Introduction

Endolymph, also known as Scarpa fluid, is a clear fluid that can found in the membranous labyrinth of the inner ear. It is unique in composition compared to other extracellular fluids in the body due to its high potassium ion concentration (140 mEq/L) and low sodium ion concentration (15 mEq/L). Many tissues play key roles in the production and maintenance of the ionic composition of endolymph, including the Reissner membrane, stria vascularis, and dark cells of the vestibular organs. Endolymph is separated from surrounding perilymph by the Reissner membrane that forms a barrier between the two fluids. The Reissner membrane allows for selective ion transport and ultimately the production of endolymph from perilymph. Another important tissue involved in the production of endolymph is the stria vascularis found lining the lateral wall of the cochlear duct. The cells found in this tissue help to maintain the high membrane potential and potassium ion concentration of endolymph. Dark cells present in the cristae ampullaris of the semi-circular canals make use of the sodium/potassium ATPase pump to actively pump potassium into the endolymphatic fluid. The ion concentration created and maintained by these various tissues results in endolymph having a high positive potential relative to perilymph. The potential gradient created between the two fluids allows for high sensitivity to sound waves that result in depolarization and nerve transmission to the brain for interpretation. Many tissues play a role in the regulation and reabsorption of endolymph; however, these processes are not entirely understood.[1][2][3][4]

Structure and Function

Endolymph serves several important sensory functions based on its interaction with cells in either the vestibular apparatus or the cochlear duct.

Vestibular Apparatus

The vestibular apparatus is composed of the utricle, saccule, and three semicircular ducts. The acceleration of endolymph within regions of the vestibular apparatus allows for our perception of balance and equilibrium. This occurs through head movement that causes endolymph to move specialized cells known as hair cells. Hair cells are arranged in rows and contain "tip-links" that connect them and either depolarize or hyperpolarize afferent nerve fibers based on their direction of movement. Depolarization of hair cells results in an influx of potassium ions from endolymph that leads to calcium channel opening and, subsequently, neurotransmission along nerve fibers. These nerve fibers carry information from different regions of the vestibular apparatus that each plays a unique role in our perception of balance and equilibrium. The saccule and utricle are bones connected to the semicircular ducts that each contain a macula responsible for the perception of linear movement based on endolymph stimulation of hair cells. Likewise, rotational movement is sensed by the semicircular ducts aligned in the lateral, superior, and posterior planes. The ipsilateral semicircular ducts are functionally coupled with the contralateral semicircular ducts to provide feedback of spatial orientation of the head and equilibrium from endolymph movement via the vestibulocochlear nerve. This relays to the visual system to create the vestibulocochlear reflex, which allows one to keep their eyes fixed on an object while the head is moving in a horizontal plane.

Cochlear Duct

Endolymph in the cochlear duct plays a very important role in the perception of sound. This occurs when pressure waves travel down the external acoustic meatus and strike the tympanic membrane, causing it to vibrate. These vibrations are transferred to the ossicular chain, consisting of the malleus, incus, and stapes, to the oval window opening to the bony labyrinth vestibule. The footplate of the stapes transfers these pressure waves to the perilymph and ultimately the endolymph. Vibrations in the endolymph stimulate regional hair cells in the organ of Corti based on the frequency of the vibration, creating a tonotopic map along the cochlea. Based on the region of hair cells stimulated by vibration in the endolymph, nerve impulses are sent to the brain via the cochlear portion of the vestibulocochlear nerve. The brain then interprets these impulses as individual frequencies of sound.[5][3][6]

Embryology

The inner ear is formed embryologically from ectodermal tissue. The otic placode is formed from ectoderm and becomes the otic pit. The otic pit becomes the otic vesicle, which then forms the inner ear within the petrous temporal bone.[7]

Blood Supply and Lymphatics

The arterial supply to the vestibulocochlear system begins with the basilar artery. The basilar artery gives rise to the anterior inferior cerebellar artery (AICA), which gives off the internal auditory artery. The internal auditory artery gives rise to the common cochlear artery and anterior vestibular artery to supply the inner ear through a complex network of anastomoses.[8]

The venous drainage of the vestibulocochlear system involves two main veins. One of these veins drains the central part of the sensorial areas (maculae plus cristae) and then becomes the vein of the cochlear aqueduct. The other vein drains the peripheral part of the sensorial areas as well as the simple endolymphatic walls and becomes the vein of the vestibular aqueduct. The latter vein has a close relationship with the endolymphatic sac.[9]

Nerves

Endolymphatic fluid waves generated by mechanical vibrations result in the transmission of nerve impulses along the vestibulocochlear nerve.

Surgical Considerations

In patients with refractory Meniere disease, surgery to decompress the endolymphatic sinus or transect the vestibular nerve have been used as last-resort treatments.[10]

Clinical Significance

Expansion of the volume of endolymph in the endolymphatic space results in endolymphatic hydrops. This finding plays a role in the symptoms associated with disease; however, the presence of endolymphatic hydrops does not always result in Meniere disease. Meniere disease is characterized by a group of progressive and recurrent symptoms involving vertigo, hearing loss, tinnitus, and a sense of increased pressure in one or both ears.

Endolymphatic hydrops can be observed microscopically as an enlargement of the endolymphatic space and distention of the Reissner membrane. It is worth noting that endolymphatic hydrops occurs in the presence of volume expansion with a negligible change in pressure in both the endolymph and perilymph. This is due to the high compliance of the endolymphatic boundaries that can accommodate for volume increase without an associated substantial pressure increase.

The pathophysiology of endolymphatic hydrops was once hypothesized to be a result of impaired flow and reabsorption of endolymph through the endolymphatic sac; however, recent evidence suggests that the volume expansion is the result of ion transport and osmotic gradient dysregulation. This failure of homeostasis is now thought to be associated with hormonal dysregulation of aquaporins that play a role in fluid equilibrium across endolymphatic boundaries.[2][11][12]