An anatomical evaluation of variations in the terminal branching pattern of facial nerve

Abstract               Background: Facial nerve is the seventh cranial nerve which supplies all the muscles of facial expressions by its five terminal branches –temporal, zygomatic, buccal, marginal mandibular and cervical. In surgeries like rhytidectomy, bro uplifting procedures, tumors of the parotid gland and nerve transfer procedures, conservation of all these terminal branches is necessary. Present study is regarding the variations in the terminal branching pattern of the facial nerve. Materials and method: study was carried out on total 18 parotid regions of cadavers. Piece meal dissection of gland is carried out and nerve is traced upto its two divisions- temporofacial and cervicofacial. Results: as per Devis classification, we categorized branching pattern into six types. Our findings are – type I-5.5%, type II- 5.5%, type III-22.22%, type IV-16.66% and type VI-44.44%. We had not found any case of type V pattern. Conclusion: There are so many variations in the branching pattern of the facial nerve. It is necessary for the surgeon to be aware of such probable variations forthe right surgical approach in order to conserve the terminal branches and avoid post-operative morbidity like facial nerve paralysis.

Key Word: facial nerve.

INTRODUCTION

Facial nerve is the seventh cranial nerve. It is the nerve of second pharyngeal arch. It supplies all the muscles of facial expression that’s why also called as ‘smiling nerve’. Facial nerve emerges from the surface of pons. It passes through internal acoustic meatus and travels through the facial canal. It then exits through stylomastoid foramen. It enters the parotid gland from its posterior border and passes between superficial and deep part of the gland winding around isthmus. Within the gland, it divides into two trunks temporofacial and cervicofacial. Temporofacial trunk further divides into temporal and zygomatic branch while cervicofacial trunk divides into buccal, marginal mandibular and cervical branch. These different branches emerge out from the anterior border of the gland and supply different group of facial muscles.^1-4 Normally one eachtemporal, zygomatic, buccal, marginal mandibular and cervical branch is there. Sometimes buccal branch divides into upper and lower buccal branches. Surgeons have to preserve all these branches during surgeries of parotid region like facial rhytidectomy, bro uplifting procedures.⁵ Variations in the number of these terminal branches of facial nerve is not uncommon. Sometimes even communications between different branches present. In order to have functional conservation of facial nerve, in parotid surgeries, surgeons should be aware of possibility of such variations to avoid post-operative morbidity like facial nerve paralysis⁶ In case of tumors of parotid gland, the conventional approach is anterograde parotidectomy in which the main trunk of facial nerve is identified and subsequent meticulous dissection is carried out for the precise removal of the tumor saving all terminal branches of facial nerve.⁷ Present study is dissection based cadaveric study of terminal branching pattern of facial nerve and its associated variations carried out on 18 parotid regions.

MATERIALS AND METHOD

The present study was carried out on total 18parotid regions. We did piece meal dissection of parotid gland where in parotid region was dissected carefully preserving emerging branches of facial nerve. We traced facial nerve branches till temporofacial and cervicofacial trunk between the superficial and deep part of the parotid gland. Total number of temporal, zygomatic, buccal, marginal mandibular and cervical branches were noted. When buccal branch was divided into upper and lower buccal, it was separately mentioned and communications between them if present were noted. Communications of temporofacial and cervicofacial trunk were observed and as per Davis⁸ classification, we classified these terminal branching patterns of all the cases into six categories. We observed relation of marginal mandibular branch (superficial or deep) with the facial artery.

OBSERVATIONS AND RESULTS

In our study, we followed classification given by Davis⁸ et al for categorization of terminal branching pattern of facial nerve. Devis⁸ classified terminal branching pattern of facial nerve into six categories. As per Davis⁸ classification our results are

Table 1:  Categorization of cases as per Devis⁸ classification

Legend

Figure 1: Branching pattern type I; Figure 2: Branching pattern type II; Figure 3: Branching pattern type III; Figure 4: Branching pattern type IV; Figure 5: Branching pattern type VI; Figure 6: Branching pattern type VI; Figure 7: Branching pattern type VI; Figure 8: Branching pattern type VI

DISCUSSION

As per Devis⁸ classification, When compared with previous studies the distribution is as follows  I Table number 2

