Timothy C. Hain, MD Page last modified: June 18, 2009
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).
The endolymph in the ES is connected to that of other endolymphatic spaces of the inner ear via the endolymphatic duct (ED). The vestibular aqueduct contains both the ES and ED. Note that on the picture below, although it shows the main features, the artist has taken some liberties with the anatomy for clarity -- see Figure 7.294 of Gray's Anatomy for a more accurate rendition.
The endolymphatic sac is involved both with fluid exchange in the ear as well as immune regulation. 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 (see section below on fluid exchange), and in fact, there is considerable evidence that this surgery is a placebo treatment.
Other named ducts in the endolymphatic system (blue on figure) include the cochlear duct (within the cochlea) ductus reuniens (between the saccule and cochlear duct), the utriculosaccular duct (between utricle and saccule).
Perilymph is similar to spinal fluid. It surrounds the endolymphatic compartment, and is connected to spinal fluid through several pahways. On the diagram above, the blue is endolymph, and the light brown around it is perilymph.
The cochlear canaliculus 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.
It is thought that the cochlear canaliculus 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 progresive 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. 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 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.
Thinking from a design perspective for a moment, the multiple narrow ducts inside of the endolymphatic system would not be very conducive to providing a quick pathway for fluid exchange. In fact, the narrow ducts somewhat isolate the cochlear compartment from the vestibular compartment as well as spinal fluid. Considering this design, it seems unlikely that narrowing of the endolymphatic duct (i.e. plumbing) is the explanation for the high pressure found in Meniere's disease.