Perilymphatic Fistula

An abnormal communication between the perilymphatic space of the inner ear and the middle ear, allowing perilymph leakage and transmission of pressure changes into the labyrinth. It remains a contested diagnosis because there is no reliable non invasive confirmatory test, and both overdiagnosis and underdiagnosis occur.

Sites of leak

• Round window membrane, which is a three layered membrane approximately 70 micrometres thick and the commonest site.
• Oval window at the annular ligament, particularly after stapes surgery.
• Microfissures, including the fissula ante fenestram.
• Bony defects from cholesteatoma erosion, temporal bone fracture, or congenital malformation, especially incomplete partition type III and enlarged vestibular aqueduct where a cerebrospinal fluid gusher may occur.
• Surgical defects after stapedotomy or cochlear implantation.

Mechanisms of rupture

• Explosive: raised cerebrospinal fluid pressure from straining, coughing, sneezing, lifting or a forceful Valsalva transmitted through the cochlear aqueduct and the internal auditory canal to the perilymphatic space, rupturing the membrane outward into the middle ear.
• Implosive: raised middle ear pressure from barotrauma or Valsalva against a blocked Eustachian tube pushing the round window membrane inward.

Causes

• Barotrauma from diving, flying and forceful Valsalva.
• Head trauma and temporal bone fracture.
• Penetrating middle ear injury, including cotton bud injury with stapes subluxation.
• Acoustic trauma from blast.
• Stapes surgery and other middle ear procedures.
• Cholesteatoma with labyrinthine fistula.
• Congenital inner ear malformation, presenting with recurrent meningitis or fluctuating hearing loss in a child.
• Spontaneous, which is the most disputed category.

• Sudden sensorineural hearing loss, or fluctuating and progressive hearing loss, temporally related to a straining, barotrauma or traumatic event.
• Vertigo and disequilibrium, which may be episodic, positional or constant.
• Tinnitus and aural fullness.
• A popping sensation in the ear at the moment of the precipitating event, reported by a proportion of patients.
• Symptoms provoked by straining, coughing, sneezing, nose blowing, lifting, and by loud sounds.
• Postural aggravation, with symptoms often worse when lying on the affected side or with head position change.

The essential historical feature is the temporal relationship between a pressure event and the onset of audiovestibular symptoms. Without it, the diagnosis becomes very difficult to sustain.

Examination

• Otoscopy is usually normal unless there is a traumatic perforation or middle ear pathology.
• Fistula test: pressure applied to the external canal with a pneumatic otoscope or tragal compression produces vertigo and nystagmus. It has poor sensitivity, around 50 percent at best, and a negative test does not exclude a fistula.
• Hennebert sign, that is nystagmus induced by pressure, and Tullio phenomenon, that is vertigo and eye movement induced by loud sound, may be present but are also features of superior semicircular canal dehiscence, which must be excluded.
• Head impulse test, nystagmus assessment and test of skew.
• Full neurological examination.

There is no reliable non invasive test, and the diagnosis is made at exploratory tympanotomy in equivocal cases.

• Pure tone audiometry with masked bone conduction, repeated serially, which is the most important test. Progressive or fluctuating sensorineural loss supports the diagnosis and drives the decision to explore.
• Tympanometry and acoustic reflexes.
• Electronystagmography or videonystagmography with a fistula test using controlled pressure application.
• Vestibular evoked myogenic potentials, principally to exclude superior canal dehiscence, where thresholds are lowered and amplitudes raised.
• High resolution computed tomography of the temporal bone, seeking pneumolabyrinth, which is air within the vestibule or cochlea and is pathognomonic; a bony defect; middle ear fluid; and inner ear malformation. Reformats in the plane of the superior semicircular canal to exclude dehiscence.
• Magnetic resonance imaging with high resolution T2 sequences to assess labyrinthine fluid signal and to exclude other causes.
• Beta 2 transferrin and beta trace protein assay on middle ear fluid, which if positive confirms perilymph or cerebrospinal fluid. Sampling is technically difficult and false negatives are frequent because the volume obtained is tiny, but a positive result is highly specific. Cochlin tomoprotein assay is more specific for perilymph but is not widely available.
• Exploratory tympanotomy remains the definitive diagnostic procedure. Intraoperative identification of a leak is itself unreliable, since local anaesthetic, blood and saline can be mistaken for perilymph, and the absence of a visible leak does not exclude an intermittent one.

