It may be difficult in patients with poorly pneumatised mastoids and especially in those with a forward lying sigmoid sinus or low placed middle cranial fossa.

Facial paralysis appearing within 10 days of an attack of acute otitis media may be due to edema or inflammation of FN in its bony canal. Early treatment is indicated if the paralysis is not improving;

In patients with no muscle tonus, no reaction of the muscle to direct current, no random action or the fibrillation potential in electromyography, FN Decompression is contraindicated.

In patients with major temporal bone fractures, management of the potential life threatening head injury takes precedence. The facial paralysis can be addressed when the patient is stable again.

Patients with facial paralysis as well as Progressive Muscular Atrophy, hematological disease, or who are physically weak are unlikely to require FN decompression.

Incisions: The incision is placed 5–10 mm posterior to the postauricular sulcus. The skin and subcutaneous tissues are elevated from the periosteum overlying the mastoid cortex, and from the level of the top of the pinna to the mastoid tip.

Intact canal mastoidectomy: After drilling the mastoid cortex, the mastoid antrum is opened, then by thinning the posterior wall of EAC, access is enabled to the of tympanic cavity. The sigmoid sinus and middle cranial fossa plate, the ridge of the digastric muscle, the horizontal semicircular canal and short process of incus are all identified. The short process of incus should be protected to avoid dislocation and subsequent conductive hearing loss.

Exposure of the vertical segment of FN: The bony canal of FN is thinned from stylomastoid foramen to the bottom of short process of incus. The digastric ridge is a landmark for FN at stylomastoid foramen. When skeletonising the bony canal, one should use a diamond bur and drill along the longitudinal axis of FN. The blood vessels in the FN sheath give a pink colour through the bone, allowing early identification of the line and depth of the nerve. The vertical segment of FN can also be exposed via facial recess approach.

Exposure of the horizontal segment of FN: The posterior tympanic cavity is opened via a facial recess approach. The facial recess is a triangular area bounded by the chorda tympani nerve anterior-inferiorly, the superior part of the vertical segment of FN posterior-inferiorly and the fossa of the incus superiorly. An open facial recess provides access to the ossicles, stapedial tendon, round window, oval window, bony canal of the horizontal segment of FN and the cochleariform process. The FN segment from cochleariform process to geniculate ganglion can be approached through the area surrounded by the body of incus, the horizontal semicircular canal and dural plate of middle cranial fossa. The ampulla of the horizontal and superior semicircular canal should be carefully protected. Decompression of the tympanic segment of FN canal can be performed by thinning the bony canal from the inferior surface of the curve of the second genu of the nerve to the cochleariform process. After removing bone along the canal inferiorly, the pyramid segment, the vertical segment and chorda tympani nerve can be decompressed.

When the ossicular chain is intact, only a small diamond bur should be used inferiorly to the incus in the facial recess. Separation of the incudostapedial joint is usually not necessary. Sometimes the incus could be removed temporarily and stored in physiological saline. The head of malleus can then be displaced laterally or drilled down. The lateral wall of the bony canal of the horizontal segment is removed as far as the geniculate ganglion. Once the decompression of the nerve is complete, the incus is repositioned.

Spiral thinning of the bony canal of FN : The bone of the canal is thinned from the posterior part of FN canal in the vertical segment, the lateral part in the pyramid segment and inferior part in the horizontal segment in a spiral fashion to protect neighboring structures. The chorda tympani nerve may be injured if the decompression is performed laterally in the vertical segment. The horizontal semicircular canal may be injured if the decompression is performed posteriorly in the pyramid segment. The superior semicircular may could be injured if the decompression is performed posterior superiorly in the horizontal segment. A diamond bur is used to delineate the course of the nerve, but leaving an eggshell layer of bone. When the light pink FN is identified through the eggshell thick bone, the nerve can be exposed with gentle removal of the bone using a micro elevator. The nerve is exposed across its full width and for at least 5 mm beyond the pathology at each end of the decompression. The bone chips, granulation tissue and cholesteatoma are all removed from the FN.

Dissecting the FN sheath: The FN sheath is opened with a sharp knife or hook to decompress the FN. Fibrous adhesions around the nerve fibre must be divided.

Closing the operative cavity: After the FN decompression, the operative cavity is irrigated with physiological saline, clearing bone and blood debris. Gelatin sponge soaked with dexamethasone is placed on the vertical segment of FN. In an open mastoid cavity, the skin flap of the external acoustic meatus is used to cover the FN. Finally, the cavity is gently packed with iodoform gauze and the incision is sutured step by step.

If the patient has nearly normal hearing, the ossicular chain should be protected as far as possible during FN decompression to avoid sensorineural and conductive hearing loss.

Avoid contact with the incus when working close to the horizontal semicircular canal. The bone of the facial nerve canal is also thin in this region and one needs to be careful to avoid damaging the nerve and the chorda tympani.

A diamond bur should be used in a spiral fashion to drill the bone of the FN canal with suction and liberal irrigation to avoid heat and to clear bone dust, thus improving identification of the nerve through the bone. When opening the FN sheath attention should be paid not to injure the stylomastoid artery.

When dissecting the thinned bone over the sheath, care should be taken not to lever on the nerve or injure it with the dissector.

If a bone spicule is found in the FN, it should be removed and the integrity of the nerve should be assessed. If the nerve is substantially intact, the sheath should be opened to decompress the nerve. If there is separation of the nerve, it should be repaired with re-routing and primary repair or grafting with a nerve cable graft. Re-routing is the first option as there will be only one anastomosis. This must be done without tension, and if this is not possible, a cable graft is used.

Sensorineural hearing impairment due to direct injury to inner ear or drill contact with the incus causing transmitting intense vibration to the inner ear.

Vertigo due to injury of the semicircular canals.

Conductive hearing impairment. There are two mechanisms: