Anatomy
Cranial Nerve Lesions
Which of the following nerves is the efferent pathway of the corneal blink reflex:
Answer:
The facial nerve is the efferent pathway of the corneal blink reflex. The ophthalmic division of the trigeminal nerve is the afferent pathway of the corneal blink reflex.Cranial Nerve VII: Facial Nerve
Anatomy / Cranial Nerve Lesions / Head and Neck / Face
Last Updated: 11th April 2019
The facial nerve (CN VII) mediates facial movements, taste, salivation and lacrimation.
Table: Overview of the Facial Nerve
| Cranial Nerve | Facial Nerve (CN VII) |
|---|---|
| Key anatomy | Exits brainstem in cerebellopontine angle, enters internal auditory meatus and facial canal, exits facial canal and skull via stylomastoid foramen |
| Motor function | Muscles of facial expression, posterior belly of digastric muscle, stylohyoid muscle, stapedius muscle, parasympathetic innervation to lacrimal, salivary, oral, pharyngeal and nasal glands, efferent pathway of corneal blink reflex |
| Sensory function | Taste to anterior two-thirds of tongue |
| Assessment | Facial movements, corneal blink reflex |
| Clinical effects of injury | Facial weakness, loss of efferent corneal reflex, impaired lacrimal fluid production, hyperacusis, impaired sense of taste to anterior two-thirds of tongue, impaired salivation |
| Causes of injury | Bell’s palsy, Ramsay-Hunt syndrome, Guillain-Barre syndrome, mumps, middle ear disease, tumours, trauma |
Function
The facial nerve provides motor innervation to the muscles of facial expression, the posterior belly of the digastric, the stylohyoid and the stapedius muscles. The chorda tympani branch supplies taste to the anterior two-thirds of the tongue. The facial nerve also carries parasympathetic innervation to the lacrimal glands, salivary glands, nasal, palatine and pharyngeal mucous glands.
Anatomical Course
The facial nerve arises in the pons, leaves the brainstem in the cerebellopontine angle and exits the posterior cranial fossa through the internal acoustic meatus in the temporal bone before entering the facial canal still within the temporal bone where it gives rise to three main branches:
- The nerve to the stapedius (innervating the stapedius muscle)
- The greater petrosal nerve (supplying parasympathetic innervation to the lacrimal gland and the mucous glands of the oral cavity, nose and pharynx)
- The chorda tympani (supplying taste to the anterior two-thirds of the tongue and parasympathetic innervation to all salivary glands below the level of the oral fissure)
The facial nerve exits the facial canal (and the basal skull) through the stylomastoid foramen between the styloid and mastoid processes of the temporal bone, at which point it gives off the posterior auricular nerve (innervating the occipital belly of the occipitofrontalis muscle of the scalp and external ear muscles).
Anatomical Course of the Facial Nerve. (Image modified by FRCEM Success. Original by Patrick J. Lynch, medical illustrator (Patrick J. Lynch, medical illustrator) [CC BY 2.5 , via Wikimedia Commons)
The facial nerve then gives off motor branches (innervating the posterior belly of the digastric muscle and the stylohyoid muscle) before entering the deep surface of the parotid gland.
Once in the parotid gland, the facial nerve divides into five terminal branches:
- The temporal branch (innervating muscles in the temple, forehead and supraorbital areas)
- The zygomatic branch (innervating muscles in the infraorbital area, the lateral nasal area and the upper lip)
- The buccal branch (innervating muscles in the cheek, the upper lip and the corner of the mouth)
- The marginal mandibular branch (innervating muscles of the lower lip and chin)
- The cervical branch (innervating the platysma muscle)
Terminal Branches of the Facial Nerve. (Image by Patrick J. Lynch, medical illustrator (Patrick J. Lynch, medical illustrator) [CC BY 2.5 , via Wikimedia Commons)
Assessment
The facial nerve can be assessed by:
- Looking for symmetry in the face at rest
- Asking the patient to perform the following movements
- Raising their eyebrows
- Closing their eyes tightly
- Blowing out their cheeks
- Smiling
Likely Causes of Disease or Injury
Causes of CN VII palsy include:
- Bell's palsy (idiopathic)
- Ramsay-Hunt syndrome (herpes zoster infection of the CN VII motor ganglion)
- Guillain-Barre syndrome
- Botulism
- Infection e.g. mumps, measles, chickenpox, otitis externa/media, encephalitis, mastoiditis
- Tumours e.g. parotid tumours, cerebellopontine angle tumours
- Fractures of the petrous temporal bone
- Blunt/penetrating trauma to the face or during parotid surgery
- Penetrating injury to the middle ear or barotrauma
- Brainstem injury
Upper motor neuron (UMN) facial nerve palsy warrants CT head to exclude cerebrovascular events and other intracranial causes such as tumours, particularly cerebellopontine angle tumours.
Common Clinical Effects
Injury to the facial nerve may result in:
- Ipsilateral facial weakness with flattening of the nasolabial fold and dropping of the corners of the mouth, drooping of the lower eyelid and inability to close the eye
- Loss of corneal reflex (due to paralysis of the orbicularis oculi muscle)
- Impaired lacrimal fluid production (due to impaired function of the greater petrosal nerve)
- Hyperacusis (hypersensitivity to sound due to impaired function of the nerve to the stapedius)
- Impaired sense of taste to anterior two-thirds of tongue and impaired salivation (due to impaired function of the chorda tympani)
If the damage is peripheral (LMN), the forehead will be involved and there will be an inability to close the eyes or raise the eyebrows. If the damage is central (UMN) there is forehead sparing as the frontalis and orbicularis oculi muscles are innervated bilaterally.
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
