Surgery of the Endolymphatic SAC

Introduction

The endolymphatic sac is an important organ in maintaining fluid homeostasis within the inner ear system. Alterations in this process can result in a state of endolymphatic hydrops (ELH). Ménière disease (MD), a clinical syndrome of fluctuating, progressive hearing loss; episodic vertigo lasting 20 minutes to 24 hours; tinnitus; and aural fullness is the most common condition resulting from a hydropic state. Aggressive neoplasms may also develop from the endolymphatic sac. Surgery of the endolymphatic sac can be considered part of the treatment algorithm for MD and for endolymphatic sac tumors.

Prior to discussing the approach and technical aspects of endolymphatic sac surgery in this chapter, we review the anatomy, physiology, and pathophysiology of the endolymphatic sac. The diagnosis and management strategies for MD are discussed and patient selection, complications, and outcomes are reviewed. Although this chapter primarily focuses on MD, a brief discussion on endolymphatic sac tumors is also included.

Endolymphatic Anatomy

The endolymphatic sac and duct, the saccule and utricle, the membranous semicircular canals, and the cochlear duct or scala media are membranous structures filled with potassium-rich fluid known as endolymph. These structures are interconnected by the smaller utricular duct, saccular duct, and ductus reuniens. The membranous endolymphatic structures are surrounded by the sodium-rich perilymph that fills the periotic spaces within the bony labyrinth.

The position of the endolymphatic sac along the posterior fossa dura is relatively constant, but its size and the amount of bony covering by the operculum are variable. In most patients, 50% of the sac lies outside of the temporal bone and 50% of the sac is intraosseous. Approximately 10% of sacs are completely extraosseous along the posterior fossa dura. The sac can extend posterolaterally to cover the lateral sinus. The duct narrows at its isthmus to 0.1 to 0.2 mm in diameter.

Surgical Anatomy

From a surgical perspective, the endolymphatic sac is most often approached through the mastoid bone, and isolated on the lateral surface of the posterior fossa dura. Gibson noted that the extraosseous portion may be difficult to define surgically, appearing only as a thickened area of dura. Huang attributed the higher success in endolymphatic sac surgery to the definite identification of the sac, with entry into the true sac lumen, and preservation of the sac anatomy. Similarly, Kitahara et al. found that positive identification of the operculum was associated with better hearing outcomes after surgery. Ammirati et al. advocated preserving the integrity of the endolymphatic system, suggesting severe audiovestibular disturbances that may follow sac disturbance; however, others have found that patients tolerate the complete excision of the sac without hearing loss, whether the excision is incidental in other cranial base procedures, or intentional for complete endolymphatic sac ablation.

Important topographic landmarks for identifying the endolymphatic sac exist from the transmastoid or posterior fossa approach. The transmastoid extradural landmarks for localization of the endolymphatic sac and for preservation of the labyrinthine structures include Donaldson’s line, an imaginary line drawn posteriorly through the plane of the horizontal semicircular canal ( Fig. 32.1 ), and measurements delineating the hard angle. The endolymphatic sac is generally found along the posterior fossa dura inferior to Donaldson’s line. Caution must be used when identifying the sac to avoid damage to the facial nerve and the posterior semicircular canal. Anatomical variants of the normal temporal bone anatomy have been associated with MD and an understanding of the potentially altered anatomy is important for surgical planning of sac procedures. Hypoplasia of the mastoid air-cell system, hypocellularity of the periaqueductal cells around the endolymphatic duct and sac, reduction of the aditus ad antrum, and hypoplasia of the facial recess have all been described.

Intradural identification of the endolymphatic sac in relation to the anatomical structures of the posterior fossa places the sac 10 to 15 mm lateral to the internal auditory meatus, and 11 to 17 mm posterosuperior to the 11th cranial nerve in the jugular foramen. Typically, the thickening of the dura and the bony ledge of the operculum pinpoint the location of the sac.

Endolymphatic Physiology

The maintenance of a potassium-rich endolymph produces the endocochlear potential, a DC voltage gradient, to drive the transduction process which is important in the detection of sound, motion, and position. ^, There are several theories on how the endolymphatic sac contributes to the electrolyte and fluid homeostasis within the endolymphatic system. Understanding this physiology, and therefore pathophysiology, informs the rationale of treatment options for symptomatic ELH.

The oldest and predominant theory on the role of the endolymphatic sac suggests that it is an organ for the resorption of endolymph produced elsewhere within the system. Endolymph is produced by dark cells located largely in the stria vascularis, but also in the vestibular ampullae, within the maculae of the saccule and utricle, and along the endolymphatic duct. Active ion transport through a sodium-hydrogen exchange system within the vestibular dark cells along the endolymphatic system promotes a dynamic linear flow toward the sac where the fluid is reabsorbed, maintaining the fluid homeostasis.

Salt proposed another theory in which there is negligible flow of endolymph in a normal state. This was based on the findings that direct measurements of the dispersal markers in the endolymph failed to support dynamic flow. Rather than linear flow of endolymph toward the endolymphatic sac, he proposed that the endolymphatic sac acts as a master volume regulator, promoting bidirectional endolymphatic flow in response to volume needs. The ionic component of the endolymph is maintained through single cell transport of ions. In contrast to the endolymph throughout the inner ear, a gradient exists along the duct, leaving the electrolyte state in the endolymphatic sac high in sodium and low in potassium. An active equilibration of ions creates an osmotic potential that may influence the transepithelial flow of fluids. Higher concentrations of Na ⁺ /K ⁺ -ATPase are seen in the endolymphatic sac but diminish proximally along the endolymphatic duct.

