H&N8 - Cranial Nerves I-VI

Question 1

5 features of the olfactory nerve (CN I)

Origin
Route
1 Function
Examination
Clinical Condition

Answer 1

1. ) Origin - forebrain
   - anterior extension of the brain itself (not ‘true’ CN)
2. ) Route - roof of nasal cavity –> cribriform foramina (ethmoid bone) –> olfactory bulb –> olfactory tract –> temporal lobe
3. ) Special Sensory - sense of smell (olfaction)
4. ) Examination - not often tested, ask about changes in sense of smell, if examined, test one nostril at a time
5. ) Anosmia - loss of sense of smell
   - often caused by upper respiratory tract infections
   - also caused by head injuries and tumours at the base of the frontal lobes (anterior cranial fossa/basilar skull fracture)

Question 2

5 features of the optic nerve (CN II)

Origin
Route
1 Function
Examination x4
Clinical Conditions x2

Answer 2

1. ) Origin - forebrain, part of the visual pathway
   - anterior extension of the brain itself (not ‘true’ CN)

2.) Route - retinal ganglion cells –> optic disc –> optic nerve –> optic canal –> optic chiasm –> optic tract

3. ) Special Sensory - vision
   - communication from the optic tracts with the midbrain allows for visual reflexes (from occipital lobe)
4. ) Examination - seen directly with opthalmoscope
   - visual acuity tests, visual fields, pupillary light responses
5. ) Clinical Conditions
   - papilledema is swollen optic disc due to raised ICP
   - optic nerve carries extension of the meninges which explains photophobia in meningitis

Question 3

3 features of the optic chiasm

Definition
Function
Lesions

Answer 3

1.) Definition - X-shaped structure formed by the crossing of optic nerves

2. ) Function - where the sensory fibres from the right and left optic nerves mix together
   - the optic tract then contains sensory information from both the right and left eye (optic nerve)
3. ) Lesions - retinal or optic nerve lesions affect one eye
   - lesions affecting the optic chiasm or tract (e.g. pituitary tumours) cause bitemporal hemianopia which is bilateral visual symptoms

Question 4

5 features of the oculomotor nerve (CN III)

Origin
Route
2 Functions
Examinations

Answer 4

1. ) Origin - midbrain
2. ) Route - cavernous sinus –> superior orbital fissure
3. ) Motor Function - eyeball (extraocular) muscles and eyelid muscles (levator palpebrae superioris)
4. ) Autonomic Function - parasympa fibres innervate the sphincter pupillae muscles (constricts pupil) and ciliary muscles (alters lens shape)
5. ) Examination - inspect eyelids and pupil size
   - test eye movements and pupillary reflexes
   - ask about double vision (diplopia)
   - down and out position due to unopposed action of LR6 SO4
   - severe ptosis due to loss of skeletal component to LPS

Question 5

4 causes of pathology of the oculomotor nerve

Answer 5

1. ) Vulnerability to Raised ICP - due to compression between tentorium cerebelli and petrous bone
   - raised ICP due to tumours or haemorrhages
   - causes a blown pupil because the parasympa nerve fibres are first compressed leading to pupil dilation
2. ) Aneurysms - of the posterior communicating artery
3. ) Cavernous Sinus Thrombosis - causing compression
4. ) Microvasculopathy - diabetes and/or hypertension
   - vascular causes tend to spare the pupils (autonomic)

Question 6

5 features of the trochlear nerve (CN IV)

Origin
Route
1 Function
Examination
Pathology

Answer 6

1. ) Origin - midbrain
   - emerges from the dorsal aspect of the brainstem making it the longest cranial nerve
2. ) Route - cavernous sinus –> superior orbital fissure
3. ) Motor Function - superior oblique (extraocular)
4. ) Examination - test eye movements and diplopia
   - diplopia is rare and often subtle (corrected w/ head tilt)
5. ) Pathology - caused by head injury –> raised ICP
   - other causes are congenital palsies

Question 7

6 features of the trigeminal nerve (CN V)

Origin
Routes
2 Functions
Examination
Pathology

Answer 7

1. ) Origin - pons
   - emerges laterally from the pontomedullary junction
2. ) Routes - pons –> trigeminal ganglion –> 3 branches:
   - ophthalmic (Va) –> superior orbital fissure –> orbit
   - maxillary (Vb) –> foramen rotundum –> pterygopalatine fossa
   - mandibular (Vc) –> foramen ovale –> infratemporal fossa
3. ) General Sensory - supplies skin and part of the scalp
   - also supplies deeper structures e.g. paranasal sinuses, nasal and oral cavity, anterior part of tongue, meninges

4.) Motor Function - muscles of mastication (Vc only)

5. ) Examination - check sensation by touching dermatomes for Va (forehead), Vb (cheek), and Vc (jaw)
   - test motor (mandibular) using jaw jerk
   - corneal reflex, Va is afferent limb (facial is efferent)
6. ) Pathology - due to orbital/facial trauma and fractures
   - conditions include trigeminal neuralgia and shingles
   - ophthalmic shingles can lead to vision problems because the nerve innervates the conjunctiva and cornea

Question 8

Important branches of the trigeminal divisions
(CN Va, Vb, Vc)

3 Ophthalmic
2 Maxillary
3 Mandibular

Answer 8

1. ) Ophthalmic - frontal, lacrimal, and nasociliary branch
   - supplies the eye, conjunctiva, orbital contents, and structures within or deep to its dermatomal distribution
   - frontal –> supraorbital + supratrochlear which goes to supply the forehead
2. ) Maxillary - many branches, important ones are:
   - infraorbital supplies the cheek and lower eyelid
   - superior alveolar has 3 divisions which supply the nasopharynx, nasal cavity, upper teeth and gums
3. ) Mandibular - inferior alveolar, auriculotemporal, lingual
   - inferior alveolar becomes the mental nerve supplying the mental protuberance (chin), lower lip and gums
   - auriculotemporal supplies the temple area and TMJ
   - lingual supplies general sensory to anterior 2/3 of the tongue

Question 9

5 features of the abducens nerve (CN VI)

Origin
Route
1 Function
Examination
Pathology

Answer 9

1. ) Origin - pons
   - emerges ventrally from tune pontomedullary junction
2. ) Route - cavernous sinus –> superior orbital fissure
3. ) Motor Function - lateral rectus (extraocular muscle)
4. ) Examination - test eye movements
   - patients present with diplopia
5. ) Pathology - mainly due to raised ICP
   - easily stretched due to emerging at ponto-medullary junction before running under the surface of the pons
   - can also get microvascular complications from diabetes and hypertension