2.2: optic disc and optic nerve

list abnormal findings you may see on fundoscope associated with conditions

1. bartonella neuroretinitis 2. diabetic papillopathy 3. optic disc coloboma

where is the optic disc?

3.5 mm (15 degrees) nasal to the fovea (fovea is temporal FT) -the fovea is always temporal to the optic nerve -thus, you can tell whether you are looking at the R or L eye by seeing where the fovea is

what are the different divisions of the optic nerve?

4 divisions: intra ocular, orbital, canalicular, cranial 1. intraocular-at the optic disc, in the eye 2. intraorbital-from posterior part of eyeball to optic canal 3. intracanalicular-inside optic canal of sphenoid 4. intracranial-travels inside skull superior to diaphragma sellae and the cavernous sinus, ultimately forming the optic chiasm. passing through subarachnoid space back to brain.

bartonella neuroretinitis

Bartonella Neuroretinitis is the bacteria that leads to cat scratch disease 1. swollen optic disc 2. exudate in macula with protein deposits

Bitemporal hemianopsia

bilateral loss of vision out of the temporal field of each eye due to compressive lesion from a pituitary tumor -pituitary adenomas extend into the extracellular space and push up on the optic chiasm -the crossing nasal retinal fibers are put under tension and can't work correctly

optic tract

continuation of crossed nasal and uncrossed temporal optic nerve fibers running back to the brain -contains ipsilateral eye temporal retinal fibers (nasal visual field) and contralateral eye nasal retinal fibers (temporal visual field) destined for the LGN -thus, the right optic tract carries information from the L visual field of each eye -the left optic tract carries information from the R visual field of each eye -wraps around the midbrain cerebral peduncle to reach LGN

what is a visual field test?

creates an inverted detailed map of areas where you can and can't see -areas where you can't see well (blind spot) appear very dark -the peripheral visual field is slightly darker than the central visual field -blind spot should be located 15 degrees temporal fixation to fovea (b/c the temporal visual field projects to the nasal retina--where the optic disc/blind spot is)

optic disc coloboma

developmental abnormality of the optic nerve -carved out area where there should be neural tissue but there's not -excavation in area of the eye b/c the optic nerve didn't close properly

what happens if the myelinated nerve fiber layer extends into the eyeball itself?

if the myelinated nerve fibers develop incorrectly, the myelinated nerve fiber layer can extend into the eyeball itself (you would see white myelin in the nerve fiber layer of the retina which is abnormal) -this is not a problem or pathological -white myelin will be visible on upon fundoscopic exam

if you have a lesion in the optic tract or anywhere behind the optic tract, the visual symptoms are all on the ___ side

if you have a lesion in the optic tract or anywhere behind the optic tract, the visual symptoms are all on the contralateral side visual field; homonymous defect

how does the optic tract reach the LGN?

it loops around the midbrain cerebral peduncle

diabetic papillopathy

much more rare than diabetic retinopathy 1. rarely occurs in diabetes 2. significant swelling of optic nerve 3. white cotton wool spots that represent infarcts of the retinal nerve fiber layer 4. hemorrhages in retinal nerve fiber layer

what designates nerve fibers as nasal vs temporal?

nasal nerve fibers = fibers that originate nasal to the fovea temporal nerve fibers=fibers that originate temporal to the fovea

what happens to the nasal retinal fibers and temporal retinal fibers at the optic chiasm?

nasal retinal fibers (fibers that originate nasal to the fovea in either eye): cross over and find the optic tract just posterior to the chiasm temporal retinal fibers: stay ipsilateral at the chiasm

optic disc

optic nerve head/disc: the region of the retina where axonal projections from the retinal ganglion cells converge to make up the optic nerve

what is the blindspot?

optic nerve head/disc: the region of the retina where axonal projections from the retinal ganglion cells converge to make up the optic nerve 1. physiologic blind spot exists temporal to fixation in the visual field because there are no photoreceptors in the optic disc 2. the optic nerve has no visual receptors and is just a pathway back to the brain

starting at the optic tract, all of the visual information from the ___ side of the world will be represented in the___ side of the brain

starting at the optic tract, all of the visual information from the contralateral side of the world will be represented in the ipsilateral side of the brain

which part of the visual field does each occipital lobe receive and why does this make sense?

the R occipital lobe gets the left visual field of each eye the L occipital lobe gets the right visual field of each eye -this makes sense b/c the right brain (via the R frontal cortex) moves the left side of the body

are the optic nerve fibers myelinated?

the axons become myelinated at the lamina cribrosa CT meshwork 1. the retinal ganglion cell axon nerve fibers in the eyeball itself are not myelinated 2. they become myelinated when they reach the lamina cribrosa CT meshwork 3. when they are myelinated, they can conduct signals faster

what usually causes problems at the optic chiasm?

the chiasm is most commonly affected by compressive lesions such as pituitary tumors -optic chiasm lies dorsal and just above the pituitary gland -this creates a bitemporal hemianopia: bilateral loss of vision out of the temporal field of each eye (b/c the temporal field projects back to the nasal retina and this info is conveyed via the nasal retinal fibers that are getting compressed at the optic chiasm)

how is the optic nerve formed?

the retinal ganglion cell axons converge at the optic disc and create the optic nerve

how are the retinal nerve fibers spread throughout the retina?

the retinal nerve fibers have striations that allow them to be spread throughout the nerve fiber layer

optic chiasm -what is it -what happens here -what is it part of -what is it located next to

the two optic nerves meet and there is a crossing of the nasal retinal fibers 1. the two optic nerves meet behind the eye 2. part of the diencephalon (which also includes hypothalamus) 3. creates part of the floor of the 3rd ventricle 4. lies dorsal and just above the pituitary gland