://www.astrocyte.in/text.asp?2015/2/2/60/172685
| Introduction|| |
Bell's palsy accounts for 60%–75% of all cases of sudden-onset unilateral facial paralysis  and has an annual incidence of 20–30 cases per 100,000 individuals. Although the etiological basis of this common condition has been extensively studied, there is still no definite consensus on this subject. Various causes have been hypothesized, including microcirculatory failure of the vasa nervosum, ischemic neuropathy, viral infections with herpes simplex type 1 (HSV 1), and immunological reactions. The pathophysiological correlate for all these etiologies is edema of the facial nerve. The edema of the facial nerve in a rigid noncompliant bony facial canal may lead to damning obstructionof the axonal flow and compromise of the neural vasculature, and subsequent neural ischemia and axonal degeneration.
To further understand whether there are any predisposing factors that make a nerve susceptible to compression by edema, we conducted this study to assess the diameter of the bony facial canal in cases of Bell's palsy on both the affected and unaffected sides with the help of high-resolution computed tomography (HRCT) and observe whether any anatomical differences in the size of the bony canal predispose a particular side to this pathology. Additionally, an attempt was made to assess whether the degree of facial nerve degeneration in a case of Bell's palsy was related to the diameter of the fallopian canal.
| Material and Methods|| |
The study was conducted at a tertiary care university hospital during the period August 2011 to March 2013. All patients with unilateral Bell's palsy were included in the study, after obtaining consent from the patient and obtaining approval of the institutional review board. A detailed history and clinical examination ruled out any cause for facial palsy. The facial paresis/palsy was graded using the House and Brackman system [Table 1].
All patients underwent HRCT of the temporal bone with 1-mm-thick contiguous axial sections. The labyrinthine portion of the facial canals was measured in each patient—both at the meatal foramen (junction of the internal auditory meatus and the fallopian canal) and at the mid-portion of the labyrinthine canal, on both the sides. The diameters of the facial nerve canal on both the affected and the unaffected sides were measured using the paired t tests.
Facial nerve conduction was measured using the Viking Quest four-channel system. The electrode was placed just outside the stylomastoid foramen. For recording, the surface electrode was placed on the nasolabial fold. The compound action potentials on both the affected and the normal sides were recorded. The ratio of the compound action potentials on the affected and the unaffected side (n) was calculated.
Facial nerve degeneration = (1 − n) × 100%
The compound action potentials generated were compared with the measurement of the labyrinthine segment of the facial nerve both at the meatal foramen and at the mid-labyrinthine segment using the paired and unpaired t tests.
| Results|| |
A total of 30 patients were included in the study. The patients were in the age range of 11–60 years, with the maximum patients in the age-group of 21–30 years (n =12). Of these 30 patients, 19 were male (63%) and 11 were female (37%). Right-sided palsy was observed in 12 patients (40%), whereas left-sided palsy was observed in 18 (60%). On otoscopic examination, 73% of all patients (n = 22) showed congestion of the chorda tympani nerve.
Facial palsy in all patients was graded in accordance with the House–Brackman grading system; it ranged from grade 2 to grade 4, with maximum patients having grade 4 palsy (n = 25).
On HRCT of the temporal bones, the mean width of the meatal foramen on the affected side was 1.08 mm (range, 0.56–1.95 mm), and on the unaffected side, it was 1.23 mm (range, 0.7–1.54 mm; P = 0.001). The width of the mid-labyrinthine portion on the affected side ranged from 0.86 to 1.98 mm (mean, 1.2 mm), and on the unaffected side, it ranged from 1.06 to 1.77 mm (mean, 1.42 mm; P = 0.0003).
Nerve conduction studies were done and compound action potentials were recorded at the orbicularis oculi and orbicularis oris muscles. In both the muscles, both facial nerve conduction latency and duration (in milliseconds) were observed to be increased [Table 2] and [Table 3] on the affected side [Table 2] and [Table 3], while the amplitude was reduced on the affected side.
|Table 2: Facial Nerve Conduction Study Findings for the Orbicularis Oris Muscle|
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|Table 3: Facial Nerve Conduction Studies for the Orbicularis Oculi Muscle|
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In all, 50% of the patients had 50%–75% degeneration of the facial nerve; 30% had less than 50% degeneration; and 20% had more than 75% degeneration. It was observed that 88.9% (n = 8) of the patients with less than 50% facial degeneration recovered within 1 month of presentation; 26.7% (n = 4) with 50%–75% facial degeneration recovered within 1 month and 73.3% (n = 11) recovered within 1–2 months; and 100% with more than 75% degeneration recovered within 1–2 months.
