The cranial nerves

CliniCal anaesthesia

The cranial nervesJohn Craven

Abstractthe cranial nerves are described in this article with emphasis on their

intracranial and extracranial relationships. their function and distribution

is also emphasized, as is the assessment of their integrity. a summary

of how their clinical assessment is made and the consequences of their

most common pathology is included.

Keywords assessment of cranial nerves; cranial nerves; distribution of

cranial nerves; injury

The 12 pairs of cranial nerves originate in the brain and leave the cranial cavity through its basal foramina. Each has motor and sensory fibres. The attachments of the nerves to the brainstem are shown in Figure 1.

CI The olfactory nerve

The olfactory nerve is an outgrowth of the forebrain and supplies the upper olfactory mucous membrane. Its fibres originate in the mucosa and join to form 15–20 bundles, which pass through the cribriform plate of each ethmoid to reach the olfactory bulb. An extension of the meninges surrounds them. Fractures of the anterior cranial fossa involving the cribriform plates may be accompanied by a loss of cerebrospinal fluid from the nose (CSF rhinorrhoea) and, if the nerves are also damaged, long-term anosmia.

CII The optic nerve

The optic nerve is the sensory nerve of the retina. Its fibres origi-nate in the ganglion layer and converge on the posterior part of the eyeball. It passes through the orbit and optic canal to the middle cranial fossa, where it decussates with that of the opposite side, forming the optic chiasma. The macula, the most sensitive part of the retina, accounts for about 25% of the visual cortex. The nerve within the orbit is enclosed in its meningeal sheath and surrounded by a cone of extraocular muscles. The ciliary ganglion is posterolateral and the ophthalmic artery and nasocili-ary nerve medial. The central artery of the retina (a branch of the ophthalmic artery) enters the nerve in this part of its course. The

John Craven, FRCS, was formerly Consultant Surgeon at York Hospital,

York. He is past chairman of the primary examiners of the Royal

College of Surgeons of England.

anaesthesia anD intensiVe CaRe MeDiCine 8:12 49

intracranial course is short and lies on the sphenoid bone medial to the internal carotid artery. The optic chiasma lies above and anterior to the pituitary gland.

Because the meninges surround the nerve, an increase in CSF pressure causes venous stasis and oedema (swelling) of the optic nerve and retina, recognized ophthalmoscopically as papilloedema. Visual defects may result from pressure on the optic nerves or chiasma from pituitary tumours or aneurysms (swellings) of the internal carotid arteries. Figure 2 explains why specific visual field defects follow section of the optic nerves, optic chiasma and optic tracts.

CIII The oculomotor nerve

The oculomotor nerve has somatic motor and parasympathetic motor fibres. The somatic fibres supply the bulbar muscles, except for superior oblique and lateral rectus. The parasympa-thetic fibres synapse in the ciliary ganglion and supply sphincter pupillae and the ciliary muscle. The nerve leaves the midbrain between the cerebral peduncles and passes through the posterior and middle cranial fossae to divide into superior and inferior divisions near the superior orbital fissure.

Superficial origins of the cranial nerves

Longitudinal fissure Olfactory bulb

Olfactory tract

Optic nerve (CII)

Optic

chiasm

Optic

tract

Trochlear

nerve

(CIV)

Trigeminal nerve (CV)

Hypoglossal nerve (CXII)

Sensory root

Motor root

Oculomotor

nerve

(CIII)

Olfactory

nerves (CI)

Infundibulum

Mammillary

body

Pons

Abducent

nerve

(CVI)

Vestibulocochlear

nerve (CVIII)

Glossopharyngeal

nerve (CIX)

Vagus nerve (CX)

Accessory

nerve (CXI)

Facial nerve (CVII)

Figure 1

9 © 2007 elsevier ltd. all rights reserved.


Visual field defects

Visual field defects result from lesions that affect different parts of the visual pathway

Right-side blindness

Visual

fields

Bitemporal hemianopia Left homonymous hemianopia

a Section of right optic nerve results in

monocular blindness in temporal (T) and

nasal (N) visual fields of the right eye

b Section of optic chiasm reduces peripheral

vision, resulting in bitemporal hemianopia

c Section of right optic tract eliminates vision

from the left temporal and right nasal visual

fields, a contralateral homonymous

hemianopia. It is often observed in patients

with a stroke

T N

Visual

fields N N

Visual

fields TN

Figure 2

In the posterior cranial fossa the nerve is close to the ten-torium cerebelli. In the middle cranial fossa it passes forwards on the lateral wall of the cavernous sinus. Its superior division traverses the superior orbital fissure within the tendinous ring of the extraocular muscles and supplies superior rectus and levator palpebrae superioris. The inferior division also passes through the fissure and supplies medial and inferior recti and inferior oblique muscles. It also carries a parasympathetic branch to the ciliary ganglion.

