Detection frequency of cystic rhomboid lip in a nonoperative clinical population: an MR imaging morphologic study

The cystic rhomboid lip (CRL), a diverticular expansion of the fourth ventricular lateral recess at the foramen of Luschka, is familiar to posterior fossa surgeons but remains incompletely characterized on Magnetic Resonance (MR) imaging. We aimed to determine its MR imaging detection frequency and morphologic features in a nonoperative referral population and to evaluate a practical recognition framework against surgically confirmed cystic cerebellopontine angle (CPA) mimics.

In this retrospective 2-center study, 662 consecutive patients who underwent thin-section skull base MR imaging for suspected posterior fossa cranial nerve disorders (2022-2024) were independently reviewed by 2 neuroradiologists. Based on anatomic and surgical literature, CRL was defined a priori by two required features at the foramen of Luschka: anterior displacement of CN IX-X and posterior displacement of the choroid plexus (the CN-choroid plexus separation sign), with a CSF-equivalent signal in the intervening space. Ipsilateral lateral recess dilatation was considered a supportive but nonmandatory feature. Extension pattern was classified with a radiologic adaptation of the Akiyama framework. The same features were then assessed in 54 surgically and histopathologically confirmed cystic or solid-cystic lesions at the level of the lower cranial nerves.

CRL was detected in 42/662 patients (6.3%; 95% CI, 4.7-8.5%), comprising 48 lesions (6 bilateral). Mean age was 55.8 ± 15.8 years; 61.9% were women. Mean maximum diameter was 15.7 ± 2.8 mm. A jugular foramen extension pattern (JFT) was present in 43/48 lesions (89.6%) and a lateral extension pattern (LET) in 5/48 (10.4%); JFT lesions were larger than LET lesions. Lateral recess dilatation was present in 32/48 lesions (66.7%). The CN-choroid plexus separation sign was present in 48/48 CRLs and in 0/54 comparison lesions. CSF-equivalent signal was present in 14/54 comparison lesions (25.9%), all arachnoid cysts accompanying a solid tumor, none of which showed the separation sign. Interobserver agreement was perfect for CRL detection (κ = 1.00) and extension pattern (κ = 1.00) and excellent for diameter measurements (ICC, 0.91-0.93).

CRL is a relatively common finding on thin-section skull base MR imaging (6.3%). The CN-choroid plexus separation sign with CSF-equivalent signal reliably distinguishes it from cystic CPA mimics.