Facial Nerve Anatomy and Diseases

Facial nerve is a mixed nerve and it is the seventh paired cranial nerve. The course of the nerve is very complex and during the entire course the facial nerve releases many branches that transmit a combination of sensory, motor and parasympathetic fibres. During its course, any compression to the facial nerve or its branches due to any tumour or some other reason can lead to multiple symptoms. This is the reason it is very important to study the facial nerve’s complex course.

Facial Nerve Anatomy

There are three parts of Facial nerve:

1. Intracranial
2. Intratemporal
3. Extracranial

1. Intracranial: 
   • Facial nerve nucleus is located in the brainstem ( Pons) and from brainstem the intracranial part of the facial nerve starts.
   • After emerging from the nucleus , the facial nerve winds around the abducens (6^th nerve) nerve nucleus and exits the pontomedullary junction.
   • Then it enters the internal auditory meatus in the petrous part of the temporal bone.
   • The section from the pons to the internal auditory meatus is called as the intracranial segment of the facial nerve.
2. Intratemporal:
   • The intratemporal segment of the facial nerve is the longest portion of the facial nerve. This part runs in a bony canal called as Fallopian canal. 
   • It has four segments- Meatal segment, Labrynthine segment, Tympanic segment, Mastoid segment.
   • Meatal segment- 
     • It is 8 to 10 mm segment.
     • It has a vertical projection in the internal acoustic meatus called Bill’s bar. The horizontal projection is called as falcifrom crust in the internal acoustic meatus.
   • Labrynthine segment-
     • It is the shortest and the narrowest segment of the facial nerve measuring 3-4 mm in length.
     • It is present in the inner ear.
     • It is very important to note that this segment of nerve is the narrowest and that makes it prone for ischemia and edema. That can lead to Bell’s Palsy.
     • Important landmarks of facial nerve are- Cochlea ( Anteriorly), Vestibule (middle) and Semicircular canals ( Posteriorly).
   • Tympanic segment-
     • The tympanic segment of the nerve takes on the first turn (called as First genu or geniculate ganglion) on passing from the inner ear to the middle ear.
     • It lies in the medial wall of the middle ear.
     • It runs above the oval window and below the Lateral Semicircular canal.
     • The next destination of the nerve is the mastoid and the horizontal segment when entering the mastoid takes the second turn called as second genu.
     • Pyramid is the landmark for the 2^nd genu of the facial nerve.
   • Mastoid segment -
     • It is a vertical segment and measures 13 mm in length.

3. Extracranial Segment:

• The facial nerve exits the temporal bone via the stylomastoid foramen and then it enters the parotid gland where it runs between its superficial and deep lobes.
• It exits the parotid gland after traversing within it into 5 terminal branches. The branches are:
• Temporal
• Zygomatic
• Buccal
• Marginal mandibular
• Cervical
• Other branches of the Facial Nerve: 
  • Greater superficial petrosal nerve - Its origin is First genu and it is responsible for lacrimation.
  • Nerve to stapedius - Its origin is 2^nd genu and it supplies the stapedius muscle.
  • Chorda tympani nerve - It supplies taste to the anterior two-thirds of the tongue.

Also Read: Dysphagia: Pathophysiology/Causes, Diagnosis, Treatment

Facial Nerve Diseases

• Bell’s palsy- Bells Palsy is an idiopathic facial nerve palsy and due to lots of branches there is more risk of peripheral nerve lesions than central facial nerve palsy. Other features are :
• Its risk is 3 times greater in pregnancy and is associated with HSV and EBV.
• Increased risk with Diabetes.
• It is usually associated with Upper respiratory tract infection.
• The clinical features are 
  • It is unilateral
  • Inability to close the eye
  • Decreased teraing
  • Hyperacusis 
  • Mouth drop
  • Loss of taste to anterior 2/3^rd of the tongue
• Treatment includes supportive treatment that is physiotherapy and taking care of the eye.
• Medical management can be done by giving Prednisolone.
• If still the symptoms are present then Surgical decompression of the segments.
• Melkersson-Rosenthal Syndrome- It is a triad of Recurrent facial nerve palsy, Swelling of lips and Fissured tongue.
• Ramsay-Hunt syndrome/Herpes Zoster Oticus- The pathophysiology of Ramsay-Hunt syndrome is the reactivation of the Herpes Zoster virus which was already present in the geniculate ganglion. It leads to vesicles in the facial nerve distribution and accompanies the Lower motor neuron type facial palsy. Treatment is Antiviral drugs.
• Unilateral Supranuclear Palsy - It is alesion above the facial nerve nucleus and it has characteristics of the Upper motor neuron palsy. The clinical features include lower face paralysis while sparing the forehead.
• Bilateral Supranuclear Palsy- Affects the entire face including the forehead.
• LMN Lesion- Complete ipsilateral palsy.

Also Read: Gene Therapy: A Journey into the Future of Medicine

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