Because of its close relationship to the edge of the tentorium cerebelli, the oculomotor nerve may be damaged if there is lat-eral shift of the brain, such as may occur with an intracranial haemorrhage. One of the earliest localizing signs of this may be a selective palsy of the parasympathetic fibres, causing dilatation of the pupils due to damage to the parasympathetic fibres pass-ing to the ciliary ganglion.

Signs of a complete palsy of the nerve are: ptosis (drooping eyelid), loss of pupillary light reflexes, dilatation of pupil, abduc-tion and downward deviation of eye because of unopposed action of superior oblique and lateral rectus and loss of accommodation in the eye because of paralysis of ciliary muscles (Figure 3).

CIV The trochlear nerve

The trochlear nerve is the thinnest cranial nerve and supplies the superior oblique muscle. Emerging from the lower dorsal midbrain, the nerve passes forwards through the posterior and middle cranial fossae to enter the orbit through the superior orbital fissure.

In the posterior cranial fossa it passes forwards around the midbrain, following the edge of the tentorium, lateral to the oculomotor nerve. In the middle cranial fossa it lies in the lateral wall of the cavernous sinus. It traverses the superior orbital fis-sure and gains the roof of the orbit to supply the superior oblique muscle. The nerve has a long intracranial course and may be injured if a severe head injury and intracranial bleeding produces


lateral shift of the brain. If the superior oblique muscle is para-lysed and no other extraocular muscle is affected (this is rare) then diplopia occurs when the patient is looking downwards. The affected eye is pulled downwards only by the inferior rectus and thus in a slightly different direction to that of the uninjured side.

CV The trigeminal nerve

The trigeminal nerve conveys somatic sensory and visceral motor fibres. The sensory fibres innervate the anterior part of the scalp and the dura, the face, nasopharynx, nasal and oral cavities and the paranasal air sinuses. They are derived from cell bodies in the trigeminal ganglion and form a large sensory root from which fibres enter the lateral side of the pons. The motor fibres emerge from the pons as a smaller motor root and supply the muscles of mastication. The dural sheath containing the two roots crosses the petrous bone from the posterior into the middle

Right ocular nerve palsy

Right oculomotor nerve palsy producing a downward and

outward gaze and dilated pupil in the right eye. The right eyelid

is elevated by a finger to overcome the right ptosis.

Figure 3



cranial fossa. The trigeminal ganglion, which can be thought to be equivalent to the dorsal sensory ganglion of a spinal nerve is crescent-shaped and lies partly within an evagination of the dura mater of the middle cranial fossa, the cavum trigeminale. The motor root of the nerve and the greater superficial petrosal nerve both lie deep to the ganglion. Above it lies the temporal lobe of the cerebrum, medially is the internal carotid artery and the cavernous sinus. The dural sheath partially enveloping the nerve fuses with that of the middle cranial fossa to form the lateral wall of the cavernous sinus. It covers only the posterior half of the ganglion so that the posterior half of the ganglion and the sen-sory and motor roots are bathed in CSF. The ganglion receives ophthalmic, maxillary and mandibular divisions from their wide peripheral distribution. The motor root passes by the ganglion to join the mandibular division.

The ophthalmic division is the smallest division and passes for-wards on the lateral wall of the cavernous sinus and, near the superior orbital fissure, divides into lacrimal, frontal and naso-ciliary nerves, which each pass through the fissure into the orbit.• The lacrimal nerve supplies the lacrimal gland and the skin and conjunctiva of the lateral part of the upper lid and adjacent conjunctiva. Parasympathetic secretomotor fibres are carried to the gland by a branch of the zygomaticotemporal nerve.• The frontal nerve divides into supraorbital and supratrochlear nerves. The former leaves the orbit by the supraorbital notch to supply the upper eyelid, frontal sinuses and scalp as far back as the vertex. The supratrochlear nerve supplies the skin of the upper eyelid and medial forehead.• The nasociliary nerve traverses the superior orbital fissure to gain the medial wall of the orbit, where it divides into anterior and posterior ethmoidal, infratrochlear and ciliary nerves.• The ethmoidal nerves leave the orbit through a foramen on its medial wall to reach the anterior cranial fossa and descend through the cribriform, supplying anterior cranial fossa dura, ethmoidal and sphenoidal air sinuses, the upper anterior nasal cavity and the skin of the tip of the nose.• The infratrochlear nerve supplies the medial part of the up-per eyelid, conjunctiva and adjacent nose, and the long ciliary nerves supply the sclera and cornea. Sympathetic fibres are con-veyed to the dilator pupillae muscle. Ophthalmic herpes zoster (shingles) involving the corneal branch gives a sensory loss, leading to ulceration.