Table 2: Distribution of cases as per Devis⁸ classification in various studies

Figure 1:  showing distribution of cases in different studies as per Devis⁽⁸⁾ classification

In most of the studies (Devis⁸, Park and Lee⁹, Bernstein¹⁰, Katz¹¹, Mynt¹² and Gataa and Faris¹⁵⁾ the maximum terminal branching pattern was type III, where only single anastomotic branch was present between temporofacial and cervicofacial trunk. Maliq¹⁴, Khaliq⁶, Singh¹⁶ and Thuku¹⁷ had found maximum number of cases as normal pattern without any anastomosis which is regarded as type I in the classification. In our study we got maximum number of cases (44.44%) showing complex anastomosis between different branches of temporofacial and cervicofacial trunk which is type VI in the classification. The difference in the findings of study may be due to difference in the study population. Kwak HH(18) classified the terminalbranching pattern of facial nerve into four types depending upon the origin of buccal branches-

Table 3: Categorization of cases as per Kwak’s classification

Kwak HH¹⁸ found that maximum times buccal branch was arising from two main divisions and it was interconnected to the zygomatic branch which is type II as per the classification. Our study also is in agreement with this finding. we also found maximum number of cases (33.33%) belonging to type II. There were four cases which showed combination of type II and type III where buccal branch was arising from two divisions and it was interconnected to the zygomatic branch plus received one nerve twig from the mandibular branch. Such cases accounted for 22.22% of total number of cases. We found variations in the number of temporal branches. In maximum number of cases (28%) the number of temporal branches were 4. Only 3 cases (17%) showed single temporal branch. Hwang K ⁽¹⁹⁾ in his article mentioned that he found 4 temporal branches in maximum number of cases. As per author highest order of ramification of temporal branch was 7. We also found variations in the number of zygomatic branches. Out of total 18 cases, single zygomatic branch was found in 6 cases (33%) while 7 cases (39%) showed 2 zygomatic branches. Maximum branch order was 5 which was found in 3 cases. We found single marginal mandibular branch in all cases. While Saylam C²⁰ had reported two ramifications of this branch in 4% of cases while Karapinar²¹ in his study found such two branches in 63.6% of cases. In all the cases we studied, we observed that marginal mandibular branch was superficial to facial artery and vein.This finding was in agreement with the finding by Liu AT²².Karapinar²¹found two cases where marginal mandibular branch ran between facial artery and vein.Gossain A K²³ found interconnections between zygomatic and buccal branches in over 70% of cases while we found such connections in 55.55% cases. Gossain K²³ also found interconnections between temporal and marginal mandibular branches of facial nerve in 15% of cases. We had not found any such interconnections between temporal and marginal mandibular branch. As per Saylam C²⁰, mandibular branch is the most vulnerable to surgical injuries among all the branches of facial nerve due to its location. Woltmann M²⁴ mentioned in his article that in surgeries of submandibular region, an incision 3 cm below the inferior margin of mandible as a submandibular incision is expected to reduce the risk of neuropraxia of the marginal mandibular nerve. Knowledge of anatomy of marginal mandibular nerve and its probable variations is especially useful in surgeries related to submandibular region like rhytidectomies.²⁵ All these findings indicate that zygomatic, upper and lower buccal and marginal mandibular branches are in intimate relations with each other and with the retaining ligaments of face. So while release of these ligaments like zygomatic ligament or masseteric ligament or mandibular ligament in surgeries, surgeons should be aware of such probable interconnections. The presence of interconnections between the branches reduces the risk of facial palsy after transection of terminal branch.²⁶ We found less interconnections or anastomosis between lower branches of facial nerve than the upper branches. This is in agreement with the findings of Sadina rana⁶, Nicoli F²⁷ and Khanfour AA²⁸.

CONCLUSION

There are so many variations in the terminal branching pattern of facial nerve. Facial nerve paralysis is the stressful complication of such surgeries and damage to the nerve has both functional and communal consequences to the patient.²⁹ So the knowledge of variations in the pattern will help the surgeon in deciding the right surgical approach in various surgeries like facial reconstructive surgery, neck dissection, nerve transfer procedures for conservation of the facial nerve. It is also useful for understanding the pathophysiology of various cranial, skull base and neck disorders.^30,31

REFERENCES