Magnetic resonance imaging [MRI] is widely used to visualize this anatomical region. Thin slice T2-weighted MRI images employing 3-dimensional transformation [3D T2 DRIVE images]were evaluated in the axial plane.

The AICA follows a highly variable course and vascular loops can be identified in many asymptomatic individuals.

The correlation of anterior inferior cerebellar artery [AICA] vascular loop around cerebellopontine angle [CPA] and otologic symptoms remains controversial. Vascular compression of the seventh and eighth cranial nerves in the CPA may lead to hearing loss, tinnitus, vertigo, dizziness, imbalance and hemifacial spasm [nerve vascular compression syndrome].

Fig 5: Microvascular compression syndrome. [A] Source MRI [axial thin-section...

Based on the MRI findings, several grading systems had been proposed to describe the course of vascular loops within IAC-CPA region.

Chavda classification, based on the anatomic location of the AICA loop and describing the depth of extension of the AICA loop into the IAC: 

• Type I: AICA loop within the CPA but outside the IAC.
• Type II: AICA loop extending into the IAC but is less than 50% the length of the IAC.
• Type III: AICA loop with greater than 50% extension into the IAC.
  

  Fig 6: Diagrammatic representation of type I, II and III AICA loops [Chavda...
  

  Fig 7: Display of vascular loop type 1–3 with a scheme [Chavda...
  

  Fig 8: Chavda classification of AICA loop. [A] AICA loop [arrow] observed in...
  

  Fig 9: Chavda classification of AICA loop. [a] Type I: AICA loop [arrow] is observed...
  

  Fig 10: Chavda classification 1. Type 1 vascular loop on the left [white arrows]. 2....
  

  Fig 11: Grade I Chavda vascular loop in the right cerebellopontine angle. AICA...

Gorrie grading system [second classification system], describes the extent of contact between the AICA loop and the cranial nerves: 

• Type A: AICA loop runs separately from cranial nerves.
• Type B: AICA loop runs adjacent to the cranial nerves.
• Type C: AICA loop runs between the VII and VIII cranial nerves.
• Type D: AICA loop displaces the cranial nerves, resulting in bowing of the nerves.
  

  Fig 12: Gorrie classification of AICA loop. [A] AICA loop [arrow] running separate...
  

  Fig 13: Gorrie classification of AICA loop. [a, e] Type A: AICA loop [A] runs...
  

  Fig 14: Magnetic resonance imaging brain axial sections showing relationship between...
  

  Fig 15: Chavda and Gorrie classifications 1. A, Anterior inferior cerebellar artery;...
  

  Fig 16: Axial T2 SPACE sequence demonstrating a classic loop of the AICA [arrow] within...

The position of the loop relative to nerve[s] may also be evaluated in transverse view and described in terms of the type of contact to the nerve[s]: 

• Type A: crossing 2 [vestibular and cochlear nerve] or 4 nerves in the IAC [as a bridge].
• Type B: crossing only one nerve.
• Type C: running parallel to the nerves but not crossing them.
  

  Fig 17: AICA loop classification according to Chavda classification and type of...

Kazawa system, describes the existence of loop formation and the IAC extension: 

• Type IA: non-loop AICA in the cistern.
• Type IB: non-loop AICA enters the IAC.
• Type IIA: loop type AICA in the CPA cistern.
• Type IIB: loop type AICA enters the IAC.
  

  Fig 18: Kazawa classification system Drawing schemas of the AICA/PICA course and shape...
  

  Fig 19: Kazawa classification system [A] Type IIB loop-type AICA/PICA entering into...

The size of the AICA loop was also classified into two groups based on the caliber of the facial nerve [comparing the thickness of the loop with adjacent facial nerve] 

• AICA loops with a caliber greater than the facial nerve are large [L].
• AICA loops with a diameter less than the adjacent facial nerve are small [S].
  

  Fig 20: Typical presentations of anterior inferior cerebellar artery loop sorted by its...

Possible theories of audio-vestibular symptoms associated with AICA loops:

1. Compression of the vascular loop with impaired blood flow can result in ischemia of the labyrinthine artery, a branch of AICA, with impaired perfusion of the inner ear structures [cochlea and vestibule] and resultant vestibulocochlear nerve compression syndrome.
2. Changes within the blood stream disturb the laminar stream and existing turbulence can cause an audible sound [tinnitus].
3. Vascular loops in the IAC could generate arterial pulse synchronous tinnitus by transmission of vibrations, possibly through the VIII cranial nerve into the cochlea.
4. Neurovascular compression may result in gliosis, edema, regional demyelination, and eventually fibrotic changes induced by the axonal degeneration with aberrant nerve conduction of the cochlear nerve and alteration of central plasticity or impairment of blood flow on the nerve.
5. Pulsatile irritation can cause demyelination of the nerve at the root entry zone [REZ]. The REZ is the transition area where the myelination changes from central nervous system myelination to peripheral nervous system myelination. In the VIII cranial nerve, the REZ occurs in a length stretching between the brainstem and the fundus of the IAC. Demyelination could cause decreased conduction velocity, leading to the formation of false synapses and ephaptic transmission of nerve potentials. Contact alone would not be sufficient for symptoms, and an episode of vestibular neuritis leading to axonal loss and swelling is required to establish attachment between the nerve and the vessel.
6. Brain stem auditory nuclei may be secondarily affected by changes in the auditory nerve and be the actual generator site[s] for the tinnitus.
7. The primary abnormality lies in the nucleus of the cranial nerve and that vascular compression is secondary.
8. A common developmental abnormality leading to both the formation of a vascular loop and cochlear malfunction.