Incidence and Possible Predictive Factors of Facial Nerve Paralysis after Superficial Parotidectomy for Benign Tumours: Our Experience

Abstract

Salivary gland tumors represent 3–10% of all head and neck neoplasms. Most of the tumours are benign with parotid gland being most commonly affected. Surgical intervention in the form of parotidectomy forms the mainstay of treatment. Among the various postoperative complications that may occur after parotidectomy, facial nerve weakness is the most dreaded one for both the surgeon as well as the patient. In the literature, the incidence of transient facial weakness after parotid surgery ranges from 10 to 68%, and long-term dysfunction ranges from 0 to 19%. Apart from injury mechanisms such as nerve division, stretch, thermal injuries, ischemia, several other risk factors such as tumour size, disease duration etc. have also been identified for post parotidectomy facial nerve dysfunction which vary considerably from study to study. Thus, facial nerve injury despite being a common and dreaded complication is also a preventable one. We in our institute conducted a retrospective study from January 2018 to December 2021 to evaluate the incidence of facial nerve weakness and possible predictive factors among 60 patients who underwent superficial parotidectomy for benign tumours and found that when it comes to benign tumours surgical technique rather than tumour factors are more important in preventing nerve injury. Undertaking this study was important in order to emphasize the importance of extensive preoperative planning and better surgical practices among budding head and neck surgeons in our country.

Introduction

Salivary gland tumors represent 3–10% of all head and neck neoplasms [1]. The parotid gland is most commonly affected, ranging between 36.6 and 83%. Bulk of the tumours are benign with pleomorphic adenoma being most common comprising 45 to 60% of all salivary gland tumours. Malignant tumors are relatively rare, occurring in 15–32% of cases with mucoepidermoid carcinoma being the most common malignant tumour seen [1, 2].

Approximately 80% of all pleomorphic adenomas occur in the parotid gland mostly arising from the inferior pole of the superficial lobe [3]. Surgical intervention in the form of parotidectomy is the mainstay of treatment. Parotidectomy was first introduced into the world literature by Berard in 1823 who removed a parotid tumor of 8 years’ duration. Since then a number of different surgeries have come up such as superficial parotidectomy, subtotal parotidectomy, and total parotidectomy [4].

Among the various postoperative complications that may occur after parotidectomy, the well documented ones include facial nerve paresis or paralysis, salivary fistula, Frey’s syndrome, infection, and recurrence of the tumour. Among these, facial palsy is the most dreaded complication for both surgeon as well as the patient. In the literature, the incidence of transient facial weakness after parotid surgery ranges from 10 to 68%, and long-term dysfunction ranges from 0–19%⁵.

Facial nerve injury mechanisms during parotidectomy include nerve division, stretch, compression, ligature entrapment, thermal and electrical injuries, and ischemia [5].

Several risk factors shave been identified for post parotidectomy facial nerve dysfunction which vary considerably from study to study.

Anatomic Consideration

The parotid gland is a bilateral salivary gland located in the face.

It lies within a deep hollow, known as the parotid region. The parotid region is bounded as follows:

• Superiorly – Zygomatic arch.

• Inferiorly – Inferior border of the mandible.

• Anteriorly – Masseter muscle.

• Posteriorly – External ear and sternocleidomastoid.

It produces serous saliva.The secretions of the parotid gland are transported to the oral cavity by the Stensen’s duct. It arises from the anterior surface of the gland, traversing the masseter muscle. The duct then pierces the buccinator, moving medially. It opens out into the oral cavity near the second upper molar. It can be divided into deep and superficial lobes, separated by the facial nerve .

The parotid gland and the facial nerve are very closely related. Both the structures grow at the same time during fetal development. As the facial nerve exits the skull and travels underneath the ear and towards the muscles of facial expression, the parotid gland continues to grow and ultimately wraps around the nerve. Once inside the parotid, the facial nerve travels deeper into the gland, with three-quarters of the gland being superficial to it. After a short distance inside the parotid, it divides into two major branches, called the superior and inferior divisions. Those two divisions then branch off even further into 5 terminal branches: temporal, zygomatic, buccal, marginal mandibular, and cervical and supply the muscles of facial expression. Additionally, the nerve also carries branches that progress further into the parotid and instruct the gland as to when it should produce and secrete saliva.

The purpose of our study was to evaluate the incidence of postoperative facial weakness related to superficial parotidectomy in benign tumours and identifying the possible predictive factors such as tumour size, disease duration, diagnosis, age etc. and our experience of using surgical magnification with surgical loupes to prevent facial nerve injury.

