Timothy C. Hain, MD • Page last modified: August 23, 2020
Within the inner ear, there are two types of fluid -- endolymph (inner fluid), and perilymph (outer fluid), separated by a membrane. Something like a "balloon within a balloon" arrangement.
There are several ear disorders in which it is generally thought that pressure anomalies within the inner ear contribute to ear symptoms -- these include Meniere's disease, Perilymph fistula, Enlarged vestibular aqueduct syndrome, superior canal dehiscence, and low spinal-fluid pressure syndromes. There are also endolymphatic sac tumors associated with or without von Hippel-Lindau Disease (VHL)
The endolymphatic sac (ES) is a membranous structure in the inner ear located partly in the temporal bone and partly within the dura of the posterior fossa. It contains endolymph, which is similar in chemical makeup to intracellular fluid (high in K, low in Na). It was first described by Cotugno, in 1760 (Corrales and Mudry, 2017).
Artist's rendition of endolymphatic system. Blue indicates endolymph. ES (endolymphatic sac), ED (Endolymphatic duct). Diagram of endolymphatic compartments, originally due to Spalteholtz (1914). This illustration is from Bast (1937)
The endolymph in the ES is connected to that of other endolymphatic spaces of the inner ear via the endolymphatic duct (ED) as well as the utricular duct and the duct between the saccule and cochlear duct (ductus reuniens). The vestibular aqueduct, a bony canal, contains both the endolymphatic sac and duct. Note that on the picture on the left above, although it shows the main features, the artist has taken some liberties with the anatomy for clarity.
A more detailed diagram due to Spalteholtz (1914) is shown on the right. Here it can be seen that the endolymphatic system is broadly separated into two compartments, one vestibular on the left (excepting the saccule), and the other cochlear + saccule on the right. Between the two compartments are several narrow ducts -- namely the utriculosaccular duct (containing the "valve of Bast", and possibly the entrance to the endolymphatic duct), and the ductus reuniens between the saccule and the cochlear duct. These ducts originate from the areas containing the otolithic maculae -- the utricle in one case, and the saccule in another, and presumably loose otoconia from time to time. Some have suggested that otoconia might plug these ducts up causing pressure irregularities within the labyrinth.
The endolymphatic sac is possibly involved with fluid exchange in the ear (endolymph absoprtion, regulation of endolymphatic volume and pressure, fluid and ion transporter, mechanical valve, endolymph production), as well as immune regulation. (Corrales and Mudry, 2017)
The endolymphatic sac is sometimes surgically obliterated, shunted, or decompressed in an attempt to change the natural history of Meniere's disease. The rationale for this "endolymphatic shunt" surgery is difficult to follow, and in fact, there is considerable evidence that this surgery is a placebo treatment.
As in any plumbing system, the narrow spots are of particular interest when one is considering blockages.
Regarding the utriculosaccular duct, Bast (1937) states that the utricular side is a "slit" measuring about 1200 microns in length. The lumen varies from being oval to round, the latter having a diameter of about 25 to 35u. This is roughly the width of several otoconia (which are about 6-15 u), and it would seem possible that several together might block the duct. The valve, according to Bast, and shown below on the right, is formed by the reflection of the wall of the utricular duct and that portion of the antero-medial wall of the utriculus. Again according to Bast, the valve might aid in closing of the utricular end of the utricular duct.
Anatomic formulation of Monsanto et al (2016), showing structures involved in Endolymphatic flow. Illustration from Bast, 1937, showing a different arrangement of structures around the endolymphatic duct. U.D. refers to utricular duct, U.V. to utricular valve, E.D. to endolymphatic duct.
Monsanto et al (2016), provided a slightly different anatomical formulation than the two illustrations above, concerning the critical area around the endolymphatic duct. They show a separate opening for the endolymphatic duct (called the "sinus") and the utricular duct. If this anatomic organization is correct, then pressure in the superior labyrinthine compartment might, in theory, be different than pre sure in the cochlea and saccule, given that they are separated by a the valve of Bast. Also, the lower cochlear compartment might be protected from pressure in the endolymphatic duct by the valve.
