Congenital facial nerve aplasia is a rare condition where the patient presents with signs of facial nerve paralysis.
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Clinical presentation
Patients present with unilateral or bilateral facial weakness characterized by facial asymmetry, incomplete closure of eyelid, deviation of angle of mouth and loss of nasolabial furrow.
Pathology
The cause can be intrapartum trauma or developmental. This article focuses on developmental facial nerve aplasia.
Radiographic features
CT
Although the facial nerve itself is not seen on CT, developmental abnormalities of the facial canal such as hypoplasia or aplasia can be seen provided high-quality thin section bone algorithm imaging of the temporal bones is obtained.
MRI
The key to accurate diagnosis is the ability to directly image the facial nerve and demonstrate it's absence/hypoplasia. As such, high-resolution imaging such as 3D balanced steady-state gradient-echo sequences like 3D FIESTA (GE), TrueFISP (Siemens) or balanced FFE (Philips) are the mainstays for visualization of the facial nerve.
Other routine MRI sequences are used to rule out brain parenchymal or brainstem abnormalities.
On 3D sequence, the 7th nerve which usually accompanies the 8th nerve in the cistern, as well as the internal auditory canal, is not visualized on the affected side.
The nerves are seen at Bill's bar in the internal auditory canal, where the facial nerve is located anterosuperiorly. It may be difficult to appreciate the facial nerve in this segment and it should be traced from its origin, in the cisternal as well as the canalicular segments.
Treatment and prognosis
Physiotherapy is the mainstay of treatment. Most patients regain some function in follow-up which may be due to aberrant innervations of some of the facial muscles by other cranial nerves such as the trigeminal, hypoglossal, or glossopharyngeal nerves.
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