Differential diagnosis and the discriminating feature

• Superior semicircular canal dehiscence: autophony, air bone gap with present acoustic reflexes, lowered vestibular evoked myogenic potential thresholds, and a visible dehiscence on reformatted computed tomography. This is the most important condition to exclude, since the symptoms overlap almost completely and the treatment differs entirely.
• Ménière disease: episodic vertigo of characteristic duration with fluctuating low frequency loss and no precipitating pressure event.
• Sudden idiopathic sensorineural hearing loss: no precipitating event and no pressure sensitivity.
• Labyrinthine concussion after head trauma, without a fistula.
• Inner ear decompression sickness in a diver, distinguished by the dive profile and timing.
• Enlarged vestibular aqueduct, which produces fluctuating loss after minor head trauma and is seen on imaging.

Conservative management, which is the first line in most cases

• Strict bed rest with the head elevated 30 to 45 degrees for 5 to 10 days.
• Absolute avoidance of straining: stool softeners such as lactulose 15 mL twice daily or a macrogol preparation, avoidance of heavy lifting, no nose blowing, no Valsalva, and instruction to sneeze with the mouth open.
• Avoidance of coughing where possible, with antitussives if needed.
• No flying, diving or exertion.
• Corticosteroids are frequently given, using prednisolone 1 mg/kg orally daily to a maximum of 60 to 80 mg for 7 to 14 days with a taper, on the same basis as for sudden sensorineural hearing loss, though evidence specific to fistula is limited.
• Serial audiometry, at least twice weekly initially, since the trajectory of hearing determines whether to explore.
• Many fistulae seal spontaneously within 1 to 2 weeks with these measures.

Indications for exploratory tympanotomy

• Progressive sensorineural hearing loss despite 5 to 10 days of strict conservative management.
• Persistent or disabling vertigo.
• Pneumolabyrinth on imaging, which is a definite fistula.
• Known penetrating middle ear trauma with stapes subluxation.
• Post stapedectomy patients with vertigo and hearing deterioration, where a fistula or an overlong prosthesis is likely and where early exploration is indicated.
• Recurrent meningitis in a child with an inner ear malformation, where the leak is a route for infection.
• Deteriorating hearing in an only hearing ear, where the threshold for exploration is lower.

Operative approach

• Transcanal tympanotomy under local or general anaesthesia, elevating a tympanomeatal flap.
• Careful inspection of the round and oval window niches under high magnification after removing mucosa and any obscuring tissue. Use a Valsalva or jugular compression manoeuvre with the head down to provoke a visible leak.
• Seal both round and oval windows regardless of whether a leak is identified, using autologous soft tissue: perichondrium, temporalis fascia, fat or areolar tissue, placed after abrading the mucosal surfaces to promote adhesion.
• In post stapedectomy cases, assess the prosthesis length and position and replace it if it is too long or displaced.
• Endoscopic approaches offer superior visualisation of the round window niche.
• Where there is a bony defect from trauma or malformation, a formal repair with fascia, cartilage and bone pate is required, and in a cerebrospinal fluid leak with recurrent meningitis, obliteration of the middle ear with blind sac closure of the canal may be necessary.

Postoperative care

• Continue head elevation, avoidance of straining and stool softeners for 1 to 2 weeks.
• No flying, diving or heavy exertion for at least 6 weeks.
• Serial audiometry at 2 weeks, 6 weeks and 3 months.
• Vestibular rehabilitation once the acute phase has passed.

Outcomes and counselling

• Vertigo improves in the majority after repair, in the region of 70 to 90 percent.
• Hearing recovery is far less predictable and often does not occur, particularly where the loss has been present for weeks. Counsel patients that the primary aim of surgery is to stop further deterioration and to control vertigo rather than to restore hearing.
• Recurrence occurs and re exploration is sometimes needed.
• Permanent avoidance of diving is advised after a confirmed fistula.
• Where hearing is not recovered, proceed to hearing rehabilitation with an aid, a contralateral routing device or cochlear implantation as appropriate.