Cranial nerves and their assessment Flashcards
Cranial nerves
12 pairs
Part of peripheral NS
Some sensory, motor or mixed
Some have autonomic functions (III, VII, IX, X)
Except for vagus (X), all others serve only structures in head and neck
Where do the cranial nerves originate from?
I and II from pons
V - VII originate from medulla
IX and XII originate from medulla
Sensory motor mixed pnemonic
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Cranial nerves in order
Olfactory Optic Oculomotor Trochlear Trigeminal Abducens Facial Acoustic Glossopharyngeal Vagus Accessory Hypoglossal
Olfactory nerve
- function
- origin and destination
CN I
Sense of smell
Arises from receptor cells in the nasal epithelium
Travels via olfactory nerve filaments through the cribiform plate of ethmoid bone to the olfactory cerebral cortex (especially temporal lobe)
Responsible for sense of smell
Damage to olfactory nerve
Causes impaired sense of smell
Testing olfactory nerve
Not tested routinely
Ask if any change in sense of smell or taste
If testing indicated
-get pt to clear nose
-ask pt to hold one nostril closed and close eyes
-place recognisable scent under nostril (e.g. coffee, peppermint, lemon, vanilla, chocolate)
Repeat with other nostril
Nb. avoid irritating substance like vinegar - these may stimulate pain receptors of trigeminal nerve
Anosmia
Loss of sense of smell (e.g. flu, nasal polyps)
Most commonly results from nasal congestion
Other causes are
-basal or frontal skull fracture
-nasal or frontal lobe (olfactory groove) tumour
CN I lesion
Nose, cribiform plate of ethmoid bone, base of skull
E.g. meningioma, early sign of Parkinson
The optic nerve
- function
- origin and destination
CN II
Provides vision
Sensory
From retinal euro-epithelium
-rods and cones activate bilpolar cells (first order neurones)
-synapsing on ganglion cells in retina
To: second order neurones converge on optic disc/ where optic nerve partially crosses over at optic chiasma to opposite side of brain
Interpretation of vision (central and peripheral) occurs in occipital cortex
Damage to the optic nerve
Damage causes blindness in visual field
Testing optic nerve
- Visual acuity: central vision (Snellen chart)
- position pt 6m from chart
- pt covers one eye at time
- record smallest line
- record as fraction: upper (1st) number normally 6 (m away from chart), lower (2nd) number tiny number under smallest line read - Peripheral vision
- sit arms length and at eye level
- aim of test to compare your vision with pts
- to check left eye, pt covers right eye with card and you do to, ask pt to look directly into your eye
- move your wagging finger from periphery towards centre and ask pt to tell you when their finger moves
- should be tested in horizontal plane and in upper and lower temporal quadrants
- change hands and repeat on nasal side - Fundoscopy
- pt gazes into distance
- look at cornea (for ulcers), iris and then lens for red reflex (absent if pt has cataracts)
- search for optic disc by following large retinal vein back towards disc, where they all converge
- look at edges of disc and colour - Light reflex
- shine light in one eye (stimulate optic nerve) and both pupils constrict (both oculomotor nerves)
Cranial nerve III, IV and VI
Controls eye movement and are tested together
III: oculomotor nerve
IV: trochlear nerve
VI: abducen nerve
All have sensory component for muscle proprioception
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Oculomotor nerve
Somatic and autonomic motor function
Eye movement, opening of eyelid, focusing
Muscles it controls:
Medial rectus (looking inwards)
Superior rectus (looking upwards)
Inferior oblique (looking upwards and inwards)
Inferior rectus (looking downwards)
Trochlear nerve
-function
-damage and causes
Origin and destination
Eye movement:
Superior oblique (downwards and inwards)
Mixed nerve fibres, mainly motor
Originates from midbrain and travels alongside oculomotor nerve to supply SO muscle
Damage causes double vision on looking straight down and inability to rotate eye laterally
Causes:head injuries, aneurysms of internal carotid artery
Abducens nerve
- function
- damage and causes
Provides eye movement:
Lateral rectus (outwards)
Damage results in inability to rotate eye laterally and at rest eye rotates medially
Causes: head injuries, aneurysms of internal carotid artery
Complete and partial 3rd nerve palsy
- symptoms
- causes
Symptoms:
-ptosis
-eyes down and out
-failure of adduction
Causes:
-complete palsy commonly caused by compressive lesion
-partial III nerve palsy spares pupil (vascular link, diabetes mellitus, aneurysm, tumour, trauma)
Common causes of abnormal eye movements
Central (brainstem) lesions Vascular e.g. basilar thrombosis, pontine haemorrhage Tumour e.g. pontine glioma Demyemlination Wernickes Encephalopathy Peripheral Lesions Vascular e.g. hypertension Basal skull fracture Vasculitis GBS (Miller-Fisher variant) Complete Opthalmoplegias Thyroid eye disease, myasthenia Gravis, myopathies.
