Flashcards in Vision Deck (33):
How is light sensed by the rods and cones in the retina?
1. Ganglion cell of optic nerve senses light
2. APs transmitted distally to amacrine cells
3. APs transmitted to bipolar cells
4. APs travel horizontally to horizontal cells
5. APs reach rods and cones
6. APs return proximally to the ganglion cell and travel down the optic nerve
Describe the anatomy of the retina.
Inner retinal layers and outer retinal layers
Fovea = part of retina which has the smallest diameter of retinal layers (cones in highest density)
What parts of the eye does light have to pass through to reach the photoreceptors?
Light focused by cornea and lens
Light moves through the vitreous humour
Light passes through layers of the retina
Light reaches photoreceptors
What is the result of foveal hypoplasia? In what condition does this occur?
Common in albinism
Contrast the rods and cones.
- not present in central retina/fovea
- adapt to darkness
- many rods converge into single bipolar cell
- concentrated in central retina/fovea
- high acuity
- day vision
- colour vision (red, blue, and green cones)
Which cells make up the interneurones combining signals from photoreceptors?
What are the different types of ganglion cells?
Magnocellular (similar to rods):
- in dorsal stream (parietal lobe)
- high luminance contrast
- sensitive to motion
- responsible for object location and motion
Parvocellular (similar to cones):
- in ventral stream (temporal lobe)
- colour contrast
- high fine detail
- responsible for object recognition and colour
What is amblyopia? What is the most common type? How is it treated?
Poor sight not due to any detectable disease of the eyeball or visual system ("lazy eye")
Commonest type is amblyopia ex anopsia = factors (e.g. squint, cataract, refractive differences) impair the normal use of the eye during childhood by preventing formation of clear image on the retina, causing cortical visual impairment
Treat with glasses and a patch over the lazy eye (poor compliance)
What is strabismus? What is the consequence of untreated childhood strabismus?
Abnormal alignment of two eyes
One eye can fixate on objects but one eye cannot
If this occurs during childhood, the brain no longer recognises signals from abnormal eye (no diplopia)
note: easier to detect by covering one eye at a time whilst doing eye movements (cover test) - removes compensation
What is anisometropia?
Refractive differences between two eyes
What are some causes of deprivation of vision which may cause the brain to no longer recognise signals from the abnormal eye?
Describe the normal appearance of the retina on fundoscopy.
Fovea in centre surrounded by macula
Blind spot on NASAL side
- exit of ganglion cells from optic disc (no photoreceptors present)
Give some examples of abnormal findings in funduscopy.
Macula: age-related macular degeneration
- irregular with prominent rim = glaucoma causing papilloedema
- swollen optic nerve = optic neuritis (MS)
What is the visual field abnormality when there is a lesion in the right optic nerve prior to the optic chiasm? Give an example of a cause.
Right-sided total anopia
- optic neuritis (MS)
- anterior ischaemic optic neuropathy
- idiopathic intracranial hypertension
What is the visual field abnormality when there is a lesion in the optic chiasm? Give an example of a cause.
Bitemporal hemianopia (fibres from nasal hemiretinae cross over in optic chiasm)
- pituitary adenoma
- cyst of Rathke's pouch
What is the visual field abnormality that results from a retrochiasmal lesion in the right optic tract? Give an example of a cause.
Left-sided homonymous hemianopia
- nasal hemiretina fibres from left eye have crossed over in the optic chiasm (loss of temporal hemifield of left eye)
- temporal hemiretina fibres from right eye have remained ipsilateral (loss of nasal hemifield of right eye)
- infection e.g. encephalitis
- AV malformation
What is the visual field abnormality that results from a lesion in the right-sided optic radiation in the temporal lobe? Give an example of a cause.
Left-sided superior homonymous quadrantanopia
- temporal lobe receives vision from superior visual quadrants
- inflammatory process
- infection e.g. encephalitis
What is the visual field abnormality that results from a lesion in the right-sided optic radiation in the parietal lobe? Give an example of a cause.
Left-sided inferior homonymous quadrantanopia
- parietal lobe receives vision from inferior quadrants
- infection e.g. encephalitis
What is the visual field abnormality that results from a lesion in the right visual cortex?
Right-sided central loss
Loss of vision is small, central, and congruent between the eyes (stricter arrangement of fibres)
Give an example of a cause of a pupil which has accommodation but is not reactive. What is the pathophysiology?
