Facial Nerve Flashcards
Fully describe the 4 types of fibres that the facial nerve carries
- Efferent somato-motor:
This the most important function. Provides innervation to:
- mimetic facial muscles
- Platysma
- posterior belly of digastric muscle
- stapedius muscle (stapedius nerve, which is involved in the stapedial reflex)
Note:The 3rd branch of the trigeminal nerve provides innervation to the anterior belly of digastric muscle
- Efferent viscero-secretory:
i. Greater petrosal nerve
innervates lacrimal, nasal, and palatal glands
ii. Chorda tympani nerve
innervates submandibular and sublingual glands. - Afferent gustatory:
Chorda tympani nerve -innervates taste buds on anterior 2/3 of tongue. - Afferent somatosensory:
Posterior auricular nerve and “Ramsay-Hunt’s fibers” which provide innervation to the external auditory canal, in particular to:
a. the posterior auricle
b. concha of auricle (Ramsay-Hunt area)
c. posterior meatal wall of the external auditory canal.
- The larger part of the facial nerve is involved efferent somatomotor function
- The nervus intermedius or Wrisberg’s nerve instead is the one related to the efferent viscerosecretory, afferent gustatory, and afferent somatosensory functions.
Detail the origin of facial nerve fibres
Origin of the facial nerve fibers:
o Efferent somatomotor fibers - originate from motor facial nucleus.
o Efferent viscerosecretory fibers - originate from the salivary nucleus and lacrimal-nasal-palatal nucleus.
o The afferent gustatory fibers and the afferent somatosensory fibers - originate from the solitary tract nucleus
Describe motor innervation by the facial nerve (from cortex to muscle)
The corticonuclear fibers of the facial motor nucleus are bilaterally represented, from the left and right corticonuclear tract. The only exception is the innervation of the lower half of the face, which is all innervated unilaterally
This has clinical implications:
- A palsy of the facial nerve of a central nature is characterized by a palsy of only the lower half of the face because the other muscles are innervated bilaterally
Fully describe the anatomical path and branchings of the facial nerve.
The facial nerve is anatomically divided into 3 portions:
- Intracranial
- Intratemporal bone tract
- Extracranial
In the temporal bone there are 3 segments, 2 genua:
The first segment:
• between the end of the inner ear canal and the 1st genu (located in the geniculate canal)
• From the geniculate ganglion originates the greater petrosal nerve (the first branch of the facial nerve)
The second segment:
• between the 1st and the 2nd
• called tympanic tract because it runs in the tympanic cavity, over the stapes.
The third tract: • called mastoid tract • between the 2nd genu and the stylomastoid foramen • Gives origin to 2 efferent branches: 1. the stapedial nerve 2. the chorda tympani
In the inner ear canal [internal acoustic meatus?], the facial nerve runs very close to the 8th cranial nerve (vestibulocochlear). A disorder that involves the 8th cranial nerve may also lead to an involvement of the facial nerve and vice versa
List the causes of peripheral palsy of the facial nerve
- Idiopathic (Bell’s Palsy)
- Traumatic
- Infectious (e.g borreliosis, herpes zoster)
- Inflammatory (e.g cholesteatoma, sarcoidosis)
- Neoplastic
- Metabolic
Generally describe Bell’s Palsy, its clinical history and clinical examination (central vs peripheral)
It’s an acute but limited facial paralysis, usually unilateral. It is a diagnosis of exclusion (historically thought to be idiopathic however know we know most cases are a reactivation of a Herpes Simplex Virus (HSV)
Incidence: 30 per 100 000
Pregnant women experience it 3.3X more frequently
Diabetics: 4-5X
- Bilateral involvement is very uncommon (in less than 1%)
- Recovery: 1-2 months
- No other symptoms
CLINICAL HISTORY
- Ask if there are otologic symptoms (hearing loss, tinnitus, vertigo, ear discharge). These SHOULD NOT be present in a peripheral palsy
- Ask about trauma event in recent past
- Is this the first episodde?
