Radiological analysis of the ethmoid roof in the Malaysian population

Abstract

Objective

To ascertain the prevalence of the lateral lamella of the cribriform plate height according to Keros classification in the Malaysian population, and to find if there is any difference between the major ethnic groups in Malaysia (Malay, Chinese, Indian).

Methods

Retrospective analysis of 150 CT scan studies of the paranasal sinuses.

Results

The mean height of the lateral lamella of the cribriform plate (LLCP) in 300 sides was 2.64 mm. Keros type I was seen in 240 sides (80%), while, Keros type II was seen in 60 sides (20%). There was no significant difference in the distribution of Keros classification among the major ethnic groups of Malaysia (Malay, Chinese, Indian). Keros type I was seen in 103 sides in males and 137 sides in females. Significant difference was observed in type II between the male and female (47 sides in males and 13 sides in female; p-value <0.001). When comparing the difference in the height of the LLCP in the same individual, asymmetry was observed in 139 patients. The LLCP height was higher on the left side in 71 patients and higher on the right in 68.

Conclusion

The understanding of the anatomy of the ethmoid roof with its possible variation is crucial to give the surgeon the optimal information about the possible risk that one can face during the surgery.

Introduction

Nowadays, endoscopic sinus surgery (ESS) is a common operation which is not only indicated for the management of chronic rhinosinusitis that resists medical treatment, but it extends to include a number of conditions and approaches like: nasal polyposis, mucocele, sellar and parasellar tumours, and optic nerve decompression [1].

Although ESS is widely performed, it is not devoid of complications. These can be divided into minor and major complications. Minor complications occur in 1.1–20.8% of functional endoscopic sinus surgery cases [2]. They include bleeding, infection, crusting, synechiae formation, ostial stenosis, tooth or lip numbness, and recurrence of the disease [2]. Major complications occur in 0–1.5% of such operations. They include, cerebrospinal fluid leak, ocular injury (herniation of orbital fat, extra-ocular muscle injury, ocular motility dysfunction, optic nerve injury, and peri-orbital hematoma or peri-orbital emphysema), and intracranial injury (brain or major blood vessels injury) [2], [3].

Most of the major complications are related to the ethmoid bone. In a study done by Dessi et al. [4], they reviewed the complications in 1192 endoscopic sinus procedure. Ethmoidectomy was the most hazardous procedure.

The ethmoid is one of the complex bones of the skull base. The roof of the ethmoidal labyrinth is formed by the fovea ethmoidalis, which is part of the frontal bone separating the ethmoidal cells from the anterior cranial fossa. Medially the fovea ethmoidalis attaches to the lateral lamella of the cribriform plate (LLCP), which is the thinnest bone of the skull base [5], [6].

It is already established that the area at risk of the ethmoid sinus is not the roof, that formed by the fovea ethmoidalis but, the LLCP, which is considered the thinnest and most vulnerable structure in the whole skull base [7], [8].

Keros [9] classified the depth of the olfactory fossa into three groups according to the height of the lateral lamella of the cribriform plate. In Type I, the olfactory fossa is 1–3 mm deep (the ethmoid roof is almost in the same plane as the cribriform plate). For Type II, the olfactory fossa is 4–7 mm deep. In Type III, the olfactory fossa is 8–16 mm deep (the ethmoid roof lies significantly above the cribriform plate).

He also found that as the height of the LLCP increases, the risk of penetrating the anterior cranial fossa increases [10].

The development and refinement of computerized tomography (CT) imaging has allowed detailed assessment of not only the sinonasal diseases, but also the characterization of the paranasal sinuses anatomy.

Understanding this complex anatomic relationship of the ethmoid roof and its variability is crucial to avoid potential intracranial complications during endoscopic sinus surgery.

In this study, we focus on the ethmoid roof because of its importance in ESS. Using the OsiriX software, we performed a quantitative CT scan analysis to categorize the lateral lamella height according to Keros classification in the Malaysian population, and to find, if there is any significant difference among the major ethnic groups in Malaysia (Malay, Chinese and Indian).