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.