Microsurgical Anatomy of Rhomboid Lip around the Lateral Recess of Fourth Ventricle with Clinical Consideration

Introduction: The rhomboid lip is a sheet-like neural tissue layer that forms the ventral walls of the fourth ventricle and the foramen of Luschka. The rhomboid lip is often observed in posterior fossa surgery such as microvascular decompression for hemifacial spasm and resection of vestibular schwannoma. The Rhomboid lip is an inconspicuous structure that is anatomically unique, but its function remains unclear.

Objective: To describe the microsurgical anatomy of the rhomboid lip around the lateral recess of fourth ventricle and to consider its role by presenting histology and clinical cases.

Methods: Twelve formalin-fixed adult human cadaveric heads injected with colored silicone were examined with microsurgical techniques. Totally 20 rhomboid lips and the expansion of the rhomboid lip in the cerebellopontine cistern were examined posteriorly. The relationship between the rhomboid lip and choroid plexus, as well as rhomboid lip and cranial nerve IX were assessed. A histological examination of clinical sample of rhomboid lip was investigated.

Results: The rhomboid lip was divided into three types: nonextension type, lateral extension type, and jugular foramen type, according to the relationship between the rhomboid lip and choroid plexus. The jugular foramen type was rare. Rhomboid lip was attached with cranial nerve IX in all specimens. In the histological examination of the rhomboid lip, the rhomboid lip consisted of two layers. The inner surface layer was covered by a single layer of consecutive ciliated cells and the deep layer consisted of glial cells and neuropil. Histology showed that the rhomboid lip consisted of neural tissue with glial cells covered by ciliated ependymal cells.

Conclusion: Histologically, the rhomboid lip may be the remnant of the fourth ventricle covered with ependymal cells. Knowledge of the detailed anatomy and proper dissection of the rhomboid lip may help surgeons to obtain good visualization of structures around the lateral recess.

No conflict of interest has been declared by the author(s).