Discussion
Typically, Bell’s palsy does not progress and it is rather unilateral and self-limiting. In fact, the majority of cases recover spontaneously within 3 weeks, even if untreated 7, or remit within 4-6 months, but nearly always remission is complete by 1 year.8 In 2013 the specialists’ panel made recommendations that diagnostic imaging for patients with new-onset Bell’s palsy should not be performed routinely. However, clinicians should reassess or refer to a facial nerve specialist those Bell’s palsy patients with (1) new or worsening neurologic findings at any point, (2) ocular symptoms developing at any point, or (3) incomplete facial recovery 3 months after initial symptom onset.9
The differential diagnosis of permanent FNP, with particular emphasis on expansive processes, including cancers, is mostly discussed in the literature basing on case reports.10-12 We present a set of patients with irreversible FNP and under-diagnosed malignancies finally found out in the deep lobe of the parotid gland. All of them were primarily misdiagnosed with Bell’s palsy. While detailed diagnostics implemented 3 to 8 months after FNP onset excluded masked ear infection and head pathology, as well as did not reveal pathology in ultrasound examination of the neck and laboratory tests, further examinations were discontinued.
The most common imaging techniques used in the assessment of salivary glands are ultrasound, CT and MRI. Ultrasound is a quick and relatively inexpensive method that can accurately depict most salivary gland neoplasms.13However, it should be underlined that it provides limited visualization of the deep lobe of the parotid gland. Moreover, penetration of ultrasound can be hindered in case of high content of fat in the parenchyma.14 CT is frequently used as a more precise tool, but, due to relatively poor soft-tissue contrast, sometimes the accurate assessment of the tumor extent and differentiation of its malignancy potential can be impossible.13 Nowadays MRI is a gold standard in evaluation of parotid gland.15Our study shows that it is very important to include MRI in the diagnostics of all patients with irreversible FNP and, what important, even to repeat the imaging in uncertain / untypical clinical picture of the disease. MRI should cover all anatomical segments of facial nerve, both intra- and extratemporal. In 3 out of 4 presented patients with malignancies of the parotid gland, MRI of the head and neck was not included in the diagnostic algorithm. In one patient MRI scanning did not cover the region of parotid gland where the malignant tumor was located, which was an obvious shortcoming. Adenoid cystic carcinoma of the deep lobe of the parotid gland, which was finally confirmed in all presented cases, is a slow-growing but very aggressive tumor characterized, among others, by perineural spread.16It is estimated that FNP is an initial symptom in about 60% of cases.17 Referring to our study group, early diagnosis with MRI of the neck could have improved oncological results in all of the analyzed patients, especially those with the distant spread of the disease.
Zimmerman et al 18 concluded that MRI is a useful tool for exclusion of otogenic and neoplastic processes with a sensitivity of 83% and 88%. Quesnel et al 19 indicate imaging of parotid gland, brain and temporal bone at 4 months for patients with irreversible idiopathic facial paralysis. They support that recommendation by the natural history of Bell’s palsy, in which early signs of recovery are seen in the majority of cases by this time. We strongly agree with such an algorithm. In case of no lesion identified, they suggest to repeat the imaging in 3 months or even to consider explorative surgery and biopsy of tissues adjacent to the facial nerve. In our opinion, surgical procedures, due to the potential complications and difficulties, should be performed only in selected cases.
Although there are a few papers on magnetic resonance (MRI) protocols for the malignancies manifesting as facial palsy20-22, they are universally suboptimal and thus may fail to reliably differentiate neoplastic from inflammatory process even when interpreted by experienced radiologists. Chhabda et al23 underline that selected segments of the nerve are better evaluated on certain imaging modalities, so radiologists should try to combine clinical picture with specific facial nerve pathologies to tailor the imaging test to best answer the clinical question. Gupta el al 24 recommend contrast-enhanced MRI as the first imaging in cases when the palsy cannot be definitively localized. They indicate high-resolution temporal bone CT to evaluate the fallopian canal if the lesion can be localized in the region of the mastoid, tympanic, or labyrinthine segments of the facial nerve and contrast-enhanced MRI if the cisternal or intracanalicular segments of the facial nerve or the pontine nuclei should be assessed. In our opinion, such recommendations are very good. The intraparotid facial nerve is best evaluated by gadolinium-enhanced MRI, with T1-weighted gadolinium-enhanced images.12 In the inner ear pathologies the combined non-contrast and postcontrast T1-weighted and T2-weighted sequences are mostly used.24
Our paper has strong points and some limitations. Our attempt to create diagnostic algorithm in irreversible Bell’s palsy to avoid missing a parotid gland cancer is unique. The main limitation is a small sample size. Nevertheless, deep lobe adenoid cystic carcinoma is a rare disease and it is difficult to expand the study group. The algorithm can be used for clinical practice and initiation of further studies.