Occipital emissary foramina in human skulls: review of literature and proposal of a classification scheme of the occipital venous anastomoses in the posterior cranial fossa

Abstract

Objectives: The present study aims to explain the interesting discrepancy between the occipital emissary foramina and the respective emissary veins in the literature. Majority of the studies report that the foramina have a low and variable frequency, but the emissary veins are reported to be disproportionately present in quite large number of patients in some diagnostic imaging studies. Methods: Seventy-five adult skulls were examined for the presence of occipital foramina. Results: A complete occipital emissary foramen was found only in one skull (1.33%), but a number of other skulls also showed some foramina on the external and internal surfaces of squamous part of the occipital bone. Conclusion: It can be concluded that foramina of another vein which is related to the squamous part of occipital bone, the occipital diploic vein, might be the main reason for the discrepancies present in the literature. The suggested classification scheme of venous anastomoses in the posterior cranial fossa can explain the variable bony foramina observed in skull series.

Prevalence and Morphometry of Occipital Foramen in Dry Skulls and its Clinical Implications

“…25 As far as EOC is concerned, in the present study, OEF was found in 3 skulls on EOC and on both sides of EOC in 3 skulls constituting 3.75% which is lower than that detected by Alghamdi et al 25 Single OEF was found on the left side of EOC at a distance of 9 mm from the border of the foramen magnum in 1.33% of skulls. 17 In the present study, a single OEF was detected on the left side of EOC in 4 skulls, constituting 5% which is higher than the above study.…”

“…The incidence of OEF in the present study is higher than that observed by Kadanoff and Mutafov 15 in Bulgarian skulls and by Sharma et al 16 in Indian skulls but lower than that detected by Subashri and Thenmozhi 14 . Variability in the incidence of this foramen was because some investigators designated all openings on occipital bone as OEF irrespective of their patency 17 . Only patent foramina transmit emissary veins, and so do true OEF 17 .…”

“…17 While the foramina on the occipital bone, which was not patent, transmits occipital diploic vein and so cannot be called OEF. 17 This indicates why incidences of OEF showed higher incidences in studies on dry skulls. 18 Moreover, the incidence of OEF is observed to be 28% in magnetic resonance imaging (MRI) studies on axial images 19 and 65% in patients with pseudotumor cerebri syndrome (PTCS) along with 30% in healthy individuals on subtracted computed tomography (CT) venography.…”

“…If the OEF is present near or at the EOP, it connects the occipital vein to the confluence of sinuses, 17 if situated on the EOC, it connects the occipital vein with the occipital sinus and if located near the foramen magnus, it connects the occipital vein with the marginal sinus. 17 As is clear, the OEV passing through OEF whether present on EOP, EOC or near the foramen magnum, it leads into one of major venous sinuses of cranial cavity. Thus, injury to OEV passing through OEF may cause catastrophic hemorrhage during suboccipital craniotomy.…”

“…Variability in the incidence of this foramen was because some investigators designated all openings on occipital bone as OEF irrespective of their patency 17 . Only patent foramina transmit emissary veins, and so do true OEF 17 . While the foramina on the occipital bone, which was not patent, transmits occipital diploic vein and so cannot be called OEF 17 .…”