Cranial Nerves Flashcards
What does the olfactory nerve do?
Responsible for smell
What is the most common cause of loss of smell and other causes?
URT infection is the most common cause
Other causes: increasing age, smoking, skull fracture (frontal and basal), SOL in temporal/frontal lobe
How do we examine olfactory nerve?
Sense of smell on each side separately Familiar smells (eg. peppermint/vanilla) and px must identify smell Quick and less formal bedside test involves substances e,g coffee, soap, orange peel
What do we assess for the optic nerve?
Visual acuity
Visual fields by confrontation
Fundoscopy (exam of optic fundus using opthalmoscope)
How do we assess for visual acuity?
Snellen chart
Px stands 4 m from chart
Determine fraction
Top number = distance between chart and px
Bottom number = lowest line accurately read
Describe normal, best, abnormal vision fractions from the snellen chart.
Normal vision: 6/6 or 4/4
Better than normal: 6/<6, 4/<4
Abnormal: 6/>6, 4/>4
Why does the patient not remove their glasses for the snellen chart exam in neuro examination?
We are assessing optic nerve, not abnormalities of cornea/lens/optic fundus
Scotoma
Area of visual loss surrounded by a field of normal vision
Quadrantanopia
Loss of 1/4 of field of vision
Hemianopia
Loss of 1/2 field of vision
Homonymous hemianopia
Loss of the same half of the visual field in each eye
Monocular blindness
Loss of vision in one eye
What is the optic fundus?
Interior surface of the eye opposite the lens and includes the retina, optic disc, fovea, macula and posterior poles
What is the optic disc?
Circular area in the retina where retinal ganglion cells leave the eye to form the optic nerve
Name the ocular nerves.
III - oculomotor
IV - trochlear
VI - abducens
What are the ocular nerves responsible for?
Eye movement
Eyelid opening
Pupillary constriction
What do we examine for the ocular nerves?
Examine the eyes
Examine eye movements, look for nystagmus, ask about diplopia
Examine pupillary light and accommodation reflex
Describe the components of observing the eyes.
Pupils - size, shape, equality
Eyelids - opening, closing, ptosis (levator palpebrae superioris)
Alignment of eyes - conjugate gaze vs strabismus
Describe how we examine eye movements, nystagmus and diplopia.
Moving target using finger or pen & ask patient to follow
Look for reduction of eye movement and nystagmus
Nystagmus = rapid uncontrolled eye movement (horizontal, vertical, rotatory)
Diplopia - both eyes cannot aim at the same target
What does the light reflex depend on?
Afferent optic nerve (II) and efferent oculomotor nerve (III)
Pupil constricts due to stimulation of the parasympathetic fibres supplied by the Edinger Westphal nucleus of CN III
What causes the accommodation reflex?
Stimulation of parasympathetic fibres of CN III = pupil constricts due to action of pupillary constrictor muscle and lens becomes more rounded due to action of the ciliary muscle
PERLA
Pupils equal and reactive to light and accommodation
What are the signs of oculomotor nerve palsy (III)?
Dilated pupil on affected side
Ptosis (eyelid droop) on affected side (inactivation of levator palpebrae superioris)
Compensate for ptosis by contracting muscles of forehead
Strabismus (eyes not aligned) with a downward abducted eye on the affected side = unopposed actions of the superior oblique and lateral rectus
Diplopia, very little eye movement on affected side
Loss of accommodation and light reflex on affected side
What are the signs of trochlear nerve palsy (IV)?
Extortion (outward rotation) of the affected eye - unopposed action of the inferior oblique muscle
Vertical diplopia
Weakness of downward gaze when eye is directed medially
E.g. difficulty in descending stairs or reading a book
What are the signs of abducens nerve palsy (VI)?
Stabismus with the affected eye directed medially (inward deviation) - unopposed action of the medial rectus muscle
Look at an object laterally on the side of the lesion - affected eye will be unable to be abducted beyond the midline of gaze
Horizontal diplopia
What is the trigeminal nerve responsible for?
Corneal reflex and jaw jerk
What are the signs of a dysfunctional trigeminal nerve?
Sensory problems e.g. pain/numbness in face or motor problems causing trouble with chewing and talking
Describe the examination of the trigeminal nerve.
Examine sensory function (divisions = ophthalmic, maxillary, mandibular) - light touch, superficial pain, corneal reflex
- light touch and superficial pain = same as sensory exam
- corneal reflex = blinking in both eyes when cornea is touched
Examine motor function - muscles of mastication
Examine jaw jerk
What are the nerves supplying the cornea?
Sensation from cornea - ophthalmic division of ipsilateral trigeminal nerve (CN V)
Efferent pathway on facial nerves (CN VII) of both sides
What is responsible for the jaw jerk?
Mandibular division of the trigeminal nerve & efferent limb by the trigeminal motor root.