Argyll-Robertson pupil (bilateral ptosis, small irregular pupils, constrict only on convergence)
- autonomic neuropathy e.g. diabetes
- syphilis (lesion in ciliary ganglion)
Lesion damaging fibres Edinger-Westphal nucleus (parasympathetic) responsible for reaction to light but not accommodation
Give an example of a cause of a pupil with absent accommodation but is reactive. What is the pathophysiology?
Give examples of causes of eyes with absent direct pupillary reflex.
Optic nerve problem (ABSENT consensual light reflex)
- optic neuritis
- angle closure glaucoma (increased intraocular pressure)
Oculomotor nerve problem (PRESENT consensual light reflex)
Give some examples of causes of a loss of central vision.
Retrobulbar neuritis (MS)
What are the features of oculomotor nerve palsy?
Partial ptosis (levator palpebrae superioris lost)
"Down and out" pupil (loss of all extraocular muscles except superior oblique and lateral rectus)
- medial rectus loss ---> pupil "out"
- inferior oblique + superior rectus + inferior rectus ---> pupil "down"
Pupil dilatation (loss of Edinger-Westphal nucleus fibres)
note: pupil sparing if parasympathetic fibres are affected only e.g.
- aneurysm of posterior communicating artery
- cavernous sinus thrombosis
OR if the lesion is post division at ciliary ganglion
Diplopia (horizontal and vertical)
note: oculomotor nerve in anterior midbrain
- vascular e.g. diabetes, hypertension, collagen vascular disease
What are the features of trochlear nerve palsy?
Diplopia when looking down and in e.g. walking down stairs (loss of superior oblique - loss of intorsion on looking down)
Tend to tilt head to side opposite affected eye to compensate
Affected pupil may be slightly elevated whilst adducted
note: trochlear nerve has long course including the posterior midbrain (vulnerable in trauma e.g. whiplash)
- vascular e.g. diabetes, hypertension, giant cell arteritis
What are the features of abducens nerve palsy?
Loss of abduction on affected side (loss of lateral rectus)
Horizontal diplopia on lateral gaze
note: abducens nerve in pons
- vascular e.g. diabetes, hypertension, aneurysm, giant cell arteritis, collagen vascular disease
How can you assess which image is abnormal in diplopia on lateral gaze?
Cover both eyes - the image that is seen furthest laterally is in the abnormal eye
Reminder: what is the function of the different extraocular muscles?
- in ABduction = elevation
- in ADduction = intorsion
LATERAL RECTUS = ABduction
- in ABduction = depression
- in ADduction = extorsion
- in ABduction = intorsion
- in ADduction = depression
MEDIAL RECTUS = ADduction
- in ABduction = extorsion
- in ADduction = elevation
Outline the steps of the visual pathway.
- nasal hemiretinae receive light from temporal hemifield
- temporal hemiretinae receive light from nasal hemifield
2. Optic nerve
3. Optic chiasm
- fibres from nasal hemiretinae cross
- fibres from temporal hemiretinae do not cross
4. Optic tract (CNII is a brain tract; not a peripheral nerve - encased in meninges and based in CSF)
- right optic tract receives light from left hemifield of both eyes
- left optic tract receives light from right hemifield of both eyes §
5. Lateral geniculate body (part of thalamus - receives feedback from reticular formation, brainstem, and cortex)
6. Optic radiation/geniculocalcarine tract
- ventral part (inferior temporal lobe)
- dorsal part (posterior parietal lobe)
7. Primary visual cortex (occipital lobe)
- higher proportion devoted to central vision (higher density of photoreceptors); visual cortex compares size of surrounding objects to central object
- image is inverted
Give examples of causes of nystagmus.
- oculocutaneous albinism
- poor vision in early infancy (e.g. cataracts, optic atrophy, bilateral macular disease)
- retinal diseases (e.g. achromatopsia, congenital stationary night blindness)
- congenital squint syndrome
- MS ---> oscillopsia
- cerebellar dysfunction
What is the treatment of cataracts?
1. Liquefaction of hard lens
2. Aspiration of cataract fragments
3. Replacement of lens
What is the pathophysiology of myopia?
Optical power of the eye is too strong for corresponding axial length ---> light rays converge too strongly ---> visual acuity decreases as objects are located further away
Far point is located in front of the eye