Saturday, March 13, 2021

Bell Palsy Casel File

Posted By: Medical Group - 3/13/2021 Post Author : Medical Group Post Date : Saturday, March 13, 2021 Post Time : 3/13/2021
Bell Palsy Case File
Eugene C. Toy, MD, Lawrence M. Ross, MD, PhD, Han Zhang, MD, Cristo Papasakelariou, MD, FACOG

CASE 40
A 28-year-old woman at 19 weeks of pregnancy complains of acute onset of numbness of the right cheek and drooping of the right face that occurred over 1 h. She denies trauma to the head. On examination, the patient has difficulty closing her right eyelid, and her right nasolabial fold is smoother than the left one. She also is drooling from the right side of her mouth. The remainder of the neurological examination is normal.

 What is the most likely diagnosis?
 What is the anatomical mechanism for this condition?


ANSWER TO CASE 40:

Bell Palsy
Summary: A 28-year-old woman at 19 weeks of pregnancy complains of acute-onset numbness of the right cheek and drooping of the right face. She denies trauma to the head. On examination, the patient has difficulty closing her right eyelid and has blunting of the right nasolabial fold. The remainder of the neurological examination is normal.

• Most likely diagnosis: Bell palsy (idiopathic seventh cranial nerve palsy)

• Anatomical mechanism for this condition: Dysfunction of the peripheral portion of the seventh cranial nerve


CLINICAL CORRELATION
Bell palsy is an idiopathic form of facial nerve paralysis that usually manifests as sudden-onset unilateral facial weakness. The peripheral portion of the facial nerve (CN VII) is affected, which may lead to loss of taste to one side of the tongue, weakness of the orbicularis oculi muscle (inability to close one’s eyes), and weakness of the orbicularis oris muscle (inability to purse the lips). The upper and lower portions of the face are affected, which is consistent with a peripheral neuropathy. In contrast, lower facial weakness alone may indicate an upper motor neuron lesion. Maximal weakness usually evolves over several hours and resolves by 1 week. Although patients may experience a sensation of facial numbness, there is generally no sensory loss. Pregnancy seems to increase the incidence of Bell palsy. Keeping the eye moist and protected is an important part of therapy. The eye is vulnerable to dryness due to impaired blinking. Damage to the intracranial course of parasympathetic fibers in the greater petrosal nerve may also contribute to decreased stimulation of the lacrimal gland. Oral corticosteroid therapy may accelerate recovery. Full recovery almost always occurs.


APPROACH TO:
The Facial Nerve

OBJECTIVES
1. Be able to describe the course of the facial nerve (CN VII)
2. Be able to list the functional components of the facial nerve

DEFINITIONS
BELL PALSY: Idiopathic palsy of peripheral CN VII leading to ipsilateral facial weakness.

CHORDA TYMPANI: Small branch of the facial nerve that supplies taste receptors in the anterior two-thirds of the tongue.

UPPER MOTOR NEURON: Neurons that conduct information from motor areas of the brain to the spinal cord. The lower motor neurons project from gray matter in the spinal cord to peripheral muscle.

VIDIAN NERVE: Nerve of the pterygoid canal.

BRANCHIOMERIC MUSCLE: Skeletal muscle derived from one of the branchial arches. In general, this muscle is innervated by cranial nerves


DISCUSSION
The facial nerve (CN VII) originates from the lateral surface of the caudal pons, at the cerebellopontine junction. There are two roots to the nerve: the large branchiomeric motor root and the small nervus intermedius, which contains sensory and visceral motor fibers. The facial nerve runs laterally with the vestibulocochlear nerve (CN VIII) to enter the internal acoustic meatus (Figure 40-1). The meatus is sometimes described as having four quadrants. The facial nerve goes through the anterosuperior quadrant, whereas divisions of the vestibulocochlear nerve go through the other three.

The facial nerve continues laterally until it reaches the bony labyrinth of the inner ear. At this point, the main trunk bends sharply in a posterior direction to enter the facial canal of the temporal bone. The bend is called the genu. The fibers comprising the greater petrosal nerve arise from the genu and course anteriorly

facial nerve anatomy

Figure 40-1. The facial nerve. (Reproduced, with permission, from Lindner HH. Clinical Anatomy. East Norwalk, CT: Appleton & Lange, 1989:49.)

(described further below). Also located at the genu is the geniculate ganglion, the cluster of sensory cell bodies that course in the facial nerve.

The facial nerve passes through the facial canal as it courses posteriorly within the medial wall of the tympanic cavity inferior to the lateral semicircular canal. As the canal reaches the posterior wall of the tympanic cavity, it turns inferiorly, giving off two notable branches, described below. The nerve exits the cranium through the stylomastoid foramen, located between the styloid and mastoid processes.

The facial nerve then courses anteriorly through the parotid gland and splits the gland into superficial and deep lobes. The nerve diverges in variable patterns to form five major branches that supply the muscles of facial expression: the temporal, zygomatic, buccal, mandibular, and cervical branches. There is also a smaller posterior auricular branch that supplies the extra-auricular muscles. Sensory nerves may innervate a small patch of skin on the posterior surface of the auricle.

