What is the significance of an Anterior Inferior Cerebellar Artery (AICA) loop near the internal auditory canal?

From the Guidelines

The Anterior Inferior Cerebellar Artery (AICA) loop near the internal auditory canal is a normal anatomical variant that can occasionally cause symptoms such as pulsatile tinnitus, vertigo, or hearing loss due to compression of the vestibulocochlear nerve (cranial nerve VIII), and diagnosis is typically made through high-resolution MRI with specific sequences like CISS or FIESTA 1.

Key Points

• The AICA loop is a normal variant that can be seen in up to one-third of normal patients, but may contribute to otological symptoms due to neurovascular compression 1.
• Patients with pulsatile tinnitus are 80 times more likely to have vascular loops in contact with CN VIII than patients without pulsatile tinnitus 1.
• Heavily T2-weighted thin-section sequences of MRI head can detect the neurovascular loops in patients with pulsatile tinnitus 1.
• Microvascular decompression surgery may be considered if symptoms are severe and clearly attributable to the vascular compression, where a small pad is placed between the artery and the nerve to relieve pressure.

Diagnosis and Treatment

• High-resolution MRI with specific sequences like CISS or FIESTA is the preferred diagnostic method for detecting AICA loops and evaluating their relationship with the vestibulocochlear nerve 1.
• Most AICA loops are asymptomatic and require no treatment, but if symptoms are severe and clearly attributable to the vascular compression, microvascular decompression surgery may be considered 1.

Importance of Recognition

• The AICA loop is an important anatomical variant to recognize during evaluation of patients with audiovestibular symptoms and when planning surgical approaches to the cerebellopontine angle 1.
• Accurate diagnosis and treatment of AICA loops can improve patient outcomes and reduce the risk of complications such as hearing loss and vertigo 1.

From the Research

AICA Loop Near Internal Auditory Canal

• The anterior inferior cerebellar artery (AICA) loop near the internal auditory canal has been studied in relation to various audiovestibular symptoms such as tinnitus, vertigo, and hearing loss 2, 3, 4, 5, 6.
• A case report published in 2014 described a patient with disabling vertigo and tinnitus caused by an intrameatal compression of the AICA on the vestibulocochlear nerve, which was successfully treated with surgery 2.
• Another study published in 2004 reported a case of an intracanalicular aneurysm at the meatal loop of the distal AICA, which was treated with surgical clipping 3.
• A retrospective cohort study published in 2015 found that AICA loops were present in 23% of patients with chronic tinnitus, but there was no significant relationship between the presence of an AICA loop and the side of tinnitus or abnormalities on auditory brainstem responses 4.
• A histological study published in 1991 found that AICA loops were present in 12.3% of temporal bones, but there was no correlation between the presence of an AICA loop and unexplained hearing loss, tinnitus, vertigo, or Meniere's disease 5.
• A recent study published in 2024 found that the prevalence of AICA vascular loop in the cerebellopontine angle cistern was as high as 47.6% in asymptomatic patients, with no significant correlation between the presence of the AICA loop and otovestibular symptoms 6.

Clinical Relevance

• The presence of an AICA loop near the internal auditory canal may be an incidental finding, and its clinical relevance should be carefully evaluated in the context of audiovestibular symptoms 4, 6.
• Surgical treatment may be considered in cases where the AICA loop is causing compression of the vestibulocochlear nerve, but the decision to operate should be made on a case-by-case basis 2, 3.

Diagnostic Yield

• Magnetic resonance imaging (MRI) may be useful in detecting AICA loops near the internal auditory canal, but its diagnostic yield in patients with audiovestibular symptoms is limited 4, 6.
• The use of high-resolution volumetric T2 sequences such as three-dimensional constructive interference in steady state (3D-CISS) may improve the detection of AICA loops in the cerebellopontine angle cistern 6.