Post-operative facial nerve palsy

Although the extent of facial nerve palsies was not included due to the heterogeneity of documentation, cases of post-operative complete facial nerve palsies were rare in the study cohort. Most of the patients where detailed assessment of the facial nerve were available had only partial nerve palsy. Facial nerve palsy after parotidectomy has been reported to occur in 16–63 per cent of cases.^{Reference Brennan, Moore and Shuler2,Reference Laccourreye, Brasnu, Jouffre, Cauchois, Naudo and Laccourreye3} Our incidence of 27 per cent is therefore at the lower end of the spectrum. It should also be noted that 3.4 per cent of the cohort had pre-existing facial nerve involvement prior to the surgery. One reason for this could be that intra-operative electromyographic facial nerve monitoring is routinely utilised in our centre. Studies which have also routinely utilised this monitoring system reported similar rates of facial nerve palsy of around 20 per cent.^{Reference Brennan, Moore and Shuler2,Reference Dulguerov, Marchal and Lehmann4,Reference Bova, Saylor and Coman5}

Post-parotidectomy facial nerve palsy can be categorised into transient or persistent. However, there is no consensus on the duration of what should be classed as transient or otherwise. Current studies have defined short-term facial nerve palsy as symptoms duration ranging from 1 to 12 months post-operatively.^{Reference Gaillard, Périé, Susini and St Guily6,Reference Guntinas-Lichius, Klussmann, Schroeder, Quante, Jungehuelsing and Stennert7} In our study, facial nerve palsy was categorised into shorter or longer than one year due to the local follow-up procedure.

Neck dissections and malignant parotid lesions have been reported to be associated with various extents of facial nerve palsy^{Reference Dulguerov, Marchal and Lehmann4,Reference Bron and O'Brien8} that were not found to be statistically significant factors in our study. This could be attributed to the bigger sample size of our study and hence a higher number of facial nerve palsy cases for statistical analysis than the aforementioned studies.

In concordance with our data, advanced age has been shown to be associated with post-operative facial nerve palsy in other studies.^{Reference Guntinas-Lichius, Klussmann, Schroeder, Quante, Jungehuelsing and Stennert7,Reference Terrell, Kileny, Yian, Esclamado, Bradford and Pillsbury9} Some researchers have suggested this is because axonal regrowth of facial nerve is attenuated in older populations.^{Reference Guntinas-Lichius, Klussmann, Schroeder, Quante, Jungehuelsing and Stennert7} Similarly, studies have shown the extent of parotidectomy is associated with a higher incidence of facial nerve palsy, which is in agreement with our findings.^{Reference Bron and O'Brien8} This is likely because the risk of stretching the facial nerve is higher in a more extended parotidectomy where more tissues are resected. Although a malignant lesion in itself has not been found to be an associated factor in this study, it is potentially correlated with associated factors such as extent of parotidectomy and advanced age. This type of multicollinearity between variables can be mitigated by incorporating a larger sample size in a multivariate model in future studies. On the other hand, our data showed that patients who had undergone partial parotidectomy had a statistically lower chance of developing facial nerve palsy compared to cases of superficial or total parotidectomies. Studies which compared partial parotidectomy against more extended approaches in resecting benign parotid lesions have come to the same conclusion.^{Reference Huang, Yan, Wei and He10,Reference Stathopoulos, Igoumenakis and Smith11}

Although the length of anaesthetic time in isolate could not be used as a pre-operative marker of risk, anaesthetic time has been shown to be directly correlated with morbidity after many types of surgery.^{Reference Cheng, Clymer, Chen, Sadeghirad, Ferko and Cameron12} Prolonged operations are associated with surgical team fatigue, which can increase the risk of complications.^{Reference Janhofer, Lakhiani and Song13} It is also likely that a longer anaesthetic time is indicative of a more complex procedure where the facial nerve has a higher chance of being injured intra-operatively.

