Perilymphatic Fistula Workup

• No laboratory studies are relevant to the diagnosis of perilymphatic fistula (PLF). A possible future role for beta2-transferrin assay is discussed in Intraoperative details.

• Obtain gadolinium-enhanced MRI scans in all individuals with unilateral otologic symptoms to exclude acoustic neuroma or other structural lesions of the cerebellar pontine angle or neuraxis.
• The incidence of perilymphatic fistula (PLF) is much higher in individuals with congenital deformities of the otic capsule. Consequently, a nonenhanced fine-cut CT scan of the temporal bones can be helpful, especially in children. A child with progressive or sudden neurosensory hearing loss in an ear that is incompletely developed has a much higher risk of perilymphatic fistula (PLF) than a child with normal inner ear radiographic morphology.

Audiography

Obtain an audiogram in all patients with otologic symptomatology. Neurosensory hearing loss is a regular feature of perilymphatic fistula (PLF), but any pattern of neurosensory hearing loss can result from perilymphatic fistula (PLF). Because it is otherwise relatively uncommon, low-frequency hearing loss is perhaps a bit more suggestive of perilymphatic fistula (PLF); however, midfrequency, high-frequency, and flat losses are also consistent with the diagnosis. Fluctuating hearing loss is common with perilymphatic fistula (PLF), as it is with Ménière disease.

Some authors have reported a conductive loss in patients with perilymphatic fistula (PLF), but this probably represents a small percentage of patients.

Electrocochleography

ECoG is a method of measuring intracochlear electrical potential changes associated with hearing. Three separate responses can be obtained, as follows:

• The cochlear microphonic is a stimulus-related alternating current (AC) potential that closely mimics the stimulus and therefore is difficult to separate from stimulus artifact. The technique used to produce standard electrocochleographic tracings eliminates the cochlear microphonic from the ECoG tracing.
• The summating potential (SP) is a stimulus-related direct current (DC) potential that reflects the time-related displacement of the cochlear partition. The SP has been shown to be sensitive to inner ear fluid imbalance, particularly in Ménière disease and perilymphatic fistula (PLF).
• The action potential is an AC potential that represents the compound action potential of the fiber's eighth nerve that discharges synchronously in response to a stimulus. The initial portion of the action potential also is known as wave I of the ABR.

The 2 methods of recording ECoG are transtympanic and extratympanic.

• Transtympanic: A needle electrode passed through the tympanic membrane that rests on the promontory provides the largest and most easily readable tracings because it is a near-field potential. Fewer signals need to be averaged to obtain an interpretable response, and the potentials are much longer than in other methods of ECoG. Acceptance of transtympanic ECoG has been limited because it can be painful and because a physician's presence is required to pass the electrode through the tympanic membrane.
• Extratympanic: Extratympanic ECoG is comfortable and noninvasive, and it can be performed in a nonmedical setting. However, the size of the reported potentials is significantly smaller than with transtympanic ECoG, and, consequently, the reported potentials are more difficult to identify and interpret.

ECoG is useful in the diagnosis of both Ménière disease and PLF. Both conditions produce an elevated SP/AP ratio. Increase in the SP/AP ratio appears to result because of enlargement of the SP component in patients with Ménière disease. The mechanism by which it increases in perilymphatic fistula (PLF) is controversial. Some authors believe that the increase in SP/AP ratio in patients with perilymphatic fistula (PLF) may be the consequence of a decrease in the AP.

Several guinea pig studies have shown consistent increases in the SP/AP ratio in guinea pigs with artificially induced perilymphatic fistulas (PLFs). Gibson has demonstrated an increase in the SP/AP ratio during stapedectomy.^[14] The creation of a control hole in the footplate is not sufficient to produce a change in the SP/AP ratio; a change is noted only after some perilymph has been removed from the vestibule. Meyerhoff and Yellin have demonstrated that in individuals with surgically proven perilymphatic fistulas (PLFs) who have elevated SP/AP ratios preoperatively, the SP/AP ratio reliably returns to normal after surgical repair of the fistula.^[15]

Fistula test

Fistula testing has been shown to yield positive results in patients with perilymphatic fistula (PLF). The application of positive pressure to a tympanic membrane in an ear with a fistula is known to possibly produce nystagmus. The production of nystagmus secondary to positive pressure is referred to as a positive fistula test result. The definition actually requires the presence of documentable nystagmus. The reproduction of symptomatology secondary to positive pressure may have diagnostic importance but does not constitute a positive fistula test.

An objective record of fistula testing can be made using the electronystagmogram (ENG) and the impedance bridge. To accomplish this, the emittance probe is placed into first one ear and then the other. The pressure in the external auditory canal is varied between +200 and -200 mm of mercury. The ENG has been examined for induced nystagmus. Each ear is tested separately. A positive fistula result is identified by the production of nystagmus associated with a change in pressure on the tympanic membrane. In some cases, the nystagmus can be seen to change direction as the pressure changes from positive to negative. One would expect that the patient's subjective symptoms of vertigo, with or without nausea, would be induced during the presence of nystagmoid eye movements in a positive test result. The results of the ENG fistula test then can be compared to platform fistula test results.

Dynamic platform posturography

This test can be used to generate a sensitive test for perilymphatic fistula (PLF). In dynamic platform posturography, pressure is applied to the external auditory canal. The increase or decrease in pressure is transmitted to the tympanic membrane middle ear space and, if a fistula is present, to the inner ear. When perilymphatic fistula (PLF) is present, abnormal sway is generated by these pressure changes. Using the acoustic impedance bridge to quantify changes in external auditory canal pressure and the dynamic platform posturography to quantify anterior, posterior, and lateral sway in response to such pressure changes, a sensitive assessment for perilymphatic fistula (PLF) can be developed. Several studies have demonstrated that patients with positive results from platform fistula testing have a high likelihood of having perilymphatic fistula (PLF).

Poe et al performed middle ear endoscopy in 20 patients with suggested perilymphatic fistula (PLF).^[16] They failed to demonstrate fistulas in any patient but placed autologous blood patches in 8 patients around the round and oval windows. No change in hearing was noted postoperatively, but 3-4 patients with preoperative vertigo had relief of symptoms, and 2-3 patients with preoperative positive fistula test results had negative test results postoperatively.

The most troublesome difficulty described by Poe was obscured vision of the oval window and round window niche, secondary to mucosal adhesions. The average human ear contains only 0.07 mL (70 mcL) of perilymph; therefore, even relatively rapid leaks are, in absolute terms, quite small. Even with magnification, leaks involving only 5-10% of the perilymph are difficult to see in an operative field because local anesthetics have been injected, irrigating fluids have been used, and a minimal amount of bleeding may be present.