Minimally Invasive Endoscopic Trans-Eustachian Tube Repair of Refractory Lateral Skull Base CSF Rhinorrhea: Surgical Parameters

Cerebral spinal fluid (CSF) rhinorrhea occurs secondary to lateral skull base pathology and defects created by trauma, congenital defects, neoplasms, and iatrogenically. Refractory CSF rhinorrhea from a lateral skull base source is most commonly managed via open surgical approaches. The approach for repair is dictated by the location of the skull base and dural defect. The common pathway, however, is through the eustachian tube (ET). Given the advances in endoscopic surgery, endoscopic endonasal eustachian tube obliteration (EEETO) and endoscopic lateral skull base approaches have been utilized successfully to seal the common pathway for CSF rhinorrhea (Lemonnier et al, Am J Rhinol Allergy, 2017; Deep and Weisskopf, Otol Neurotol, 2017). While the anatomy of the ET has been studied, surgical landmarks of the Eustachian tube have not been previously described or quantified via cadaveric dissection.

The ET was cannulated bilaterally in 5 cadaveric heads with a flexible guide wire from its middle ear orifice along its course within the middle fossa to its nasopharyngeal orifice. A zero degree endoscope (Storz, Germany) was used to visualize the wire endonasally and through its entry into the middle ear orifice via the external auditory canal. An exoscope (Storz, Germany) was used to visualize as we drilled (Depuy-Synthes) to expose the length of the Eustachian tube as it extends from the middle ear, along the course of the middle fossa (lateral to the petrous carotid), and as it transitions from the horizontal to vertical segment as it leads to the nasopharynx. We defined this transition point from horizontal to vertical segment, the ET genu.

We divided the ET measurements into three sections: the total ET length (tET) from nasopharyngeal orifice to the anterior tympanic membrane ridge (just lateral to the middle ear ET orifice); the anterior segment from the nasopharyngeal orifice to the genu (aET), and the posterior ET length (pET) along the petrous carotid from the genu to the anterior TM ridge. Direct measurements of tET and pET were made, while aET was then calculated.

The mean tET length was 47.6 mm (SD ± 3.1 mm), mean pET length was 24.8 mm (SD ± 4.5 mm), and mean aET length was 21.9 mm (SD ± 5.5 mm); see.

This is the first report to summarize these studies and redefine ET anatomy, specifically the genu and its position along the length of the organ. This serves the purposes of current techniques of ET obliteration such as cauterization, packing, and for ongoing development of a catheter based repair of CSF leak either from an anterior endonasal approach or a lateral technique. Packing techniques might utilize the entire length of the ET, while cauterization-based techniques might best avoid the pET lying near to the petrous carotid. Indeed, prior studies may have underestimated these distances, as the curvilinear shape of the ET is frequently poorly accounted for. The definition of the anatomic landmarks and correlation to distance and critical structures provide the basis for minimally invasive instrumentation/techniques to treat CSF rhinorrhea from a lateral skull base source.