Several other findings support an active role of the endolymphatic sac on endolymph fluid homeostasis. Aquaporin 2; vasopressin type 2 receptor; and transient receptor potential channel vanilloid (TRPCV), subfamily type 1 and 4, were identified in the epithelial lining of the endolymphatic sac, but not in other extracellular tissues, although TRPCV 1 was observed in the surrounding vasculature. Similar findings are observed within the kidneys, suggesting a parallel role in fluid filtration and resorption. A unique protein, saccin, secreted by the endolymphatic sac, acts as an endogenous inhibitor of sodium reabsorption within the kidney. The endolymphatic sac chief cells possess organelles capable of endocrine function, and a cellular ultrastructure consistent with merocrine activity is also observed. ^,

The biochemical and cellular findings within the endolymphatic sac support its functional role in the phagocytic, immune, and allergic responses of the inner ear. Volume may also be regulated by the functional mechanical entity of an extracellular matrix of interstitial cells that support the endolymphatic duct and participate in the control of inner ear fluid dynamics and endolymph resorption. As new techniques are developed to detect and monitor minute changes in the fluid volumes, and to evaluate the chemical composition and cellular characteristics of the endolymphatic sac, a better understanding of the physiological role and pathophysiological state of the endolymphatic system in MD will be achieved. ^,

Endolymphatic Hydrops: Pathophysiology

The state of endolymphatic hydrops was correlated with the symptoms of MD in 1938 by Hallpike and Cairns. These authors identified dilated endolymphatic spaces in the temporal bone specimens of two patients with the clinical symptoms of MD who died after neurotomy of CN VIII. Their findings have been confirmed in other temporal bone studies and are described as the primary pathological correlate of MD.

The underlying cause of the hydropic state is presently unknown, although many theories exist. Conceivably, MD may represent a common end-organ condition of the endolymphatic system which may have many etiologies. Hydrops is seen more often in the cochlea and saccule, structures derived from the later developing pars inferior. Congenital insults or developmental aberrations later in embryogenesis could presumably account for this difference. A familial connection is seen in the history of 20% of patients clinically diagnosed with MD. Considering the embryological and anatomical findings associated with MD, this connection suggests a multifactorial predisposition to developing ELH. Whether this precondition is genetic or related to shared environmental factors or insults is yet to be determined.

The pathophysiological state of the endolymphatic system has been partially modeled by experimental destruction and obstruction of the sac or duct in attempts to pinpoint the underlying mechanism of the hydropic condition. In experiments in guinea pigs wherein hydrops and audiometric findings similar to those of MD were reproduced, vestibular dysfunction or vertigo was noticed only after placing the animals in a theoretical head-down position by inducing additional pressure within the inner ear fluids. The predominant theories that could explain the symptom complex of MD are based on the observed pathological and induced experimental evidence of hydrops. The temporal bone findings are summarized by da Costa et al. and include ruptures of the membranous labyrinth, fistulas of the membranous labyrinth, collapse of the membranous labyrinth, obstruction of longitudinal flow, vestibular fibrosis, sensory lesions, and neural lesions. The rupture, distention, and drainage theories are described.

Schuknecht proposed the rupture theory . He reasoned that distention of the endolymphatic space with eventual membrane rupture could cross-contaminate the perilymphatic spaces and toxify the delicate sensory hair cells with the potassium-rich endolymph. In the distention theory, Paparella described the decompensation of radial flow as the perilymphatic spaces ebb, leading to largely longitudinal flow along the hydropic membranous pathway with the saccule acting as a reservoir for the excess endolymph. As the dilated saccule encroaches on the confines of the vestibule, mechanical interference of the cochlear and vestibular function occurs by inhibition of traveling waves and physical contact with the crista ampullaris.

The drainage theory , as presented by Gibson and Arenberg, suggests that the obstruction of a narrowed endolymphatic duct divests the sac of endolymph. The endolymphatic sac responds by secreting glycoproteins and saccin. Glycoproteins act osmotically by “pulling” endolymph toward the sac, and saccin stimulates the secretion of endolymph from dark cells that distend the endolymphatic spaces “pushing” against the obstruction toward the sac. The obstructing debris ultimately and suddenly passes, and the resultant rapid flow of endolymph purportedly brings on an acute vertiginous episode. Gibson and Arenberg suggested that patients with MD with larger vestibular aqueducts could experience the resolution of auditory symptoms, as in Lermoyez syndrome, after clearance of the obstructing debris. Patients with MD have been shown to have widening of the vestibular aqueduct aperture; however, although it is enticing to establish a mechanism of pathology, patients with Lermoyez syndrome have not been shown to have wider vestibular aqueducts than do other patients with MD. Gibson and Arenberg also theorized that Tumarkin crises could be the effect of a membrane rupture in the overdistended endolymphatic space.

Although individuals with MD often have no apparent cause, there are several predisposing causes of secondary Ménière syndromes. Ménière syndrome may be precipitated by trauma (inner ear fracture, perilymphatic fistula), infections (viral, syphilis), inflammation (autoimmune, allergy, Cogan syndrome, sarcoidosis), metabolic diseases (diabetes), congenital or genetic predisposition (inner ear malformations, enlarged vestibular aqueduct), neoplasm (vestibular schwannoma), vascular disease (migraine, hemorrhage), or iatrogenic causes (stapes or mastoid surgery). These disease processes may lead to secondary ELH by causing scarring within the labyrinth, thereby activating inflammatory cytokines or complement pathways which yield edema, fibrosis, and altered extracellular matrices. Altering the hemodynamics may affect the transcellular ionic exchange and endolymph homeostasis. Creating obstructive immune complexes or cellular debris may activate cellular responses through humoral messengers. The result is the distention or obstruction of the endolymphatic flow, which disturbs the sensation of the cochleovestibular signals and results in distorted messages of sound, station, and motion being sent.

Ménière Disease