Perilymphatic Fistula

Sustained interest in perilymphatic fistula (PLF) began in the mid 1960s. The initial focus was on perilymphatic fistulas (PLFs) that developed following stapedectomy procedures. The recognition that loss of perilymph following stapedectomy could produce hearing loss, disequilibrium, and tinnitus led to the recognition that these same symptoms could arise from perilymph loss caused by other types of trauma. In the early 1970s, Victor Goodhill first distinguished between implosive and explosive forces as potential causes for rupture of the round or oval window membrane leading to PLF. ^[4]

General agreement exists that perilymphatic fistula (PLF) occurs and that leakage of perilymph from the inner ear into the middle ear can cause acute hearing loss, paroxysmal vertigo, disequilibrium, and tinnitus. Virtually no consensus exists regarding how frequently perilymphatic fistula (PLF) occurs, how it should be diagnosed, or how it should be treated when suspected. A high degree of controversy surrounds the appropriate indications for surgical exploration and operative repair of perilymphatic fistula (PLF).

Frequency

The frequency of perilymphatic fistula is unknown, but it is a very rare condition.

Perilymphatic fistula (PLF) occurs when perilymph leaks from the perilymphatic spaces of the bony labyrinth into the middle ear space. The loss of perilymph alters the balance between perilymph and endolymph within the membranous labyrinth. PLF is thus a form of inner ear fluid imbalance. Ménière disease, another form of inner ear fluid imbalance, occurs when endolymph is present in overabundance. Both disease entities alter the endolymph-to-perilymph ratio in the same direction, which may account for the difficulty involved with separating their clinical presentations.

Inner ear fluids are almost completely contained within a rigid bony framework. However, the perilymphatic space is connected to the subarachnoid space via the cochlear aqueduct. The size of the cochlear aqueduct varies dramatically between subjects. Even when it is relatively large, the cochlear aqueduct is often completely obstructed with arachnoid tissue. Evidence exists that the arachnoid becomes denser with increasing age. Thus, the patency of the cochlear aqueduct varies among persons. Establishing whether the cochlear aqueduct is patent in any particular individual is impossible.

A variety of indirect evidence suggests that pressure changes within the subarachnoid space can be communicated to the inner ear. Such pressure changes may be communicated through the cochlear aqueduct or alternatively through a patent lamina cribrosa, cochlear modiolus, or vestibular aqueduct. At least in some patients, stapes gushers appear to arise from a defective modiolus, allowing free communication between the spinal fluid and perilymphatic space.

The length of the cochlear aqueduct and the presence of arachnoid within its lumen tend to dampen the effects of sudden pressure changes in the subarachnoid space, thereby protecting the inner ear from rapid pressure changes. Simultaneous increases in pressure on the endolymphatic sac also protect the intracochlear membranes by equalizing endolymphatic and perilymphatic pressure. Even so, sudden increases in subarachnoid pressure can be transferred to the inner ear fluids.

At the round window membrane and the annulus of the oval window, the middle ear space is separated from the perilymphatic space by a soft tissue membrane. These areas permit pressure change between the middle ear and perilymphatic space. Excess pressure generated on either side of these membranes can result in a tear or rupture leading to the egress of perilymphatic fluid from the membranous labyrinth. In addition to the oval window annulus and round window membrane, which are present in every person with normal hearing, a number of other sites for perilymphatic fistula (PLF) are possible in some individuals.

• The fistula ante fenestram lies anterior to the anterior portion of the stapes footplate and is an embryonic residua formed by the resorption of precartilage in the fetus.

• Microfissures have been noted with regularity extending from the ampulla of the posterior semicircular canal to the round window.

• Congenital anomalies of the inner ear are associated with a much higher incidence of perilymphatic fistula (PLF). Incomplete partitioning of the cochlea (Mondini dysplasia) is often accompanied by incomplete formation of the stapedial footplate and a high incidence of perilymphatic fistula (PLF).

