Diagnosis: Congenital Cyst of the Cerebellopontine Angle
Facial paralysis is usually caused by a reactivation of the herpes simplex virus (HSV), also called Bell's palsy. Recurrent facial paralysis, however, requires further workup and should not be ignored nor considered to be viral in origin. In general, HSV-related facial paralysis is much more common among adults. In adults, an MRI is done if the paralysis has the following characteristics: slow onset, recurrent, involves only some of the nerve branches, and lasts over six months, among others. The workup threshold for a child with facial paralysis is a bit lower, but no criteria have been developed for when a workup should be performed on children.
Unilateral sensorineural hearing loss in a child is most commonly congenital in origin. While universal newborn hearing screening is implemented across the United States, it is not uncommon for some parents to miss that their child has unilateral hearing loss or for them to become complacent in making a follow-up of the newborn screening after their child's birth. It becomes problematic when a school-age child is found to have hearing loss. The issue at this point is whether the condition is old or new.
In situations wherein a child had passed his or her newborn hearing screening but was later diagnosed with unilateral hearing loss, a further workup is generally warranted, even though tumors of the cerebellopontine angle are rare in children. In general, the most common cause of unilateral profound hearing loss is a congenital anomaly of the inner ear. Another condition that requires further examination is the progression of unilateral hearing loss in a child. Even though most of these cases are genetic in nature, additional workup is usually warranted.
When facial paralysis and hearing loss occur together on the same side, it is an ominous sign that requires an expeditious MRI scan of the internal auditory canals with gadolinium. It is extremely uncommon and highly unlikely that an acoustic neuroma (vestibular schwannoma) would cause facial paralysis. In fact, if facial paralysis is seen with cerebellopontine angle tumor, the condition is more likely to be a facial schwannoma or a malignancy and highly unlikely to be a vestibular schwannoma. In a patient with a history of malignancy, facial paralysis and sensory neural hearing loss on the same side warrants a lumbar puncture with cytology examination to evaluate for malignant cells in addition to looking for a mass via an MRI.
In this child, the recurrent nature of the facial paralysis should have been the first sign that further workup is warranted. The combination of hearing loss and recurrent facial paralysis generally indicates that a mass is likely present. This would be true for patients of any age.
The MRI constructive interference in steady state (CISS) sequence in Fig. 1 shows a mass in the right cerebellopontine angle. The mass appears very bright (hyperintense) on T2, indicating that it is brighter than the consistency of the brain. The bright color of the mass has similar consistency as cerebrospinal fluid, which also appears bright white on these images. On the T1 sequence (Fig. 2), the mass appears to have the same consistency as cerebrospinal fluid. Just based on the information, this mass is consistent with an arachnoid cyst, a congenital cyst that contains cerebrospinal fluid. However, it would be extremely rare that an arachnoid cyst would lead to cranial nerve damage such as facial paralysis and sensorineural hearing loss.
The other possibility would be an epidermoid cyst, which are congenital cysts otherwise called epidermoids. These lesions are created when the squamous epithelium that was intended to eventually make it to the skin or other structures gets trapped in the epidural space. The squamous epithelium makes new layers and sheds the dead layer of skin (keratin). This causes an accumulation of keratin around the brain. On the skin, this layer is washed away on a daily basis. In the ear canal, it migrates outward and is mixed with the cerumen. Like the ear as in cholesteatoma, the accumulation of keratin around the brain causes mass effect and destruction. Epidermoids appear like arachnoid cysts on T1 and T2 imaging. Diffusion-weighted imaging (DWI) is a type of MRI sequence that can help distinguish avascular (dead) tissue. It is used in the diagnosis of strokes. Given that keratin debris is made up of dead tissues with no vascularity, it will appear bright (hyperintense) on DWI sequence. The coronal DWI sequence (HASTE) in Fig. 3 does not show any hyperintensity, indicating that keratin was likely absent.
CT of the temporal bones showed that the patient's right internal auditory canal was enlarged and smooth-walled. This suggested a gradual widening of the internal auditory canal, essentially ruling out a malignancy. A malignant process tends to cause destruction and invasion of the bone, which will usually look moth-eaten. The patient had a craniotomy for resection, which found a fluid-filled cyst with a mixed lining that was not arachnoid in origin.
BONUS VIDEOS: VISUAL DIAGNOSIS
Read this month's Clinical Consultation case, then watch the accompanying videos from Hamid R. Djalilian, MD, to review the patient's imaging for yourself.
- Video 1. Axial CISS MRI showing the mass and its relationship with the IAC.
- Video 2. Axial T1 sequence MRI demonstrating no enhancement or hyperintensity (brightness) of the mass.
- Video 3. Coronal CISS MRI showing the relationship between the mass and the brainstem.
- Video 4. Coronal HASTE sequence showing no hyperintensity in the entire field of study.
- Video 5. Axial CT of temporal bones showing the internal auditory canal changes.
- Video 6. Coronal CT of right temporal bone showing some widening of the cochlear nerve canal at the cochlea.
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