The maxillary division passes forwards through the middle cra-nial fossa, for a short distance in the cavernous sinus to the pter-ygopalatine fossa, where the pterygopalatine ganglion is attached to it, It then passes through the inferior orbital fissure to become the infraorbital nerve. Parasympathetic fibres to the pterygopala-tine ganglion pass onwards to the nose, palate and nasopharynx. The zygomatic nerve passes forward on the lateral wall of the orbit and supplies the skin of the temple and cheek. A commu-nication with the lacrimal nerve conveys parasympathetic fibres to the lacrimal gland. The posterior superior alveolar nerve sup-plies the upper molar and premolar teeth. The infraorbital nerve emerges from the infraorbital foramen onto the face to supply the lower eyelid and conjunctiva, the side of the nose and upper lip. Its anterior superior alveolar branch supplies the canine and inci-sor teeth, the side of the nose and the maxillary air sinus.


The mandibular division carries the motor root of the trigeminal nerve. It leaves the middle cranial fossa via the foramen ovale and divides into motor branches and sensory branches. Motor branches supply the muscles of mastication: masseter, tempor-alis, medial and lateral pterygoids’ tensor tympani and tensor veli palatini, mylohyoid and the anterior belly of digastric. The sensory branches have a wide distribution (Figure 4).• Meningeal branches pass through the foramen spinosum, supplying the dura of the middle cranial fossa.• The buccal nerve passes forwards on the inner side of the mandible, supplying the skin and mucous membrane of the cheek.• The auriculotemporal nerve passes laterally, anterior to the neck of the mandible and ascends to the temporal region of the scalp. It supplies the tympanic membrane, external acoustic meatus and auricle, the temporomandibular joint, the temporal region of the scalp and conveys parasympathetic fibres to the parotid gland.• The inferior alveolar nerve descends to the mandibular fo-ramen and enters the mandibular canal to supply gums and teeth. A branch, the mylohyoid nerve, passes forwards close to the mandible to supply the muscle and the anterior belly of digastric. A terminal branch, the mental nerve, emerges through the mental foramen to supply the skin and mucous membrane of the lower lip and chin.• The lingual nerve passes forwards along the side of the tongue, supplying sensory branches to the anterior two-thirds of the tongue, the floor of the mouth and the lingual gum. It is the largest branch of the mandibular division and receives the chorda tympani branch of the facial nerve about 3 cm below the base of the skull. This carries parasympathetic fibres to the sub-mandibular and sublingual salivary glands, and taste fibres from the anterior two-thirds of the tongue.

Damage to the trunk of the trigeminal nerve may result from pressure from an aneurysm, infiltration by a tumour and, rarely, from injury. This results in paralysis of the mastication muscles and loss of sensation on the side of the face, with loss of the cor-neal reflex (no blink when the cornea is touched). A lower-third molar tooth extraction may result in trauma to the lingual nerve

Cutaneous distribution of the trigeminal nerve

Ophthalmic

Maxillary

Mandibular

division

division

division

Figure 4



and thus loss of sensation to the anterior tongue and reduced sali-vation due to chorda tympani involvement. Trigeminal neuralgia (severe pain affecting the maxillary and/or mandibular divisions of the nerve) may require surgical treatment by ablation of the division affected or, less commonly, ablation of the ganglion.

CVI The abducent nerve

The abducent nerve is a somatic motor nerve. It leaves the in-ferior border of the pons, passes forwards through the poste-rior and middle cranial fossae, the cavernous sinus and orbit, to supply the lateral rectus muscle. In the posterior cranial fossa it lies between the pons and the occiput. It passes through the cavernous sinus to enter the orbit via the superior orbital fis-sure within the tendinous ring. This thin nerve with a long intra-cranial course is susceptible to damage by stretching in patients with raised intracranial pressure or by pituitary tumours close to the course of the nerve. Paralysis, either partial or complete, results in failure of abduction of the eye and diplopia.

CVII The facial nerve

The facial nerve (Figure 5) carries visceral motor, visceral sen-sory and visceral parasympathetic fibres. The motor supply is to the facial muscles. The visceral sensory fibres carry taste sensa-tion from the anterior two-thirds of the tongue to its nucleus in the medulla. The parasympathetic fibres are secretomotor to the submandibular and sublingual salivary glands, the lacrimal gland and the mucous membrane of the nose, pharynx and mouth.

