Elsevier

World Neurosurgery

Volume 112, April 2018, Pages e288-e297
World Neurosurgery

Original Article
Primary Obstruction of the Foramen of Luschka: Anatomy, Histology, and Clinical Significance

https://doi.org/10.1016/j.wneu.2018.01.037Get rights and content

Highlights

  • Features of primary obstruction of the foramen of Luschka are described.

  • The histology of the obstructing membrane and the rhomboid lip is identical.

  • A diverticulum of the foramen of Luschka is the result of a primary obstruction.

  • Anatomic, histologic, embryologic, and pathophysiologic descriptions are presented.

  • Clinical observations, including radiologic and surgical criteria, are provided.

Background

The foramen of Luschka is a natural aperture between the fourth ventricle and the subarachnoid space at the cerebellopontine angle. Membranous closure of this foramen is referred to as primary obstruction. Available information about this variant and its role in the development of the cysts of the posterior fossa is contradictory.

Methods

The macroscopic and histologic features of the obstructed foramina were examined in 61 formalin-fixed human brains (122 foramina). Three rhomboid lips of various sizes with lateral recess were used for comparison. Five postoperative cases of diverticulum of the foramen of Luschka were included in this study, with 1 case presented in detail to illustrate anatomic and histologic findings.

Results

Primary obstruction was present in 11 of 122 cases. In 1 case, an enlarged rigid pouch with a thick wall was found. The wall of the membrane in primary obstruction and the rhomboid lip were composed of an inner ependymal, a middle glial, and an outer leptomeningeal layer.

Conclusions

The rhomboid lip is a remnant of the roof of the fourth ventricle. Imperforation of the foramen of Luschka results in a pouch in the cerebellopontine angle that contains choroid plexus (Bochdalek's flower basket) and communicates with the fourth ventricle. This pouch has the potential to grow to a diverticulum and cause clinical symptoms. Based on our clinical observations, detailed radiologic and surgical-anatomic criteria were proposed to support the differential diagnosis of a diverticulum of the foramen of Luschka. Treatment strategies were also suggested.

Introduction

The foramen of Luschka is a natural aperture between the fourth ventricle and the subarachnoid space at the cerebellopontine angle (CPA). The microsurgical anatomy of the foramen and the related neurovascular structures is well described in the literature.1, 2, 3, 4, 5, 6 The closure of the foramen is a rare phenomenon that manifests in primary and secondary forms.7, 8, 9, 10 The secondary obstruction of the foramen is a result of various pathologies, such as tumors of the CPA and the fourth ventricle, subarachnoid hemorrhage, or inflammation of the leptomeninges. A membranous structure obstructing the foramen of Luschka without any signs of underlying pathology must be distinguished from the secondary form and referred to as primary, idiopathic, or congenital.7, 8, 9, 10 The available information about the primary form is contradictory in the literature. Some authors regard the membrane closing the foramen to be of pial,3, 11, 12 arachnoidal,6 or neural11 origin based on its macroscopic appearance. Others regard the origin to be from the imperforation of the roof of the fourth ventricle during embryonic development based on its ependymal lining.12, 13, 14 In our previous study, we hypothesized that the closure of the foramen is caused by an overspreading sheet-like neural tissue, called the rhomboid lip, that is located on the cisternal surface of choroid plexus in the CPA.2

Primary obstruction of the foramen of Luschka is clinically significant because it plays an important role in the development of various cystic structures, such as arachnoid cysts and diverticula of the CPA. Gardner et al.15 stated that arachnoid cysts of the CPA originated from the primary closure of the foramen. Fourth ventricle outlet obstruction (FVOO) is a rare and enigmatic entity caused by the simultaneous closure of the 3 apertures of the ventricle. Bilateral diverticula in the CPA communicating with the fourth ventricle are characteristic findings in FVOO.8, 9, 10, 16, 17, 18, 19, 20, 21, 22, 23 Cases of radiologically confirmed FVOO with no visible signs of an underlying pathology are referred to as primary, idiopathic, or congenital forms of FVOO.7, 8, 9, 10, 16, 18, 19, 21, 23, 24, 25, 26, 27 A subgroup of these patients have no congenital anomaly, such as Dandy-Walker or Chiari malformation. Instead, these FVOO cases are associated with atresia of the fourth ventricle outlets. Previous authors have described unilateral and bilateral pouches in the CPA arising from the foramen of Luschka and communicating with the fourth ventricle.14, 28, 29, 30, 31

To the best of our knowledge, no case of such a diverticulum has been reported in the recent literature, although there are several reports of CPA arachnoid cysts32, 33 and FVOO.9, 10, 16, 23 Only Nakahara et al.34 described a similar cystic structure recently, which was interpreted as an extremely large rhomboid lip. Motivated by the above-mentioned controversies in the recent literature as well as the poorly understood pathophysiology of the CPA arachnoid cysts and FVOO, the present study aims to give a detailed macroscopic and histologic description of the primary obstruction of the foramen of Luschka based on a large number of cadaveric examinations. Clinical cases of patients with symptomatic cystic lesions in the CPA were selected, and intraoperative and radiologic observations were analyzed. Suggestions for operative treatment strategies were made based on the analyzed clinical cases.

Section snippets

Cadaveric Examinations

The study included 61 formalin-fixed human brains (122 foramina of Luschka). The cadavers were donated for research and medical education to the Department of Anatomy, Histology and Embryology, Semmelweis University, Budapest. After removing the outer layer of the arachnoid mater at the CPA, the lower cranial nerves were elevated to visualize the foramen of Luschka. The macroscopic features of the obstructed foramina were examined. Photomicrographs were taken with a scale bar at every step of

Cadaveric Examinations

Primary obstruction of the foramen of Luschka was present in 11 of 122 cases (9.01%). All 11 closures (100%) were unilateral, 6 (54.54%) left-sided and 5 (46.46%) right-sided. The foramen of Magendie was patent in all 61 cases (100%).

The primary obstruction of the foramen of Luschka appeared as a small pouch lying on the cerebellar surface in 8 of 11 (72.72%) cases (Figure 1A). In 2 cases (18.18%), it appeared as a membrane stretched between the pons and cerebellum (Figure 1B). In 1 case

Discussion

The aperture of the lateral recess connecting the fourth ventricle and the subarachnoid space was described originally by von Luschka in the 19th century.39 The closed variant of the foramen is a known anatomic variation that has been mentioned by various other authors in the past11, 12, 14, 39, 40, 41 as well as more recently.2, 3, 6 The incidence of the primary closure varies between 0% and 20% in the literature.3, 6, 11, 12, 13, 14, 41, 42, 43, 44 Alexander13 reported that the bilateral

Conclusions

The primary membranous obstruction of the foramen of Luschka is caused by the overspreading rhomboid lip originating from the imperforation of the roof of the fourth ventricle during embryonic development. This results in a pouch in the CPA containing choroid plexus, which communicates with the fourth ventricle. This pouch carries the potential of growing into a symptomatic diverticulum. This pathology is similar to an arachnoid cyst both macroscopically and clinically, but there are some key

Acknowledgments

The authors thank Emese Palfi, assistant research fellow at the Department of Anatomy, Histology and Embryology, Semmelweis University, for her invaluable help in photographing the histologic slides; Maria Bako, histology assistant at the Department of Anatomy, Histology and Embryology, Semmelweis University, for her professional help during the histologic processing; and Istvan N. Huszar, Doctor of Philosophy student at the Nuffield Department of Clinical Neurosciences, University of Oxford,

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    Conflict of interest statement: The authors declare that the article content was composed in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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