Cerebellar Flocculus Pseudomass Protruding into the Internal Auditory Canal

There are many lesions that may exist in the cerebellopontine angle (CPA), including the lesser known cerebellar flocculus pseudomass. This report describes a case of unilateral hearing loss associated with a CPA lesion on MRI. Though initially presumed to be a vestibular schwannoma, the lesion was ultimately characterized as a cerebellar flocculus pseudomass due to key radiologic findings. The patient is a 73-year-old female referred to our practice with a preliminary diagnosis of a vestibular schwannoma after gradual onset of right-sided low frequency hearing loss. The lesion was observed with periodic magnetic resonance imaging over a period of five years with no change in appearance. Review of imaging at our institution revealed several characteristics, including lack of strong contrast enhancement, normal appearing vestibulocochlear and facial nerves, normal porus acusticus width, and cerebellar surface architecture, that are inconsistent with the diagnosis of a vestibular schwannoma. The diagnosis of cerebellar flocculus pseudomass was made, and the patient was provided reassurance regarding the lesion as well as counseling about hearing loss rehabilitation. The cerebellar flocculus can be variable in size and asymmetric, resulting in misdiagnosis as a CPA mass and inappropriate treatment. The cerebellar flocculus is a structure of the cerebellum which acts as a neural integrator and facilitates the vestibulo-ocular reflex. Reaching its final configuration several months after birth, the cerebellar flocculus, in its native position, sits between the cerebellar hemisphere and brachium pontis, projecting into the CPA cistern where it is found posterior to the facial and vestibulocochlear nerves and lateral to the choroid plexus. Little information about the cerebellar flocculus pseudomass is available in the literature, and its presentation with unilateral hearing loss is novel. In this case, the cerebellar flocculus pseudomass extended into the internal auditory canal, displacing the right vestibulocochlear nerve ([Fig. 1]). Although it is not possible to ascertain whether the internal auditory canal involvement is responsible for the patient’s hearing loss, it did contribute to uncertainty around diagnosis. To avoid misdiagnosis of CPA lesions, Calzada et al (2015) recommends a comprehensive MRI study for CPA lesions, including an axial non-fat-saturated T1-weighted image, an axial/coronal fat-saturated postcontrast T1-weighted image, a T2-weighted image, a high-resolution T2-weighted (CISS/FIESTA sequences. image, and a diffusion weighted image. Although MRI provides the best visualization of cerebellopontine angle and internal auditory canal pathologies, differentiation of abnormalities in this space can remain challenging, and a significant proportion of presumed vestibular schwannoma cases, as high as 17.6% in one study, are more consistent with alternative diagnoses. Knowledge of less common CPA lesions, such as the cerebellar flocculus pseudomass, and their variable presentations can prevent misdiagnosis and inappropriate treatment.

Fig. 1 T2 Weighted Magnetic Resonance Imaging (3D Balanced Fast Field Echo) displaying progressively cephalad cuts from A–D of; 1. cerebellar flocculus pseudomass (thin arrow) entering the; 2. internal auditory canal (outline shown with dashed line in image A); 3. arrowhead points to vestibulocochlear nerve; 4. star marks facial nerve.

 

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No conflict of interest has been declared by the author(s).

Publication History

Article published online:
05 February 2024