4.2.2 Cranial Nerves I & II Flashcards
What is different about CNI and II in comparison to all other cranial nerves?
Extension of the forebrain, has meninges surrounding it
What is the function of CNI- Olefactory?
Special sense, smell
How would you test CNI?
Not routinely tested, if tested, one nostril at a time
What is absence or reduced sense of smell called?
Anosmia- Absence
Hyposmia
Also will impair taste perception
How can CNI become damaged?
- Head/facial injury- impact (shearing olfactory neurones during passage through cribriform formaina)
- Anterior cranial fossa tumours (compression of olfactory bulb or tract)
- Associations with Parkinsons and Alzheimers (this is due to sense of smell being percieved by the temoral lobe, so effects on the brain affect sense of smell)
What is the commonest cause of anosmia?
Common cold/ URTI
What is pathway of the olefactory nerves?
After olfactory tract ends up in the temporal lobe
What is the function of CNII, optic?
- Special sense, vision
- Pupillary size and response to light
- Visual acuity and fields
Why do you have light sensitivity in meningitis?
The optic nerve is an extension of the forebrain, therefore carries extension of meningeal layers, light increases the activity of the optic nerves, thus irritating meninges more
Why does a raised intracranial pressure cause issues with vision?
CNII carries extension of meningeal layers, therefore raised ICP will affect the optic nerve leading to vision issues
Model Answer:
The optic nerve is an extension of the forebrain, so it carries an extension of meninges with it. Increased intracranial pressure increases pressure within the subarachnoid space and as this extends the length of the optic nerve the nerve becomes externally compressed by the rise in ICP
This compression of the optic nerves causes disruption of optic nerve function hence visual symptoms as axoplasmic flow within nerve fibres forming the optic nerve is interrupted by external compression.
What vision problems can occur with a pituitary adenoma/mass?
Compresses the optic chiasm leading to bitemporal hemianopia (tunnel vision)
Would have a loss of peripheral visual fields
Optic chiasm is in close proximity to the pituitary gland
How can you assess CNII?
- Pupillary size and response to light (CNII forms the afferent limb of pupillary light reflex)
- Visual acuity (snellen chart)
- Visual fields
- Opthalmoscopy- can directly visualise anterior part of optic nerve
What will patients likely experience in a CNII lesion?
Blurred vision or complete absence of vision
What clinical examination findings would you have for a CNII lesion?
- Poor visual acuity (snellen chart)
- Abnormalities in pupil size and response to light (may be very subtle)
- Evidence of pathology involving the optic nerve on opthalmoscopy (can affect the eyeball too)
What are some diseases of the optic nerve?
- Optic neuritis (20% of MS patients have history of this)
- Anterior ischaemic optic neuropathy (anterior part of optic nerve is starved of oxygen)
What are some appearance changes of CNII on opthalmoscopy?
- Papilloedema (swollen optic disc due to raised ICP)
- Pale optic disc (following neuritis?)
What is the pathway of the optic nerve?
What does the optic nerve do?
Carries sensory fibres from one eye (retina)