- Ask about history of tick bites (borreliosis)
CLINICAL EXAMINATION
Inspection: This can help us distinguish between a central and peripheral palsy
• In case of central facial nerve palsy, there is involvement of the only lower half of the face
• In case of peripheral palsy, there is a complete involvement of one half (left or right) of the face
• Diplopia is related to a central facial nerve palsy
the 6th cranial nerve fibers in the brainstem originate close to the nucleus of the 7th cranial nerve
vascular lesion of the brainstem at the origin of the facial nerve may determine an involvement of the 6th cranial nerve that leads to a palsy of the lateral rectus muscle
Describe the House-Brackmann scale of facial nerve palsy severity
There are 6 grades of facial nerve palsy
• Grade 1 is normal, and grade 6 means that there is a complete palsy of the facial nerve.
• Grades 2 and 3 are characterized by a complete closure of the eye
• In grades 4 and 5, we have an incomplete closure of the eye.
in grade 4 we have normal resting tone, whereas in grade 5 we have an asymmetric resting tone
Comment on bilateral palsy
Bilateral palsy is quite rare mainly caused by: - infections (Lyme disease) - intracranial tumor - syndromic pattern (e.g. Guillain-Barre syndrome)
Describe Ramsay-Hunt Syndrome
RHS is caused by a reactivation of Herpes Zoster. It is characterized by unilateral facial palsy (peripheral) and on the same side:
- Vesicular eruptions in the external auditory canal
- Vesicular eruptions on the soft palate
- Sensorineural hearing loss (due to CN 8 involvement)
- Vertigo (severe)
Remember that the manifestations are UNILATERAL
Describe how the parotid gland can cause facial nerve palsy
The least part of the facial nerve runs through it
- Therefore check for parotid gland tumours
Explain why an otologic examination is performed in the work up of a CNVII palsy
Inflammatory disorders in the middle ear may result in facial nerve involvement. Most frequently chronic otitis media, in particular cholesteatoma.
- e.g. cholesteatoma - an aggressive lesion that may cause an erosion of the facial nerve in the 2nd tract that runs in the tympanic cavity and also in the mastoid tract (3rd)
Evaluation encompasses both the external and middle ear and the function of the ear (audiometric test)
• e.g. an asymmetric unilateral sensorineural HL + facial nerve palsy indicate a disorder that involves the 8th cranial nerve or the facial nerve in the inner ear canal. The 7th and 8th cranial nerve run close to each other in that site
A positive acoustic reflex decay test is suggestive of a lesion in the inner ear in the portion that runs in the inner ear canal
Explain when and why radiological studies are performed
Imaging is useful to perform topographic diagnosis of facial nerve palsy
CT - Allows localization of the lesion in cases of:
i. traumatic events
ii. chronic inflammatory disorders (mastoiditis)
iii. Cholesteatoma
MRI - allows exclusion of neoplastic lesions e.g. schwannoma of the 8th cranial nerve
Fully detail the 2 types of lateral skull base fractures that can result in facial nerve palsy
Longitudinal fractures and Transverse fractures
Longitudinal fractures:
- occur parallel to the long axis of the petrous bone.
- The most frequent complications are:
a. Conductive hearing loss due to involvement of the tympanic-ossicular chain.
b. Peripheral facial palsy
c. Cerebrospinal fluid leak
Transverse fractures:
- occur perpendicular to the long axis, across the petrous bone.
- Complications, with inner ear involvement, are:
a. Sensorineural hearing loss (involvement of the cochlear system)
b. vestibular dysfunction (If the vestibular portion is involved)
c. Peripheral facial palsy
• more common than in case of longitudinal fracture due to involvement of the labyrinthine tract.
d. Cerebrospinal fluid leak (CSF rhinorrhea through the eustachian tube). Observable on CT - vestibule appears black (instead of grey) because of air
Which tests can we perform for a topographic diagnosis of peripheral facial nerve palsy?
- Stapedial reflex → stapedial branch
b. Schirmer test → greater petrosal nerve
c. Electrogustometry → chorda tympani
d. Salivary flow →chorda tympani
NOTE: THE ONLY TEST USED IN CLINICAL PRACTICE IS THE STAPEDIAL REFLEX (checking for its presence or absence)
Describe the treatment for idiopathic facial nerve palsy
The therapy is mainly just OBSERVATION as the peripheral nerve palsy usually recovers on it’s own in 2-3 months.
RARELY (e.g trauma or other causes) we administer:
- Steroids - to reduce the inflammatory process
- Antiviral drugs - to prevent reactivation of the herpes simplex virus.
- Eye care (in case of incomplete closure of the eyelid)