The greater petrosal nerve emerges from the geniculate ganglion and courses anteriorly through a small canal. It emerges through a small hiatus into the middle cranial fossa and continues anteriorly in a groove directed toward the foramen lacerum. The nerve then passes through a tunnel in the cartilage filling the foramen or through a canal in nearby bone. After exiting the basal surface of the skull posterior to the medial pterygoid plate of the sphenoid bone, the nerve heads anteriorly through the pterygoid (Vidian) canal. The pterygoid canal courses through the sphenoid bone at the base of the medial pterygoid plate. Before entering the canal, the nerve merges with the deep petrosal nerve. The newly formed nerve of the pterygoid canal (Vidian nerve) exits anteriorly into the pterygopalatine fossa. The nerve merges with the pterygopalatine ganglion, which is associated with branches of the maxillary nerve (CN V2). Sensory and sympathetic fibers pass through the ganglion and follow the branches of the maxillary nerve throughout the nasal and oral cavities. Presynaptic parasympathetic fibers synapse in the ganglion. Postsynaptic fibers project through the same nerves to innervate glands of the oral and nasal mucosa. Visceral motor fibers innervating the lacrimal gland also originate in the pterygopalatine ganglion. These fibers run from the ganglion to the infraorbital nerve (CN V2) and follow the zygomaticotemporal nerve along the lateral wall of the orbit. They then follow the lacrimal nerve (V1) to the gland. The lacrimal nerve itself is primarily sensory and innervates the periorbital skin.

As the facial nerve descends posteriorly to the tympanic cavity, two small but important branches emerge. The first is the motor branch to the stapedius muscle. The belly of the stapedius is contained within the pyramid. Its tendon emerges through the apex of the pyramid to attach to the body of the stapes. Contraction of the stapedius dampens the vibration of the ossicles, thus protecting against loud sounds. The second branch in this region is the chorda tympani. It branches from the motor trunk before it exits the stylomastoid foramen and enters the tympanic cavity through a small canal in the posterior wall. It then runs anterolaterally, deep to the tympanic membrane. As it does so, it runs between the vertical processes of the incus and the malleus. The chorda tympani courses anteriorly and inferiorly through the temporal bone and emerges from the basal surface of the skull through the petrotympanic fissure. The nerve then courses through the infratemporal fossa along the superficial surface of the medial pterygoid muscle before joining with the lingual nerve. Sensory fibers in the chorda tympani course with branches of the lingual nerve to supply taste receptors in the anterior two-thirds of the tongue. Presynaptic parasympathetic fibers synapse in the submandibular ganglion. Postsynaptic fibers supply the submandibular and sublingual salivary glands.

In addition to its complex branching pattern, the facial nerve has many functional components. To summarize, the facial nerve is primarily a motor nerve that supplies branchiomeric muscles. These are primarily the muscles of facial expression but also include the stapedius, stylohyoid, and posterior belly of the digastric muscle. Another important function of the facial nerve is to supply visceral motor fibers that supply the lacrimal gland, the submandibular and sublingual salivary glands, and mucus-secreting glands of the nasal and oral cavities. The facial nerve has an important sensory component. The special sensory component that supports taste in the anterior two-thirds of the tongue is ultimately carried by the lingual nerve. There is a minor component of general sensation from innervation of a small patch of skin on the posterior surface of the auricle.


COMPREHENSION QUESTIONS

40.1 A 44-year-old man complains of difficulty hearing from the right ear and headaches. He also has facial muscles weakness. Which of the following is the most likely explanation?
    A. Peripheral CN VII palsy
    B. Peripheral CN VIII palsy
    C. Cerebellar pontine angle lesion
    D. Trigeminal ganglion lesion
40.2 An injury to the facial nerve (CN VII) as it leaves the stylomastoid foramen would disrupt which function?
    A. Taste to the posterior tongue
    B. Sensation to the cornea
    C. Sensation to the cheek
    D. Sensation to the anterior scalp
    E. Wrinkling of the forehead

40.3 A 33-year-old woman suffered a skull fracture that led to a unilateral facialnerve palsy. Which of the following fractures was most likely responsible?
    A. Frontal calvaria
    B. Temporal bone fracture involving the squamous part
    C. Occipital fracture
    D. Basilar fracture involving the mastoid area


ANSWERS
40.1 C. When multiple nerves are affected, it is unlikely to be a peripheral disorder. Cranial nerves VII and VIII exit in close proximity from the pons. A schwannoma involving the cerebellopontine angle can affect both cranial nerves.

40.2 E. Forehead wrinkling results from contraction of the frontalis muscle, which is innervated by the facial nerve. The facial nerve is responsible for taste in the anterior two-thirds of the tongue, but the chorda tympani emerges before the main trunk exits through the stylomastoid foramen. Sensation of the cornea and sensation to the cheek are supplied by the trigeminal nerve.

40.3 D. The basilar fracture involving the mastoid region of the temporal bone may
impinge on the facial nerve as it exits the stylomastoid foramen.


ANATOMY PEARL
 Sensory fibers in the chorda tympani course with branches of the lingual nerve to supply taste receptors in the anterior two-thirds of the tongue.
 The facial nerve supplies most of the muscles involved with facial expression but also supplies the stapedius, stylohyoid, and posterior belly of the digastric muscle.
 CN VII carries visceral motor neurons that supply the lacrimal gland, the submandibular and sublingual salivary glands, and mucus-secreting glands of the nasal and oral cavities.

References

Gilroy AM, MacPherson BR, Ross LM. Atlas of Anatomy, 2nd ed. New York, NY: Thieme Medical Publishers; 2012:488−489, 492−493, 504−505, 514−515, 528−530. 

Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy, 7th ed. Baltimore, MD: Lippincott Williams & Wilkins; 2014:853−855, 861, 1068−1070. 

Netter FH. Atlas of Human Anatomy, 6th ed. Philadelphia, PA: Saunders; 2014: plates 24, 124.

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