Clinical Applications: Cranial Nerves Flashcards
What is the pathway of the olfactory nerve?
- The primary olfactory nerve cell bodies send central processes to synapse on secondary olfactory cells in the olfactory bulb.
- Seconday sensory axons then form the olfactory tract.
NOTE: The peripheral processes of the primary olfactory neurons are in the olfactory epithelium act as sensory receptors.
* Signals do not pass through thalamus
Where along the olfactory pathway can pathology occur?
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The receptor
- Temporary loss of smell results, most commonly from swelling and congestion
- In rare cases, tumors of the epithelium can arise
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The primary olfactory axon
- May result in permanent unilateral or bilateral anosmia and is frequently seen with fractures through the cribiform plate
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The central pathway
- Includes olfactory bulb, olfactory tract, and central projections
- May be compressed from tumors, aneuryms of the anterior cerebral artery, and infiltrating tumors on frontal lobe
- May result in unilateral or bilateral loss of smell
What is the pathway of the optic nerve?
CN II extends from the optic disc to the optic chiasm and continues as the optic tract to the lateral geniculate nucleus, pretectal nuclei, and superior colliculus.
*Optic nerve axons leave the eye, and blood verssels enter the eyer at the optic disc.
The retina can be divided into what two divisions?
An imaginary vertical line through the foveola divides the retina into the nasal hemiretina and the temporal hemiretina.
How is it possible for the optic nerve to be affected by MS?
The axons in the optic nerve, chiasm, and tract, like other central nervous system axons, are myelinated by oligodendrocytes. MS is a disease of oligodendrocytes.
What lesions of the visual pathway result in visual loss?
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Anterior to the chiasm
- Damge to the retina or optic nerve results in visual loss in the affected eye only (monocular visual loss)
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At the chiasm
- Damage to the optic chiasm usually results in loss of vision from both eyes
- Damage to the midline of the chiasm results in loss of peripheral fields in both eyes (bitemporal hemianopia)
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Posterior to the chiasm
- Damage tot he optic tracts, lateral geniculate body, optic radiations, or visual cortices result in visual loss from both eyes within the contralateral visual field (homonymous hemianopia)
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Lesions of Meyer’s loop
- Results in homonymous quadrantanopia (loss of the contralateral upper visual field in both eyes
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Partial loss of axons along the visual pathway
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Scotomas
- Blind spots
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Scotomas
What is macula sparing?
The sparing of the central vision within the hemianopic hemifield
*Macular sparing signifies sparing of the posterior pole of the occipital lobe
What four procedures are used to exam the optic nerve?
- Measurement of visual acuity (test macular function)
- Testing of visual fields
- Testing of the pupillary light reflex
- Fundoscopy
Lesions of the parietal Radiations/upper lip of calcarine fissure can result in…
Contralateral homonymous inferior quadrantanopsia
Lesions of the temporal Radiations/lower lip of calcarine fissure can result in…
Contralateral homonymous superior quadrantanopsia
What can cause lesions of the optic nerve or components of the optic nerve?
- Optic Neuritis- inflammation that damages the optic nerve
- Multiple Sclerosis- disease of oligodendrocytes
- Neuromyelitis Optica- inflammation and demyelination of the optic nerve (optic neuritis) and the spinal cord (myelitis)
- Amaurosis Fugax – a painless temporary loss of vision in one or both eyes (occlusion of ophthalmic artery)
- Papilledema- Increased Intracranial pressure
What disease is presented here?
Nothing. This is a normal fundoscopy
What disease is presented here?
Papilledema
*The lesion is in the optic disk
What disease is presented here?
Atrophy
Causes of ONA include heredity, trauma (including stroke), a tumor, decrease in oxygen or blood supply, infections, or disorders.
Optic neuritis takes 4-6 weeks for nerves to atrophy.
What disease is presented here?
Central Retinal Artery Occlusion
Central retinal artery occlusion occurs when the central retinal artery becomes blocked, usually due to an embolus. It causes sudden, painless, unilateral, and usually severe vision loss.
*You don’t really see the disc here
What is the pathway of the oculomotor nerve?
- Axons of the oculomotor nucleus and the Edinger- Westphal nuclei course ventrally in teh midbrain to form the oculomotor nerve
- Ocumotor nerve emerges from the interpeduncular fossa on the ventral aspect of the midbrain
- After passing between the posterior cerebral and the superior cerebellar arteries, the nerve course anteriorly
- It pierces the dura and enters the cavernous sinus an then continues toward the superior orbital fissure
Which muscles are supplied by the oculomotor nerve?
- Medial rectus muscle: Adduction
- Superior rectus muscle: Elevation
- Inferior rectus muscle: depression
- Inferior oblique muscle: Excyclotorsion
How does the posterior communicating artery aneurysm cause a lesion of the oculomotor nerve?
CN III passes close to posterior communicating artery. An aneurysm of the posterior communicating artery can compress CN III, resulting in a lower motor neuron lesion
A lesion of CN III can result in a fixed, dilated pupil. Why?
There is damaged to the Edinger- Westphal nuclei which receives bilateral innervation from the optic nerve.
Where along the course of CN II can damage occur?
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Nucleus of CN III (rare)
- Due to trauma, ischemia, or demyelination within the midbrain
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Peripheral axons
- Damage to axons in the subarachnoid space
- Diabetes
- Due to anerysms (usually of the posterior communicating artery and sometimes int he basilar artery)
- Due to uncus (of the temoral lobe) herniation
- Due to raised intracranial pressure
- Compression of axons in the cavernous sinus
- Tumors
- Inflammation
- Damage could occur at the superior orbital fissure
- Damage to axons in the subarachnoid space
Oculomotor divides into __________ and __________divisions
superior; inferior
What is the pathway of cranial nerve IV?
- Axons arising from the trochler nucleus course dorsally around the periaqueductal grey matter and cerebral aqueduct, and cross the midline.
- The crossed axons emerge from the dorsal aspect of the midbrain just caudal to the inferior colliculus to form cranial nerve IV.
- The nerve curves ventrally around the cerebral peduncle to pass between the posterior cerebral and superior cerebellar arteries.
- Cranial nerve IV runs anteriorly to pierce the dura at the angle between the free and attached borders of the tentorium cerebelli
- It enters the cavernous sinus, within the cavernous sinus, the trochlear nerve is situated between cranial nerves III and V1 and lateral to the internal carotid artery.
- It leaves the superior orbital fissure and course close to the orbit toward the superior oblique muscle