Leaving the pons the facial nerve passes laterally with the vestibulocochlear nerve through the internal acoustic meatus to the facial ganglion. The greater petrosal nerve carries secreto-motor fibres to the pterygopalatine ganglion and leaves the facial nerve at the facial ganglion, joining a sympathetic branch from the internal carotid plexus to traverse the sphenoid bone and join the pterygopalatine ganglion. The facial nerve then turns posteri-orly to descend through a bony canal in the posterior wall of the middle ear to emerge from the stylomastoid foramen. Above the stylomastoid foramen the chorda tympani nerve arises, passing

The branches of the facial nerve

Temporal

Buccal

Zygomatic

Cervical

Mandibular

Figure 5


through the middle ear to emerge from the base of the temporal bone to join the lingual nerve. It conveys taste fibres from the anterior tongue, parasympathetic fibres to the submandibular ganglion, the posterior belly of digastric, stylohyoid, occipitalis and the auricular muscles. The facial nerve then passes into the parotid gland, where it divides into several terminal branches supplying the muscles of facial expression, buccinator and platysma.

Motor function is easily assessed by observation of the sym-metry of the facial muscles during expression and whistling. The power of eye closure is a simple method of assessing the extent of a palsy. Lower motor neuron lesions produce ipsilateral facial paralysis, upper motor lesions produce contralateral paralysis of the lower face. In both types of damage the muscles of the forehead and eyebrows, which are bilaterally innervated, are spared. The cause of Bell’s palsy, a common form of facial palsy, is unknown. Inflammatory swelling of the nerve in the facial canal results in compression of the nerve and its dysfunction. Thus the cheek, lip muscles and orbicularis oculi are paralysed or weakened. Food cannot be chewed properly and the eye’s lower lid is everted and no longer able to retain the lacrimal fluid within the conjunctival sac. The cornea is at risk of drying and may ulcerate. Fortunately, most cases resolve after 2–3 months. Other causes of facial nerve paralysis include tumours in the cerebellopontine angle (acoustic neuromata) and strokes, caus-ing upper motor neuron lesions of the nerve. The peripheral branches of the nerve are vulnerable to injury during surgery on the parotid and submandibular glands.

CVIII The vestibulocochlear nerve

The vestibulocochlear nerve is a sensory nerve of two parts; the vestibular nerve concerned with balance and the cochlear nerve concerned with hearing. These unite in the internal auditory meatus before passing medially to enter the medulla. Hearing is assessed by Rinne’s test. A tuning fork on the mastoid process will detect whether vibrations are better heard by air conduction (normal) or by bone conduction (abnormal). Abnormalities of balance may, if associated with nystagmus, be caused by dam-age to CVIII.

CIX The glossopharyngeal nerve

The glossopharyngeal nerve has general and visceral sensory, motor and parasympathetic secretomotor fibres. The general sen-sory fibres supply the middle ear, the pharynx, posterior third of tongue and the carotid sinus and body. The visceral sensory fibres (taste) come from the posterior third of the tongue. Para-sympathetic fibres supply the parotid gland and motor fibres supply stylopharyngeus. The nerve rootlets join after leaving the medulla, passing through the jugular foramen with the accessory and vagus nerves and internal jugular vein. The nerve passes forwards medial to the external carotid artery, piercing the pha-ryngeal wall between superior and middle constrictors, to reach the posterior third of the tongue.

Lesions affecting the glossopharyngeal nerve alone are rare but the integrity of the nerve can be tested by touching the fau-ces and posterior tongue. The ‘gag’ reflex is absent on the side of the lesion.



CX The vagus nerve

The vagus nerve has several different components:• parasympathetic fibres supply the heart, lungs and alimentary

canal as far as the splenic flexure• visceral motor fibres supply the muscles of the larynx,

pharynx and palate• visceral sensory fibres come from the mucous membrane of

the palate, pharynx and larynx, and from the heart, lungs and alimentary canal

• special sensory fibres (taste) from the valleculae and epiglottis• somatic sensory fibres from the external acoustic meatus and

the tympanic membrane.The vagus nerve emerges from the medulla as rootlets and

leaves the skull by the jugular foramen with the glossopharyn-geal nerve, accessory nerve and internal jugular vein. It descends the neck in the posterior aspect of the carotid sheath, between the internal jugular vein laterally and the internal and common carotid arteries. It lies on the prevertebral muscles and fascia. In the root of the neck the right nerve descends in front of the subclavian artery to enter the thorax; the left nerve descends between the common carotid and left subclavian arteries. In the thorax the two vagus nerves pass posteriorly to each main bron-chus to form the pulmonary plexus, and then converge onto the oesophagus to form the oesophageal plexus, from which emerges the anterior and posterior vagal nerves to be distributed to the alimentary tract.