When using paired and unpaired t tests to correlate the temporal bone HRCT findings and compound action potentials, the P value was not observed to be significant both at the meatal foramen (P = 0.46) and at the mid-labyrinthine segment (P = 0.9).
| Discussion|| |
The fundamental factors in the pathogenesis of Bell's palsy are inflammation and edema of the facial nerve along its intratemporal course. The nerve entrapment secondary to edema would probably tend to occur in the labyrinthine segment, as this segment is the narrowest part of the fallopian canal,,, especially at the entrance (meatal foramen), where the nerve fibers are constricted by a fibrous ligament., Also, there are no anastomosing arterial arcades in this area, and the nerve and the accompanying vessels occupy a greater proportion of the bony canal as compared with other segments of the facial nerve. All the aforementioned features make the labyrinthine part of the facial nerve most vulnerable to ischemia.
Hence, in this study, the authors chose to measure the most vulnerable labyrinthine segment [Figure 1], both at the meatal foramen and at the mid-labyrinthine part, and observe whether a difference existed in the canal width between the affected and the unaffected segments in cases of Bell's palsy. This study showed a significantly smaller fallopian canal both at the meatal foramen and at the middle part of the labyrinthine segment of the facial canal on the affected side. Similar findings were documented by Kefalidis et al., who did a prospective clinical study on 25 cases of unilateral Bell's palsy and followed them up with HRCT of the temporal bone.
|Figure 1: Image of the axial section of high-resolution computed tomogram of the temporal bone showing the labyrinthine segment of the facial nerve (marked f). IAC – Internal Auditory canal, A – aditus ad antrum.|
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Murai et al. also measured the cross-sectional area of the facial nerve canal in patients with unilateral Bell's palsy and found that the area was significantly smaller on the affected side as compared with the unaffected side.
Our findings, however, differ from those previously published by Vianna et al., where no difference was found in the diameter of the facial canal on both the affected and the unaffected sides, suggesting that there is no anatomic predisposing factor for this condition.
Nerve conduction studies provide objective quantitative assessment of the facial nerve function, and the magnitude of the compound action potential represents the synchronous discharge of a group of muscle motor units resulting from supramaximal stimulation of the facial nerve. The reduction in the amplitude of the action potentials on the affected side, when compared with the normal side, is believed to reflect the number of fibers that have undergone Wallerian degeneration.
In Bell's palsy, due to inflammation, edema, and subsequent entrapment of nerves in the labyrinthine portion of the facial canal, there is nerve injury. Many studies in the past have shown a positive correlation between the nerve conduction studies and the final outcome in Bell's palsy. In the study by Yasukawa et al., 80% of the patients with less than 90% degeneration of the affected facial nerve recovered satisfactorily within 4 months. In the study by Wang et al., 83% of the patients with less than 90% loss of electroneurographic response had complete recovery, while 70% of those with more than 90% loss had incomplete recovery at 6 months after the onset of Bell's palsy.
In our study, we found that the latency and duration of nerve conduction were increased in the muscles on the affected side, while the amplitude of nerve conduction was reduced on the affected side. Those patients with less than 50% degeneration recovered faster within 1 month, while those with more than 75% degeneration recovered within 1–2 months. Therefore, the nerve conduction studies were useful in predicting the duration of recovery in Bell's palsy patients. However, we did not observe any significant correlation between the nerve conduction studies and the width of the fallopian canal on temporal bone HRCT. Also, it would be pertinent to point out that this study had limited number of subjects, and a larger study may yield more robust results.
| Conclusion|| |
The results of this study offer evidence that Bell's palsy usually coincides with the narrower fallopian canal of the patient, especially in the region of the meatal foramen and the labyrinthine segment. This anatomical deviation, thus, is a probable predisposing factor for the development of Bell's palsy.
The nerve conduction studies in patients with Bell's palsy showed it as an objective tool to help in prognosticating recovery. However, there was no significant correlation between the compound action potentials generated and the degree of degeneration of the facial nerve when compared with the size of the labyrinthine canal.
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Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3]