5 - The Eye Flashcards
What are the two layers of the retina?
Pigmented Layer: retinal pigment epithelium. contains lots of melanin to ensure no excessive refraction of light rays and stop glare. (albinos don’t have melanin so this is why they struggle with normal light being too bright)
Neural Layer: contains photoreceptors (rods and cones) and horizontal cells which do lateral inhibition that stops neighbouring cells to the highest intensity cells from detecting the light. Bipolar cells are found between photoreceptors and ganglion cells
What is the blood supply to the retina?
- Comes from the choroid later from the central retinal artery
- Can get occluded due to atherosclerosis of the ICA
How can we image the retina?
Optical Coherence Tomography
What are some pathological processes that can affect the retina?
- Hypertension and diabetes can cause retinopathy
- Amaurosis Fugax in a stroke
- Macula degeneration
- Papilloedema in raised ICP
What is Amaurosis Fugax?
- Transient monocular blindness
- CURTAIN COMING DOWN OVER VISION - THINK STROKE
- Due to blockage in ICA or retinal artery
What are the components of the visual pathway and draw a diagram displaying the pathway?
Optic Nerve –> Optic Chiasm –> Optic Tracts –> Lateral Geniculate Nucleus in the Thalamus –> Radiations (superior and inferior) –> Occipital Lobe (primary visual cortex)
What fibres are in the left superior radiation (Baum’s loop)?
- Left superior temporal fibre and right superior nasal fibre
- If in upper quadrant, in superior radiation and vice versa with lower quadrant
Where are each of the radiations?
Superior: parietal lobe
Inferior: temporal lobe
What fibres make up the optic nerve and what part of the visual field does each fibre contribute to?
- Nasal retinal fibres responsible for temporal visual field
- Temporal retinal fibres responsible for nasal visual field
- Due to the fact light travels in straight lines
- 4 fibres make up the nerve (superior and inferior)
What is the difference between temporal and nasal fibres?
Where are the optic tracts?
After the optic chiasm up to the lateral geniculate nucleus
What is the best way to figure out where a lesion is in the visual pathway?
Visual field defects are names on area of visual loss not the lesion so draw the diagram out!!!
What is monocular blindness and where is the lesion in this case?
- Complete vision loss in on eye
- Can be due to retinoblastoma, meningiomas or blockage in central retinal artery (stroke)
- Lesion in optic nerve so ipsilateral temporal and nasal fibres lost
What type of visual field loss is this, and where is the lesion in the visual pathway?
Bitemporal Hemianopia
- Lesion in optic chiasm
- Loss of nasal fibres on both sides so you lose your temporal visual field, causing tunnel vision
- Pituitary gland tumour or anterior communicating artery aneurysm
What type of visual field loss is this, and where is the lesion in the visual pathway?
Left homonymous hemianopia
- Lesion in the right optic tract. Contralateral lesion to vision loss, use nasal field loss to guide side of lesion as temporal fibres don’t decussate
- Could be neoplasia or trauma
What part of the visual field do the optic radiations supply?
- Inferior radiations supply superior visual vield and vice versa
What type of visual field loss is this and where is the lesion?
- Left homonymous inferior quadrantanopia
- Lesion in right superior optic radiation (parietal lobe)
What type of visual field loss is this and where is the lesion?
Right superior homonymous quandrantanopia
- Left inferior radiation lesion
Apart from a lesion in the optic tract, what other pathology could result in a homonymous hemianopia?
A stroke affecting the MCA could knock out both radiations ipsilaterally, have to look at the clinical history to distinguish between stroke and damage to a tract
What are these visual field defects showing and why does this occur?
- Macular Sparing: still good central vision
- Occipital lobe has dual blood supply from PCA and MCA.
- If there is a stroke of the PCA, most of the occipital lobe is lost but the MCA supplies the occipital pole(macula) so vision is spared centally
Draw the nerve pathway involved in the light reflex.
- Light stimulates CNII and goes on to synaps with Edinger Westphal nucleus in pre tectal area
- Parasympathetic respone through efferent CNIII
- Direct and consensual pupil constriction via constricter pupillae
Draw the nerve pathway involved in the accomodation reflex.
3 C’s:
- Convergence (medial rectus)
- Constriction (constrictor pupillae)
- Convexivity so fat lens (cilliary muscles)
Cerebral cortex involved as well as midbrain as some image interpretation unlike in the light reflex. Occipital lobe sends fibres back up to midbrain
Label the cranial nerves on this cadaveric specimen.
What is a scotoma?
A localised defect in the retina can cause a small patch of visual loss which can form an aura in the vision (google)
What is the MRC power scale?
What are some signs of cerebellar disease/lesions?
Why do patient’s with albinism struggle to see well in bright light?
They have less pigment in their lens so more light gets through and onto the retina. Retinal epithelium has more damage as no pigment to absorb free radicals and excess light rays so glare
Can also get glare in cataracts, presbyopia, short/long sighted, astigmatism
Label the layers of the retina on this OCT.
- Vitreous is top black bit
- Choroid is bottom black bit
What are some differentials for this case and where is the most likely site of the lesion?
- Abscess from meningitis
- Subarachnoid haemorraghe
- Left superior radiation so abscess in the parietal lobe (TIPS)
What is the doll’s eye reflex?
- Way to test the oculocephalic reflex as one test for brain stem death. Show when loss of connections between vestibular and occulomotor
- Move head to the side quickly and eyes should attempt to go back to midline position if connections in tact.
What is the medial longitudinal fasiculus?
- Midline area in the brain stem that connects occulomotor, trochlear and abducens nuclei for conjugate eye movements
- Also a connection with the vestibular nuclei to give information about the position of the head. Vestibular also give out descending fibres to muscles of the head and neck
- Stops us getting diplopia
What is internuclear ophthalmoplegia and the main cause of this?
- Often seen in MS when a new plaque from demyelination occurs in the brain stem. DIPLOPIA
- Loss of conjugate lateral eye movements with affected eye struggling to adduct slowly or absent
- When an attempt is made to gaze contralaterally (relative to the affected eye), the affected eye adducts minimally, if at all. The contralateral eye abducts, however with nystagmus
A patient has a cerebral aqueduct tumour in the midbrain. How will this affect the accomodation reflex and eye movements?
- Tumour will compress EDW and occulomotor nerve
- EDW damage will ean lack of pupil constriction and ciliary muscle contraction
- Damage to occulomotor nerve means eye held in down and out position
What are thalamoperforator arteries a branch of?
PCA!!!!