Branches• The auricular nerve supplying the external acoustic meatus and tympanic membrane.• Pharyngeal nerves passing to the pharyngeal plexus, supplying pharyngeal muscles and the mucous membrane of the pharynx.• The superior laryngeal nerve descends the lateral pharyn-geal wall and divides below the hyoid into external and internal branches; the internal piercing the thyrohyoid membrane to sup-ply the laryngeal mucosa above the vocal cords, and the external supplying cricothyroid muscle.• The right recurrent laryngeal nerve loops around the right subclavian artery, ascends in the groove between oesophagus and trachea to enter the larynx and supply all intrinsic muscles, apart from cricothyroid and the mucosa below the vocal folds.• The left recurrent laryngeal nerve arises as the vagus crosses the aortic arch, passes below the ligamentum arteriosum and as-cends behind the arch in the groove between the trachea and oesophagus. Its distribution is similar to that of the right recur-rent laryngeal nerve.

The anterior and posterior vagi enter the abdomen by the oesophageal hiatus and descend the corresponding aspects of the stomach to give branches to the coeliac, hepatic and renal plexuses, from which branches pass to the midgut, foregut and kidneys.

Injuries occur to the branches of the vagus nerve more fre-quently than the nerve trunk, usually as a result of surgery or penetrating trauma. Injury to the pharyngeal branches causes


dysphagia and aspiration of food and drink; injury to the superior laryngeal nerve causes cricothyroid paralysis and a weak voice. The recurrent laryngeal nerves are occasionally injured during thyroid surgery and may be invaded by cancers of the thyroid, larynx, oesophagus and lung. Paralysis of the vocal fold results.

The ‘gag reflex’ (see above) tests the CIX and CX and CXI; the sensory component is via CIX, the motor movements of the larynx and pharynx are via the CX and CXI.

CXI The accessory nerve

The accessory nerve has cranial and spinal roots, the cranial root arising from the medulla, the spinal by rootlets from the first five segments of the spinal cord. The cranial fibres join the vagus, supplying muscles of the pharynx, larynx and oesophagus. The spinal root of the accessory nerve supplying sternocleidomastoid and trapezius arises in the vertebral canal, ascending through the foramen magnum to the posterior cranial fossa. It leaves the cranium via the jugular foramen, passing laterally to supply sternocleidomastoid before crossing the posterior triangle of the neck to reach trapezius.

The subcutaneous course of the spinal root through the pos-terior triangle of the neck renders it vulnerable to injury during surgery in this region (e.g. lymph node biopsy). Weakness of trapezius and sternocleidomastoid muscle results. Rotation of the neck is weakened as is shrugging the shoulders.

CXII The hypoglossal nerve

The hypoglossal nerve supplies all the intrinsic and extrinsic muscles of the tongue except palatoglossus. It emerges from the medulla as 15–20 rootlets and leaves the posterior cranial fossa by the hypoglossal canal to descend behind the carotid sheath before passing forwards around the pharynx to the tongue. Below the base of the skull the nerve lies behind the internal carotid artery and vagus nerve on the prevertebral fascia. It passes for-wards between the artery and the internal jugular vein, cross-ing the external carotid artery, the loop of the lingual artery and the hyoglossus muscle before ending in the tongue. It lies medial to the digastric (posterior belly), submandibular gland and mylohyoid. It receives fibres from the first cervical nerve near its origin and conveys them through the descendens hypo-glossi branch to supply the geniohyoid and thyrohyoid muscles, before forming the ansa cervicalis with branches of the second and third cervical nerves, which supply the infrahyoid group of strap muscles.

Injury to the nerve, which may occur during tonsillectomy, produces ipsilateral paralysis of half of the tongue, resulting in the protruded tongue deviating to the paralysed side because of the action of the opposite non-paralysed genioglossus. The last four cranial nerves are similar to the upper distribution of the facial nerve; their midbrain nuclei are bilaterally distributed. Thus, damage to the midbrain by thrombosis produces a com-plete bulbar palsy, whereas an upper motor injury has to be bilateral to produce paralysis (pseudobulbar palsy). ◆


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The cranial nerves