Asymptomatic vascular compression of the cisternal segment of the facial nerve

 40 year of female patient presented with headache. No tinnitus / facial nerve palsy / hemi-facial spasm.

The above CISS 3D Axial image is showing significant anterior displacement of the cisternal segment of the right facial nerve (orange arrow) by the Anterior Inferior Cerebellar Artery (AICA) loop. AICA loop was seen coursing in between the cranial nerves VII and VIII. The focally thickened appearance of the right facial nerve is due to the AICA closely abutting the posterior aspect of the nerve at that site. No compression on the vestibulo-cochlear nerve was seen. 

Sagittal reformats of the CISS images, showing the contact of the AICA loop (orange arrow) and the facial nerve (yellow arrow). Green arrow points to the cranial nerve VIII. 

Interestingly the patient didn't have any symptoms related to the facial nerve compression at the time of the scan.

The Chavda classification of vascular loops in relation to the Internal Acoustic Canal (IAC) is: 

Type I   : Loop is present in the CP angle cistern, but not entering the IAC.

Type II  : Enters the IAC, but <50% of the length of IAC. 

Type III : Occupies > 50% of the IAC. 

Kinking or angulation of the nerve at the site of contact is taken as a sign of vascular compression.

Gorrie et al used a second classification system assessing the relationship of the vascular loop with the VIII cranial nerve. Four different relationships were categorized: 

Class A, no contact; 

Class B, vascular loop lying directly adjacent to nerve; 

Class C, loop running in between VII and VIII nerve ;

Class D, vascular loop displacing the nerve resulting in bowing of the nerve.

Gorrie et al. stated: 'A significant association was demonstrated between the AICA running between the vestibulocochlear and facial nerves. The p value was found to be 0.0162, which demonstrates a statistically significant association between the presence of a Class C loop and hearing loss.'

Studies are yet to conclusively show the significance of these 'compression' as even normal patients with no tinnitus / hearing loss have the type III AICA loop in the IAC.

See a case of of type II vascular loop here.

References

1. McDermott et al. (2003) McDermott AL, Dutt SN, Irving RM, Pahor AL, Chavda SV. Anterior inferior cerebellar artery syndrome: fact or fiction. Clinical Otolaryngology and Allied Sciences. 2003;28:75–80. doi: 10.1046/j.1365-2273.2003.00662.x.

2. Gorrie et al. (2010) Gorrie A, Warren 3rd FM, De la Garza AN, Shelton C, Wiggins 3rd RH. Is there a correlation between vascular loops in the cerebellopontine angle and unexplained unilateral hearing loss? Otology & Neurotology. 2010;31:48–52. doi: 10.1097/MAO.0b013e3181c0e63a.

3. Kim SH, Ju YR, Choi JE, Jung JY, Kim SY, Lee MY. Anatomical location of AICA loop in CPA as a prognostic factor for ISSNHL. PeerJ. 2019 Mar 11;7:e6582. doi: 10.7717/peerj.6582. PMID: 30881768; PMCID: PMC6417406.