Materials and Methods

A retrospective study was conducted in the Department of Otorhinolaryngology and Head and Neck surgery of our institute for a period of 47 months from January 2018 to December 2021. Study included 60 patients who underwent superficial parotidectomy for benign tumors arising in the parotid gland. Malignant tumours and patients who had preoperative facial weakness were excluded. Patient was subjected to a thorough clinical history and examination. Based on disease duration patients were grouped under the following categories: <1 year, 1–5 years and > 5 years duration. A CT with enhancement along with fine needle aspiration cytology was obtained preoperatively. Senior surgeons having more than 10 years of head and neck surgery experience conducted the parotidectomies. surgical loops were used to magnify surgical fields during parotidectomy.

Tumor characteristics such as tumor size was determined. Tumor size was presented by the longest diameter of the tumor (cm) size. Tumor subsites were categorized into three compartments.

based on the surgical findings (anatomical relationship to the facial nerve) as follows: superficial to the facial nerve, deep to the facial nerve and tumors located in both the superficial and deep lobes crossing the facial nerve plane.

Surgical procedure:

1. A modified Blair incision was used for accessing parotid gland. Flap dissection was kept in the subcutaneous plane outside parotid capsule.

2. Tragal pointer was used as the landmark and antegrade dissection was performed to identify the facial nerve trunk (Fig. 1).

3. After initial identification of the nerve trunk peripheral branches were identified and parotid tissue was mobilised. Once the nerve trunk was identified diathermy was avoided. Hemostasis was performed with surgical ligatures. fine curved artery forceps were placed immediately above the nerve to carefully divide the bridging tissue to avoid injury to the nerve. Saline irrigation of the surgical field was done. After removal of the gland suction drain was placed making sure that it does not overlie the trunk or any branch of the facial nerve. Wound was closed in layers.

Fig. 1.

Intraoperative image from our institute showing temporofacial and cervicofacial branches of facial nerve after superficial parotidectomy

Tumor length:

Tumour length was evaluated into the following groups : <1.0 cm, 1.0–2.0 cm, 2.0–3.0 cm and > 3 cm (Fig. 2).

Fig. 2.

Image showing pleomorphic adenoma of the parotid gland after removal

Evaluation of facial nerve function:

The function of the facial nerve was evaluated using the House-Brackmann grading system using three sites: forehead, eye, and lip (Fig. 3). Any of the sites involved was regarded as facial weakness. The patient was evaluated at day 1 followed by weekly evaluation for one month and then at 6 months and at 12 months. The facial weakness of more than 6 months duration was considered as permanent palsy in our study.

Fig. 3.

House brackmann grading for facial nerve palsy

Results

A total of 60 patients were included in this study. 36 of the patients were males and 24 were females. The youngest patient was 35 years old and the oldest was 60 years old with an average age of 50.46 years. Since the study included benign tumours only, histopathology revealed that 48 patients had pleomorphic adenoma whereas 12 patients were diagnosed to have warthin’s tumour (Fig. 4). Among the patients with warthin’s, all were males and above 45 years of age.

Fig. 4.

% of various benign tumours found in our study

Among total patients, 21 patients had temporary facial weakness and 1 patient had permanent facial weakness. Lip deviation was the most frequently affected subsite in both temporary and permanent facial weakness. Among the 22 patients with facial nerve weakness, 13 had grade II weakness, 8 patients had grade III weakness whereas only one patient with permanent facial weakness had grade IV palsy. Post operatively patient was evaluated at day 1 followed by weekly evaluation for one month and then at 6 months and at 12 months. Patients found to have facial nerve weakness were started on steroid therapy. All 22 patients had weakness on day 1. 7 patients had facial nerve weakness after 2 weeks, 3 after one month of follow up and only 1 patient after 6 months who was termed to have permanent facial nerve palsy (Fig. 5).

Fig. 5.

Number of patients with facial nerve weakness after regular follow up

We grouped the patients under various categories based on disease duration and compared it with the incidence of facial nerve weakness in post-operative patients: patients with less than one year disease duration had 29.4% incidence of facial nerve weakness, those between 1 and 5 years was 41.3% and those with more than 5 years duration had 35% (Table 1). Thus, there was no significant correlation found between facial nerve palsy and disease duration in our study.

Table 1.

Correlation between disease duration and the incidence of facial nerve palsy

Patients were also grouped based on the size of the tumour and the incidence of facial nerve weakness: those with < 1 cm had 0% incidence, for 1–2 cm was 46.1%, between 2 and 3 cm was 44.4 and for > 3 cm was 35%. Thus, increasing size of the tumour did not contribute to an increased chance of facial nerve palsy in our study (Table 2). We also experienced that the use of surgical loupes significantly improved the vision and helped prevent injury to facial nerve branches.