Smoking is a well-established risk factor for surgical complications.^{Reference Hawn, Houston, Campagna, Graham, Singh and Bishop14} We found that it is also associated with post-parotidectomy facial nerve palsy. Regional blood flow in small capillaries has been shown to be restricted in chronic smokers.^{Reference Toda and Toda15} This microvascular dysfunction and the resultant impaired tissue perfusion has been suggested to be an underlying mechanism for complications in parotidectomy.^{Reference Kim, Lim, Wood, Samant, Ver Halen and Kim16} This could also explain how hypertension is an associated risk factor as it can likewise cause vascular endothelial dysfunction.^{Reference Dharmashankar and Widlansky17}

Greater auricular nerve hypoesthesia

Commonly, the greater auricular nerve is transected in parotidectomy to allow easier access to the parotid gland. In this group of patients, numbness of the upper pinna can be found in as high as 100 per cent of cases in the immediate post-operative period, decreasing to 50 per cent at 12-month follow-up.^{Reference Ryan and Fee18} Where possible, some centres have opted to preserve branches of the greater auricular nerve in hopes of minimising post-operative hypoesthesia.^{Reference Min, Lee, Lee, Jeong, Cho and Lee19} Other centres have shown this has no impact on reducing the rate of hypoesthesia.^{Reference Porter and Wood20} Due to the heterologous format of operative notes in the eight-year study period, we were unable to determine if the greater auricular nerve was preserved in the respective cases reliably. Nonetheless, our large case numbers should offset the effect of nerve preservation not being included as a variable in the regression model.

We found that patients with diabetes and a history of percutaneous coronary intervention are more likely to develop post-operative hypoesthesia. Diabetes can cause axonotmesis through microangiopathy and has been shown to prolong the course of Bell's palsy.^{Reference Kanazawa, Haginomori, Takamaki, Nonaka, Araki and Takenaka21} The same mechanism can impede the recovery process when the greater auricular nerve is inevitably injured. Patients who have undergone percutaneous coronary intervention have been shown to have an elevated level of circulating endothelial cells in the circulation, which is a marker for endovascular damage.^{Reference Boos, Balakrishnan, Jessani, Blann and Lip22,Reference Vargová, Toth-Zsamboki, Beres, Bencze, Kerecsen and Gulacsi-Bardos23} Systemic inflammation can also result from the mechanical trauma of the procedure and the co-morbid myocardial ischaemia.^{Reference Han, Ghanim, Peleg, Uretzky and Hasin24} The resultant loss of systemic endothelial function could affect functional recovery of the greater auricular nerve and put this group of patients more at risk of hypoesthesia.

Frey syndrome

Symptomatic Frey syndrome has been reported to occur in 2–23 per cent of patients post-parotidectomy, which is in range of the 4 per cent incidence seen in our study.^{Reference Guntinas-Lichius, Klussmann, Schroeder, Quante, Jungehuelsing and Stennert7,Reference Neumann, Rosenberger, Vorsprach and Dazert25} However, there could be underestimation in the cohort due to the retrospective nature of the study. The specimen volume of the excised parotid tissue at the end of the surgery is linked with Frey syndrome, but these data were not available in our case series.^{Reference Guntinas-Lichius, Klussmann, Schroeder, Quante, Jungehuelsing and Stennert7} Consistent with our findings, one study showed tumour size to be a predictor in the development of Frey syndrome.^{Reference Lee, Chan and Chan26} Logically, the shared mechanism behind these findings is that parasympathetic fibres would be more likely to be injured from a more extensive dissection for larger lesions.

Limitation

Both radical and nerve-preserving parotidectomies were included in the study. However, due to the retrospective nature of the study, whether the facial nerve was sacrificed could not reliably be inferred from the operative record, therefore this was not included as a variable. There was also a lack of homogeneity in the documentation of the immediate post-operative ward round, therefore the House–Brackman scale of post-operative facial nerve palsy could not be reliably determined from the documentation included in the study. Future studies should be performed prospectively for more standardised data collection. Studies to investigate risk factors associated with post-operative morbidities in parotidectomy exist in the current literature but the findings have been variable. This is likely due to the heterogenous selection of cases between different centres. Similarly, although our findings have been consistent with other cited studies, our data and findings are limited to local patient selections.