Rhomboid lip

General

• Definition: a sheetlike neural tissue layer that forms the ventral walls of the fourth ventricle and the foramen of Luschka

• Thick membranous structure distinct from the arachnoid membrane

Classification

• According to the relationship between the RL and choroid plexus in the cerebellopontine cistern from the posterior view.

• Chor., choroid; CN, cranial nerve; Flocc., flocculus; Jug., jugular; For., foramen; PICA, posterior inferior cerebellar artery; Plex., plexus; Rhomb., rhomboid; Vent., ventricle.

• Non-extension type. Left RL is shown in posterior view.

• The RL spreads medially to the lateral edge of the choroid plexus.

• The RL and choroid plexus are observed in the posterior view.

 

• Lateral extension type. Left RL is shown in posterior view.

• The RL spreads laterally over the lateral edge of the choroid plexus.

• The choroid plexus is not observed in the posterior view.

 

• Jugular foramen type. Right RL is shown in posterior view.

• The RL spreads widely to the edge of the jugular foramen.

• The choroid plexus is obscured by the RL and is not observed. The RL adheres to cranial nerve IX widely.

 

• A: The rhomboid lip covers the lower CNs and obscures visualization of the root exit zone (REZ) of CN VII.

• B: After separating the rhomboid lip and arachnoid membrane from the lower CNs, the proximal portion and root exit zone of CN VII can be easily observed below the flocculus, and offending arteries (that is, those causing the HFS) are identified.

 

Anatomy of the rhomboid lip (RL)

• AICA, anterior inferior cerebellar artery; Cer., cerebello; Chor., choroid; CN, cranial nerve; Fiss., fissure; Flocc., flocculus; For., foramen; Inf., inferior; Junc., junction; Lat., lateral; Med., medullary; Plex., plexus; Rec., recess; Rhomb., rhomboid; V., vein; Vel., velum; Vent., ventricle.

• The RL is located between flocculus and medulla in the cerebellopontine cistern.

• Structures ventral to the RL include cranial nerves IX and X.

 

• The choroid plexus protrudes from the foramen of Luschka.

• The RL consists of a part of the foramen of Luschka with choroid plexus.

 

• The bilateral tonsils were removed.

• The RL originates from the dorsal medulla and courses laterally toward the cerebellopontine cistern with choroid plexus.

• Red arrowheads show the attachment and origin of the RL.

 

• The choroid plexus courses toward the cerebellopontine cistern through the lateral recess in the fourth ventricle.

• Red arrowheads show the attachment and origin of the RL that forms a part of the lateral recess.

• The dissector is going through foramen of luschka.