Goodhill separated the hydrodynamic forces that could potentially produce perilymphatic fistula (PLF) into implosive and explosive forces. Everyday activities, such as lifting, straining, coughing, and sneezing, are associated with increases in cerebrospinal fluid (CSF) pressure. Such pressure increases can be transmitted to the inner ear via a patent cochlear aqueduct or through the lamina cribrosa of the internal auditory canal. A precipitous increase in pressure within inner ear fluids can result in tears of the oval window annulus or round window membrane. Vulnerability to such tears may depend on the inherent strength of these tissues, which may vary among individuals. Goodhill termed tears produced by increased pressure within the inner ear fluid compartment as explosive. ^[4]

Conversely, pressure can increase rapidly within the middle ear space as a result of barometric pressure change, compression trauma of the ear, Valsalva maneuver, and pinched-nose sneezing. Ruptures or tears of the round window membrane, oval window annulus, or membranes protecting a fistula ante fenestram that result from increased middle ear pressure are also termed explosive forces by Goodhill.

In addition to changes related to hydrodynamic pressure, head injury may produce rupture of the membranes that seal the inner ear and prevent escape of perilymphatic fluid into the middle ear space.

The precise mechanism by which perilymph loss produces hearing loss is unclear. Decompression of the perilymphatic space may create secondary endolymphatic hydrops. Consequently, the usual symptoms of endolymphatic hydrops (Ménière disease) arise. Secondary endolymphatic hydrops has been histologically identified in experimental animals with perilymphatic fistula (PLF). Compelling indirect evidence exists that hearing loss in this circumstance is the result of loss of perilymphatic pressure. A large number of reports describe hearing loss associated with decreased subarachnoid pressure secondary to spinal anesthesia. Hardy demonstrated that such hearing loss can be reversed by injecting 20 mL of isotonic sodium chloride solution into the subarachnoid space. ^[5]

In 1974, Robertson demonstrated changes in the tuning curves of single spiral ganglion cells from the basal membrane after removal of perilymph from the tympanic scala of the guinea pig. ^[6] Flint et al showed an increase in auditory brainstem response (ABR) thresholds of 10-15 dB in guinea pigs with artificially created PLF; however, the etiology remains controversial. ^[7] Bohmer demonstrated no change in auditory thresholds measured in guinea pigs with simple round window membrane perforation, even though a decrease was noted in the normal positive hydrostatic pressure within the cochlea. ^[8]

Evidence of auditory change has also been provided by measuring electrocochleographic changes after the creation of perilymphatic fistula (PLF). Ackley demonstrated consistent increase in the ratio of the summating potential (SP) to the action potential (AP) in guinea pigs with obstructed cochlear aqueducts after the creation of PLF. ^[9] Others have subsequently confirmed these results.

Seltzer and McCabe gathered clinical data from 91 patients with surgically confirmed perilymphatic fistula (PLF). ^[10] Most patients (90%) had auditory symptoms, generally hearing loss. The character of the loss varied from sudden and profound to mild and fluctuating. Tinnitus was noted in 63% of patients, and aural fullness was noted in 25%. Glasscock et al reported that 83% of their patients with traumatic PLF had complaints of sudden or fluctuating sensorineural loss. ^[11]

Vestibular symptoms occur in most patients. Balance disturbance can be described as true rotational vertigo, lightheadedness, disequilibrium, intolerance to motion, or any combination of these symptoms. Seltzer and McCabe reported such symptoms in 80% of patients with PLF, and Glasscock et al reported 77% incidence of true vertigo or disequilibrium. ^{[10, 11]}

In obvious cases, membrane rupture is accompanied (or followed within a few minutes) by rapid, severe hearing loss; loud, roaring tinnitus; and severe rotational vertigo. Vertigo is often incapacitating and accompanied by visceral symptoms (eg, sweating, pallor, nausea, vomiting). Even cursory examination demonstrates marked instability and nystagmus. Audiometric evaluation reveals sensorineural hearing loss. Platform posturography confirms disequilibrium with a vestibular pattern, and platform fistula test results are positive. Vertigo and, to a lesser degree, tinnitus and hearing loss are sometimes exacerbated by straining (the Valsalva maneuver).

