If you are going to see a patient in the exam, than its more than likely you will be presented with a patient who has either Bell’s Palsy or an UMN 7th secondary from a stroke. Either way it is common to find these patients and you should be prepared to expect to see these patients in your finals.
The questions you might get are:
- What is the course of the facial nerve?
- What are the differences between an UMN and a LMN 7th Nerve?
- What advice would you give a patient with a 7th Nerve Palsy?
- Examine this patients neurology
- What are your differential diagnosis of a facial nerve lesion?
In order to answer these questions, by understanding the anatomy you can score full-marks because this is a very easy exam question!
Basic Anatomy- BORING AND HARD TO UNDERSTAND
- The facial nerve has three components, motor, parasympathetic (efferent) and visceral (afferent) fibres.
- The facial nucleus lies in the pons medial its motor fibres track around the 6th nerve nucleus called the facial colliculus.
- The facial nerve (motor) and its afferent fibre (nervus intermedius)
- The cross the lateral aspect of the brainstem and runs with the 8th Nerve in the cerebello-pontine angle where it enters the skull in the facial canal
- Within the facial canal is an important collection of cell bodies called the Geniculate Ganglion. It has both motor and sensory components. Sensory inputs are carried via the Nervus Intermedius to the geniculate ganglion.
- The significant motor and sensory/para-sympathetic output arising from the geniculate ganglion include: Nerve to the Stapedius, Greater Petrosal Nerve, and Chorda Tympani
- The Nerve to the Stapedius supplies motor input to the Stapedius muscle that attached to the Stapes in the middle ear
- The Chorda Tympani– has visceral afferent sensation convey sensation to anterior 2/3 of the tongue and para-sympathetic efferent fibres to the sub-lingual and sub-mandibular glands.
- The Greater Petrosal Nerve provides para-sympathetic fibres associated with Lacrimation
- The facial nerve arises at the stylomastoid foramen giving branches to the posterior belly of the digastric muscle, stylohyoid, and posterior auricular nerve.
- It than pierces the parotid gland and gives rise to five branches Cervical, Buccal, Zygomatic, Temporal and Marginal Mandibular
OKAY LETS BE HONEST THATS TO MUCH INFORMATION AND NO ONE WILL EXPECT YOU TO REAL THAT OFF IN THE EXAM BUT LETS SUMMARISE THIS!
Basic Anatomy and Simplified
- We know all the cranial nerve fibres have an UMN component supplied by the cortico-bulbar tracts
- The nucleus is in the brainstem
- It crosses the CPA
- It runs in the facial canal within the petrous temporal bone
- It exits in the stylomastoid foramen
- It runs in the Parotid gland
Therefore you can simply localise the pathology based on its course and it becomes easy!
UMN vs LMN Facial Nerve Palsy
The muscles in the lower aspect of the face are controlled by the contra-lateral hemisphere however those in the upper have bilateral cortical representation. Therefore in an UMN only the lower facial muscles are involved, in comparison to a lower facial nerve palsy where both the Upper and Lower facial musculature are involved.
Causes of Facial Nerve Palsy
1. Supra-Nuclear Lesion:
Stroke, Tumour, Lesion can cause unilateral upper 7th Nerve palsy.
2. Infra Nuclear Lesion: Brainstem
Note the brainstem have large bundle of motor tracts so these patients may present with limb weakness with either bilateral facial nerve palsies or unilateral weakness. Because the lesion is above the geniculate ganglion they will present with hyperacusis, loss of lacrimation, taste and salivation
3. Infra Nuclear Lesion: CPA
Associated with Acoustic Neuroma’s, Meningioma’s, Epidermoid cyst and will have hearing impairment
4. Facial Canal
Basal Skull Fracture, middle ear infections, Bells Palsy, Ramsay-Hunt syndrome (herpes zoster) Osteomyelitis of the temporal bone (Necrotizing Otitis Externa)
5. Stylomastoid and Parotid gland
These patients will present with the motor components but will not have the associated para-sympathetic and visceral symptoms this is because the nerves have already have been given off.
REMEMBER: PATIENTS WITH A FACIAL NERVE PALSY GET VISCO-TEARS AND EYE LUBRICANTS. LOSS OF EYE CLOSURE, LEADS TO CORNEAL ULCERATION.