CN palsies Flashcards
What is the function of CN 2?
Optic Nerve:
- transmits all visual information including brightness perception, color perception and contrast (visual acuity)
- accommodation reflex: swelling of the lens of eye that occurs when one looks at a near object as in reading
- Pupillary light reflex (afferent arm) : A greater intensity of light causes the pupil to constrict (miosis), whereas a lower intensity of light causes the pupil to dilate (mydriasis)
(the iris is the actual muscle the responds to light)
What is the blind spot?
- obscuration of the visual field
- place in the visual field that corresponds to the lack of light-detecting photoreceptor cells on the optic disc of the retina where the optic nerve passes through the optic disc
What is scotoma and causes?
- alteration in the field of vision: partially diminished or entirely degenerated visual acuity that is surrounded by a field of normal – or relatively well-preserved – vision.
- causes: demyelinating eg MS (retrobulbar neuritis), damage to nerve fiber layer in the retina (seen as cotton wool spots) due to hypertension, vascular blockages either in the retina or in the optic nerve, stroke or other brain injury, macular degeneration pituitary tumours, severe pre-eclampsia. Scintillating scotoma is a common visual aura in migraine.
CN 2 lesions - DDx
Optic neuritis
anterior fossa tumours
Optic chiasm - pituitary tumours
Summarise the visual pathway
Ganglion cells (in the retina) → optic nerve → optic canal (sphenoid bone) → left and right side of the optic chiasm (middle cranial fossa) → optic tracts → lateral geniculate nuclei (thalamus) → superior colliculus → lingual and cuneus gyrus (visual cortex - occipital lobe)
Fibers from nasal retina: cross at the optic chiasm
Fibers from temporal retina: continue ipsilaterally at the optic chiasm
Visual pathway lesions
- ipsilateral monocular blindness (optic nerve lesion)
- bitemporal hemianopia (lesion at optic chiasm) - tunnel vision
- homonymous hemianopia (lesion at optic tract)
- homonymous quadrantanopia (temporal or parietal lesion)
- homonymous hemianopia with macular sparing (lesion at optic cortex)
CN 3 - function
- Eye movement: superior rectus, inferior rectus, medial rectus, and inferior oblique muscles
- Eyelid opening: levator palpebrae superioris
- Pupillary constriction (parasym): sphincter pupillae (Edinger-Westphal nucleus and muscarinic receptors)
CN 3 palsy
- DDx = aneurysm of posterior communicating artery, nerve infarction (eg diabetes), midbrain infarction or tumour
- signs = unilateral complete ptosis (drooping eyelid), eye deviated down and out, and fixed and dilated pupil
CN 5 - function
- sensory to face: 3 branches = ophthalmic, maxillary, mandibular
- motor component to muscles of mastication
CN 5 palsy
- DDx = brainstem pathology (infarction, demyelination, tumour), CP angle tumours (acoustic neuroma), infection
- signs = unilateral sensory loss on face, scalp, anterior 2/3 of tongue and buccal mucosa
trigeminal neuralgia not necessarily a CN 5 palsy (?)
CN 6 palsy
Abducens (lateral rectus)
- DDx = MS, wernicke’s encephalopathy, pontine stroke, infections
- signs = horizontal diplopia when looking into the distance, maximal when looking at the side of the lesion, eye cannot be fully abducted and can get estropia (inwards eye deviation)
Diplopia - definition and causes
- Double vision: simultaneous perception of two images of a single object that may be displaced horizontally, vertically, diagonally or rotationally. Usually the result of impaired function of the extraocular muscles (EOMs)
- causes: EOMs may have mechanical problems, disorders of the neuromuscular junction, disorders of the cranial nerves (III, IV, and VI) and occasionally disorders of the supranuclear oculomotor pathways
CN 7 - function
- motor (somatic): muscles of facial expression
- motor (parasym): control of salivary glands (submandibular and sublingual) and lacrimal glands
- sensory: taste fibres to anterior 2/3 of tongue
CN 7 palsy
- UMN lesion: contralateral weakness of lower part of face (forehead spared) eg STROKE
- LMN lesion: ipsilateral weakness of all facial muscles (forehead included). Angle of mouth falls and taste is frequently impaired eg BELL’S PALSY
- DDx: Bell’s palsy vs Stroke, Ramsay hunt syndrome (herpes zoster infection of geniculate ganglion), trauma, tumour eg facial neuromas
Bell’s palsy - definition
Acute unilateral peripheral facial nerve palsy in patients for whom physical examination and history are otherwise unremarkable, consisting of deficits affecting all facial zones equally that fully evolve within 72 hours
- reactivation of herpes simplex virus type 1 (HSV-1) within the geniculate ganglion results in destruction of ganglion cells and infection of Schwann cells, leading to demyelination and neural inflammation
(ramsay hunt is reactivation of vzv - and presents with facial nerve palsy, auricular pain, vesicular rash around the ear)
Bell’s palsy - clinical features
single episode (recurrence very rare)
unilateral
absence of constitutional symptoms eg fever, myalgia
involvement of all nerve branches (LMN lesion)
keratoconjunctivitis sicca (dry eye - parasym dysfunction to lacrimal gland)
pain = post-auricular pain and mild/moderate otalgia
synkinesis = involuntary synchronous movement of a facial region concomitant with reflex or voluntary movement in another facial region (chronic stage only)
Bell’s palsy - investigations
- Clinical Dx
- rule out stroke, pts often concerned
- can do EMG which shows decreased amplitude of muscle action potential on affected side
Bell’s palsy - management
- oral prednisolone
- eye protection for dry eye: wear glasses and give artificial tears + ophthalmic lubricant at night
- severe: aciclovir and/or surgical decompression
CN 8 palsy
- cochlear lesion: deafness and tinnitus
- vestibular lesion: vertigo, N+V. Causes: BPPV, meniere’s, alcohol, AEDs, migraine, MS, mumps, some Abx (gentamicin)
Conductive vs Sensorineural hearing loss
- conductive = blockage in the outer or middle ear preventing conduction of sound into the inner ear
DDx - foreign bodies, otitis externa, excess wax, perforated tympanic membrane - Sensorineural = resides in the sensory (hair cells) or neural portion i.e the inner ear.
DDx - ageing, genetics eg otosclerosis, loud noises, trauma, tumours, meniere’s
CN 9 - function
- sensory: taste to posterior 1/3 of tongue, afferent arm of gag reflex, carotid sinus (baroreceptors for BP) and carotid body (chemoreceptors - partial pressure of O2 and CO2, and pH)
- motor (somatic): pharyngeal muscles (swallowing)
- motor (parasym): parotid glands (salivation)
CN 9 palsy
- unilateral lesion causes diminished sensation on same side of pharynx.
- together with CN X lesion causes absent gag reflex, palatal weakness, reduced sensation and choking
- can result in loss of taste sensation to the posterior one third of the tongue, and impaired swallowing
- usually brainstem lesion eg stroke
What is pseudobulbar palsy?
- Bilateral lesions of the corticobulbar tracts. Symptoms include slow, dysarthric speech and variable dysphagia, spasticity of the tongue and musculature of the pharynx and larynx, while pharyngeal and palatal reflexes remain intact
- can be caused by trauma, neurological disease (Parkinson’s, multiple sclerosis, etc), metabolic, vascular, tumor, nerve palsies (trigeminal, facial, vagus, hypoglossal and spinal accessory nerves)
CN 11 palsy
- results in weakness of sternocleidomastoid (rotation of head to opposite side) and trapezius (shoulder shrugging)
- injury can occur during neck surgery, trauma or spontaneously