Table 2.

Correlation between size of the tumour and the incidence of facial nerve weakness

Discussion

Parotid surgeries have always been challenging because of the intraparenchymal course of the facial nerve. This is especially true for benign parotid diseases as dissecting the facial nerve in close proximity to a benign tumor can be technically challenging more so because inflammatory lesions cause parenchymal fibrosis, which can make nerve dissection difficult [6].

We conducted a retrospective study in our institute where we evaluated 60 patients undergoing superficial parotidectomy over a 5 year period to evaluate the incidence and associated risk factors of facial nerve palsy during superifical parotidectomy for benign tumours in an effort to improve preoperative planning and better surgical practice.

We in our study found that among the benign tumours most common was pleomorphic adenoma (80%) followed by warthin’s tumour which is similar to previous studies in the past [7].

The exact mechanism of facial nerve palsy is still poorly understood However, Neural elongation might be the most probable factor. Peripheral nerves have been found to follow a peculiar curve making them more susceptible for stress-strain injuries causing mechanical rupture. Nerve elongation by approximately just 6% may cause perineurium tears disturbing the intrafascicular homeostasis and unrecoverable loss in the form of neurotmesis. In such an injury the nerve remains grossly normal but dysfunctional making it difficult for the surgeon to perceive the injury.In cases with less significant neural elongation there is no disruption of the endoneurium i.e. neuropraxia or axonotmesis, with complete recovery of the nerve function over time [8].

For this reason transient facial palsy remains the most common complication in parotid gland tumor causing distress both to patients and the surgical team. Upton et al. [9] in their study stated that temporary postoperative facial nerve weakness ranges between 18 and 65% and Nouraei et al. [10] in their study found 40% of patients with postoperative facial nerve dysfunction. Permanent palsy, according to various studies in seen in 0 to 19% of the cases [11]. Similar results were obtained in our study with 35% (21 out of 60) of the patients having transient facial palsy and 1.66% (1 out of 60) having permanent facial nerve paralysis.

The patient after the surgery may have normal facial nerve function on recovery from anesthesia, but it may subsequently deteriorate, before an eventual full recovery occus. This happens because the endoneural capillary endothelium gets impaired by anoxia and trauma of surgery. Following surgery, the endoneural capillaries become permeable, causing oedema to accumulate within the nerve. This usually takes few hours to develop and often days to resolve. More extensive compression or traction, as can occur during parotidectomy, will result in demyelination. This takes a few days to develop fully and several days or weeks before the nerve remyelination occus. Thus, this phenomenon of delayed-onset facial nerve weakness can be classified as grade 2 neuropraxia, and recovery from it is expected to occur within 4–6 weeks [12]. In our study however we found out that All 22 patients had weakness on day 1, 7 patients had facial nerve weakness after 2 weeks, 3 after one month of follow up and only 1 patient after 6 months who was termed to have permanent facial nerve palsy indicating that inflammation secondary to endoneural capillary injury could be the most probable mechanism at play with post-operative steroid therapy as a useful treatment option. This finding is similar to studies done in the past that show significant functional recovery for most patients within 3–4 months [7].

In our study marginal mandibular nerve was the mostly frequently involved branch similar to the studies in the past [13]. The possible reason behind this can be a thinner diameter, longer tracts embedded in parotid gland and fewer connecting anastomoses. We also observed that careful raising of the anterior flap during the surgery prevents injury to the marginal mandibular nerve as it runs forwards and anteriorly towards the angle of the mandible deep to platysma.

We in our study made an attempt to find out the possible predictive factors affecting the incidence of iatrogenic facial nerve injury. We compared the tumour size to the incidence of facial nerve weakness and found that the incidence of facial nerve weakness for tumours between 1 and 2 cm, 2-3 cm and > 3 cm was 46.1%, 44.4% and 40% which is not significant unlike the studies done in the past that show as the tumour size increases the likelihood of facial nerve weakness also increases [14]. We found similar results with disease duration and the incidence of facial nerve weakness which did not show a positive correlation indicating that when it comes to benign tumour surgical technique rather than tumour factor plays a more significant role in avoiding facial nerve injury.

Conclusion

Facial nerve injury during superficial parotidectomy remains a common complication. Thus, extensive preoperative counselling and proper consent of the patient is necessary. Most cases have a transient facial nerve weakness which improves over time and post-operative steroid therapy forms an important treatment option. We also found that when it comes to benign tumours surgical technique rather than tumour factors are more important in preventing nerve injury. Use of surgical loupes help in preventing damage to facial nerve branches.

Funding Information

No funding was received for this study.

Declarations

Conflict of interest

There is no conflict of interest between the authors.