The facial nerve (cranial nerve VII) is a highly complex structure.
It provides motor control of most of the muscles of facial expression and parasympathetic innervations to the salivary and lacrimal glands.
Moreover,
it receives taste sensations from the anterior two-third of the tongue,
and cutaneous sensation from the skin in and around the auricle.
Motors fibers are carried via the motor facial nerve and parasympathetic,
sensorial and general somatic afferent fibers are carried via the intermediate nerve of Wrisberg. (Fig.
2).
Cross-sectionaal imaging plays an important role in the evaluation of patients with VII cranial nerve palsy.
To image the facial nerve adequately,
it is important to understand its normal anatomy,
and some technical considerations regarding the acquisition of CT and MR to achieve high resolution images of this structure.
A segmental anatomical approach and appropriate use of CT,
MRI - or a combination of both - guided by clinical presentation,
should enable the radiologist to assess facial nerve lesions.
Neuroimaging methods for evaluating the facial nerve.
Computed Tomography (CT)
Computed tomography is the go-to method for studying the facial canal.
The evaluation of this region is performed with special middle ear protocols.
It is especially used in detectinging erosions and destruction of the facial canal.
The axial plane is the best for studying the tympanic segment in its proximal region,
the tympanic segment and the stylomastoid fossa,
the relations of the mastoid segment with the external auditory canal and the jugular fossa,
the posterior genu and the geniculate ganglion fossa.
The coronal oblique plane,
perpendicular to the lateral semicircular canal axis highlights the distal portion of the labyrinthin segment of the facial nerve next to its proximal tympanic segment (the "snake eye" sign),
the mastoid segment,
the stylomastoid fossa,
the relation between the tympanic segment and the oval window and the posterior genu of the nerve.
The oblique sagittal planes can be used to study the posterior genu of the facial nerve,
the tympanic segment,
the mastoid segment and the stylomastoid fossa.
Magnetic Resonance Imaging (MRI)
Magnetic Resonance Imaging (MRI) has the advantage of direct identification of the facial nerve and its lesions.
A specific protocol is required to syudy the VII cranial nerve that include high resolution T1 and T2-weighted 3D sequences,
diffusion-weighted sequences and contrast-enhanced T1-weighted sequences,
which should be complemented in some cases with a complete examination of the brain.