Regional Anesthesia & Pain Medicine | 2021
Novel method of locating the foramen ovale: a wobbly chair with shaky legs
Abstract
To the Editor We read with interest the article by He et al describing a novel fluoroscopic landmark to facilitate visualization of the foramen ovale (FO) and would like to submit some considerations. In our opinion, their ‘Hfigure’, recognized as two vertical lines (the medial border of the mandible and the lateral edge of the maxilla) and a horizontal one (the petrous ridge of temporal bone), is a practical mnemonic but not a new intraoperative radiological landmark. Actually, that inferior transfacial–oblique view is chapter and verse as described 5 years earlier by De Córdoba and colleagues. Furthermore, in our stepwise method of locating the FO, we quoted ‘rotate the Carm in the axial plane until the petrous apex is situated bellow the maxillary sinus and above the jaw’ and also provided a fluoroscopic projection which is the very image of the Hfigure. They are to be commended for having found that this tool can help reduce radiation and procedure time, notwithstanding. They emphasized that suboptimal FO visualization can place important structures at risk. In this regard, it should be noted that FO penetration is not only a stereotaxic procedure but also a freehand skill that must be adapted on the spot. Thus, after conclusive identification of the foramen comes the most important stage of the process: how to address it at the right angle. We name this step the ‘stall angle of attack’, in reference to the abrupt reduction in lift that a plane undergoes once it exceeds a critical point in the angle of attack. The Gasserian Ganglion and its plexus triangularis are a highly flattened structure resting close on the anterior surface of the petrous bone. Therefore, for a needle to penetrate the ganglion longitudinally, it must follow a path parallel to this surface and must not deviate from this direction. A too horizontal penetration may result in the passage of the needle being impeded by the petrous bone, but if the trajectory is too vertical, the needle may exit rapidly from the dura and strike the temporal lobe; an intermediate angle is required to penetrate the retroganglionic rootles. The ideal angle of attack for cannulating the FO passes just by the top edge of the petrous ridge and slides neatly towards the trigeminal root. For control of this, the Carm should be adjusted so that the FO appears slitlike, not oval, or, even worse, round, and is partially hidden behind the ridge, giving the appearance of a setting sun (figure 1A). Naturally, this latter projection takes the entry point higher up on the cheek. Indeed, postmortem examinations have shown that in order to enter the thin ganglion and the sensory root behind it, the angle of the needle should be kept low by entering the skin as high as possible, skirting the lower border of the maxilla and passing obliquely through the foramen (figure 1B). Admittedly, this does not match with the sagittal angulation He et al recommended because it was adjusted to obtain ‘the maximal short to long axis ratio of the FO’. This is corroborated by the fact that only 6% of their final FO views were ‘grade 1’, that is, a short to long axis ratio of ≤1/3, or likewise ‘sun not yet risen’ views. Last but not least, once the FO is pierced, lateral fluoroscopy must be used for needle advancement. The needle trajectory should be aimed directly at the intersection of the shadows of the clivus and the petrous ridge, where Meckel’s cave resides. Otherwise, numerous intracranial complications can arise from elevational displacement of the needle, mainly temporal lobe hematoma and abducens nerve injury (figure 1C). Jose Luis De Córdoba, Carlos GarcíaMarqueta, Núria Isach Anesthesiology and Pain Medicine, Hospital de Mollet, Mollet del Valles, Barcelona, Spain Anesthesiology and Pain Medicine, Hospital General de Granollers, Granollers, Catalunya, Spain