Fistula test mechanism

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Here is a thorough breakdown of the Fistula Test - its mechanism, technique, interpretation, and clinical relevance.

Fistula Test

Definition

The fistula test is a clinical bedside examination used to detect an abnormal communication (fistula) between the middle ear and the membranous labyrinth (inner ear), most commonly at the oval or round window. It is also called the Hennebert's sign test when performed in an ear with an intact tympanic membrane.

Mechanism

The key principle relies on pressure transmission to the inner ear:

When positive or negative pressure is applied to the ear canal, it moves the tympanic membrane and ossicular chain.
In a normal ear, this pressure is not transmitted to the inner ear fluid (perilymph) because the oval and round windows act as sealed membranes.
If a fistula exists (a tear or dehiscence in the oval window, round window, or bony labyrinth wall), the applied pressure is transmitted directly into the perilymphatic space.
This abnormal pressure wave causes mechanical deflection of the cupula of the semicircular canals (particularly the horizontal canal).
Cupular deflection triggers a vestibulo-ocular reflex, producing nystagmus and/or vertigo.

In superior semicircular canal dehiscence (SSCD), the same mechanism applies but via an abnormal "third window" in the bony labyrinth roof - nystagmus here is characteristic: vertical and torsional, aligned with the plane of the superior canal.

Shambaugh Surgery of the Ear: "In the presence of a fistula, or vestibulofibrosis, the applied pressure causes deviation of the cupula, resulting in nystagmus and vertigo."

Technique

A pneumatic otoscope is used to seal the external auditory canal.
The examiner applies alternating positive and negative pressure to the ear canal.
The patient is observed (or monitored with Frenzel lenses, ENG/VNG, or infrared goggles) for:
- Nystagmus - objective finding
- Vertigo - subjective complaint

A subjectively positive test = vertigo without visible nystagmus. A fully positive test = both vertigo AND nystagmus.

Variants: Tragal compression or Valsalva maneuver can also provoke nystagmus in SSCD.

Interpretation

Result	Meaning
Positive (nystagmus + vertigo)	Suggests fistula or labyrinthine abnormality
Subjectively positive (vertigo only)	Incomplete/borderline - clinical correlation needed
Negative	Does NOT exclude fistula (sensitivity only 30-50%)

Hennebert's Sign = positive fistula test in an ear with an intact tympanic membrane and no middle ear disease - implies an underlying labyrinthine abnormality.

Causes of a Positive Fistula Test

Oval window fistula
Round window fistula
Post-stapedectomy perilymph leak
Horizontal semicircular canal fistula (e.g., from cholesteatoma erosion)
Meniere's disease (vestibulofibrosis involving the saccule)
Superior semicircular canal dehiscence (SSCD) - nystagmus is in the vertical-torsional plane of the canal
Congenital syphilis / otitic syphilis
Labyrinthitis

Clinical Notes

Cholesteatoma is the most common cause of a labyrinthine fistula in chronic ear disease. The horizontal semicircular canal is the most common site (76% of cases). Of these, only about 14/19 tested had a positive fistula test - highlighting its limited sensitivity.
The test is contraindicated if a CSF fistula or active middle ear infection is present (risk of iatrogenic injury or introducing air/infection into the inner ear).
Perilymph assay (beta-2 transferrin) can supplement but has low sensitivity for small leaks.
Recently, many historically diagnosed "perilymph fistulas" have been reclassified as SSCD on further workup.

Sources:

Shambaugh Surgery of the Ear (Fistula Test section)
Cummings Otolaryngology Head and Neck Surgery (Perilymph Fistulas, Evaluation sections)
K.J. Lee's Essential Otolaryngology (Differential Diagnosis for Positive Fistula Test)

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