Unfortunately, many perilymphatic fistulas (PLFs) do not manifest in a straightforward fashion. A combination of otologic symptoms may result, and symptoms may fluctuate in complex ways that are difficult for the patient to explain. Onset of symptoms may be delayed by several days, or the acute phase may be masked by more serious injuries in other areas. Rotational vertigo may be entirely absent, and disequilibrium may be mild, vague, and episodic. Hearing loss, tinnitus, and aural fullness may come and go unpredictably. Such elusive symptomatology is partially responsible for the controversy surrounding this disorder.

The diagnosis of perilymphatic fistula (PLF) often depends on the antecedent history of otologic surgery, trauma, diving, or congenital ear malformation. Fluctuating or sudden hearing loss with or without vestibular symptoms in a patient with a previous stapedectomy, for example, is highly suggestive of perilymphatic fistula (PLF).

Trauma is frequently cited as the cause of a perilymphatic fistula (PLF). Changes in the middle ear pressure sufficient enough to implode the round window membrane can occur during descent while flying, forced Valsalva maneuver, or suppressed sneezing. Pullen reported that 48 of 62 patients with suggested PLF following scuba diving had round window membrane rupture. ^[12] Pressure sufficient to rupture the round window membrane can develop at a depth of water as shallow as 4 ft. Explosive perilymphatic fistulas (PLFs) can arise from CSF pressure increases caused by physical exertion, coughing, and straining.

Klokker and Vesterhauge have reported on 4 cases of perilymph fistula that occurred in flight attendants who flew with upper respiratory infections during a 6-month period. ^[13] All 4 flight attendants work for a major Scandinavian airline that employs approximately 3,000 flight attendants.

Perilymphatic fistula (PLF) may be the ultimate etiology of progressive sensorineural hearing loss in some children. Its frequency remains unknown and is contested. Grundfast and Bluestone reported that 66% of 33 children with progressive sensorineural hearing loss had PLF at exploratory tympanotomy. ^[14] Reilly and Kenna prospectively evaluated 244 children with sensorineural hearing loss of unknown etiology; of the 54 ears that were surgically explored, 42% demonstrated active PLF. ^[15]

Weber et al, in a follow-up article, confirmed that surgical repair of perilymph fistula does not result in a significant risk of postoperative hearing loss and that fistula repair may prevent further hearing loss, even in patients in whom a perilymph fistula was not identified at the time of surgery. ^[16]

The indications for exploratory tympanotomy are controversial. Accurate diagnosis is difficult. If the patient's history is suggestive of perilymphatic fistula (PLF), objective testing should be used to reinforce or reject the initial assessment. The signs and symptoms of perilymphatic fistula (PLF) are relatively nonspecific and overlap greatly with those seen in other otolaryngologic and neurologic diseases. Perilymphatic fistula (PLF) can be particularly difficult to differentiate from Ménière disease. Histologically, the two diseases are similar.

However, the final diagnosis of perilymphatic fistula (PLF) requires integration of all available information. Because perilymphatic fistulas (PLFs) can be difficult to diagnose, some surgeons believe that patients should be given the opportunity to choose surgical exploration even when perilymphatic fistula (PLF) is unlikely. Such an approach leads to many explorations with negative findings and many patients who do not benefit from the procedure. Conversely, a few patients benefit who would otherwise have had undiagnosed perilymphatic fistula (PLF).

Other surgeons prefer to use fairly rigid criteria for the diagnosis of PLF. These surgeons explore fewer individuals and have a much higher rate of positive intraoperative findings. A high percentage of their operative cases improve following surgery; however, some cases of perilymphatic fistula (PLF) are missed. These patients may have lost an opportunity for clinical improvement or cure.

Objective testing becomes especially important when the history of antecedent trauma is vague or remote. A perilymphatic fistula (PLF) test or elevated SP/AP ratio on electrocochleography (ECoG) significantly raises the likelihood of perilymphatic fistula (PLF).

No absolute contraindications exist to exploratory tympanotomy with round and oval window grafting in patients who can tolerate either a local or general anesthetic. A physician must exercise the usual cautions in an only-hearing ear, even though the incidence of significant hearing loss associated with this operation is low. To justify operative intervention, one must feel convinced that the likelihood of additional hearing loss is greater without surgery than with surgery.