On the right are drawings from Bast (1937) showing the more common arrangement where the duct between the utricle and saccule (utriculosaccular duct) has a "T" joint where the endolymphatic duct takes off to go to the sac. If this arrangement is correct, then one would think that the upper labyrinthine compartment might be be protected from low-pressure in the endolymphatic duct. So there are some subtle differences, and two different anatomical formulations. We are not sure which of these descriptions is correct.
Monsanto et al (2016) reported that Bast's valve was far more often open (54%) in Meniere's temporal bones than normal specimens. They speculated that Bast's valve might protect the superior portion of the labyrinth (i.e. that contains most of the vestibular system) from collapsing, in the event of decreased volume or ruptures of the membranous labyrinth. From the anatomy, one would think that this protection would apply to lowered pressure of the endolymph contained within the cochlear system, but would not provide any protection from overpressure. On the other hand, overpressure in the labyrinthine system, perhaps transmitted through the ED in the formulation of Monsanto, might not be as easily transmitted to the cochlear compartment with this valve in place.
Perilymph is similar to spinal fluid. It surrounds the endolymphatic compartment, and is connected to spinal fluid through several pahways. On the diagram on the top of this page, the blue is endolymph, and the light brown around it is perilymph.
The cochlear canaliculus (more commonly but more confusingly called "cochlear aqueduct") contains perilymph (as opposed to the endolymph containing cochlear duct). It is connected via a narrow channel containing fibrous tissue, to the spinal fluid compartment. We do not think these structures are clearly named -- aqueduct seems pretty similar to duct to us. Another way to get around this is to avoid using the term "cochlear duct".
It is thought that the cochlear canaliculus (aqueduct) is one possible route by which an ear infection can cause meningitis. Of course, for this to happen, bacteria in the middle ear would have to first enter the inner ear, and only then could they access spinal fluid. Going the other direction, meningitis may affect the inner ear when virus or bacteria from spinal fluid enter the ear via the cochlear canaliculus. Delayed hearing loss after meningitis is common. In the author's experience, this frequently presents as a slowly progressive sensorineural hearing loss.
When considering the clinical syndromes of pressure sensitivity, such as found in superior canal dehiscence, perilymph fistula, and Meniere's disease, one naturally must also think about the various interfaces that the inner ear has to other compartments -- spinal fluid and the air pressure in the middle ear.
Between the middle ear and the perilymph of the inner ear there are two moveable structures -- the oval and round windows. The oval window is coupled to external ear pressure via the tympanic membrane. The round window is coupled to middle ear pressure through it's membranous interface.
Perilymph is connected fairly directly to spinal fluid pathways via the cochlear canaliculus (aqueduct). When spinal fluid pressure changes, perilymph pressure changes within about 10 seconds. In some people, there may be also a more direct connection around the vestibular nerve (accounting for the so-called "gusher" found on fistula surgery). In SCD, the time constant between CSF and perilymph should be much shorter as there is a near direct connection between CSF and spinal fluid. In perilymph fistula, there should be a generally lower perilymph pressure than normal as well as an increased "compliance" of the perilymphatic compartment (i.e. reduced stiffness). Rapid changes in CSF pressure should be transmitted more easily to perilymph.
Endolymph is coupled more indirectly to CSF pressure and air pressure, through membranes. One pathway is via the endolymphatic duct, to the sac, and the dural membrane. Another is via the membranes that separate the endolymphatic and perilymphatic compartments. Curiously, there are many narrow tubes in the endolymphatic system - -the endolymphatic duct, and two tiny ducts between the vestibular system and the cochlea. It would seem from this design that there is something to be gained by isolating these structures from each other.
Another way to think about interfaces and coupling is to consider the frequency response of the compartments. Perilymph is the most directly coupled to spinal fluid -- and therefore has the highest frequency response. A rough estimate of the time constant is 10 seconds. Perilymph is also coupled to air via the oval and round windows.
Endolymph is not directly coupled and thus should have minimal high-frequency response, and by the same token, might not be able to easily "equalize" pressure. In persons with "hydrops", one would expect that the endolymphatic compartment would have even less than normal ability to equalize pressure.