4- Ophthalmology (visual fields defects) Flashcards
Anatomy of the central visual pathway
Comprised of the:
- Optic nerve (CNII)
- Optic chiasm
- Optic tracts
- Optic radiations
- Visual cortex of the occipital lobe
visual fields
Relate to the peripheral vision (temporal and nasal). Each eye has it own set of visual fields.
- These overlap to form binocular vision.
Defects are names based on the area of
visual loss rather than the site of lesion
- Monocular blindness
- Bitemporal hemianopia
- Homonomous hemianopia
If we want to detect something in the temporal visual field….
light will travel through the pupil straight to the** nasal retinal fibres** (temporal visual field detected by nasal retinal fibres)
If we want to detect something in the nasal visual field…
light will travel through the pupil straight to the temporal retinal fibres (nasal visual field detected by temporal retinal fibres)
- The optic nerve (CN II)
- Split into diff divisions (different fibres of the retina)
- Temporal (lateral)- orange
- Nasal (medial)- green
- Also have up and down
- The optic chiasm
- Nasal fibres decussate
- Temporal fibres remain ipsilateral
Chiasm= where the crossover happens
- The optic tracts
- From optic chiasm to lateral geniculate nucleus (LGN)
- Contains temporal fibres from the ipislateral side
- Contains nasal fibres from the contralateral side
- Optic radiations
- From LGN to primary visual cortex (x2 ) in the occipital lobe (x2 lobes)
2 routes to the occipital lobe
-
Superior route via the parietal (superior optic radiations)
- Continuation of superior quadrant fibres (temporal and nasal)
- ‘Baums loop’
2. Inferior routevia the temporal (inferior optic radiations)
- Continuation of inferior quadrant fibres (temporal and nasal)
- “meyes loop’
summary of optic tract
Visual field defects are due to pathology in the:
Optic nerve
- Monocular blindness
Optic chiasm
- Bitemporal hemianopia
Optic tract
- Homonymous hemianopia
Optic radiation
- Quadrantanopia
- Both affected: homonymous hemianopia due to stroke
Visual cortex
- Macula sparing stroke
Monocular blindness
- Temporal and nasal fibres on the ipsilateral side affected
- Therefore the nasal and temporal visual field are lost on the ipsilateral side
Bitemporal hemianopia
- Nasal fibres on both sides affected
- Temporal visual field loss on both sides
- ‘tunnel vision’
- Cause e.g. pituitary adenoma
Homonomous hemianopia
- Lesion of the optic tract on the right hand side
- Left nasal retinal fibres (contralateral) and right temporal retinal fibres (ipsilateral) affected
- Left temporal (contralateral) visual field lost and right nasal (ipsilateral) field loss
- ‘left homonomous hemianopia’- even though lesion is on the right- due to decussation
o Name the visual defect on visual loss not lesion
i.e. homo- loss of both left or loss of both right
Optic radiation lesion
- Superior visual fields are detected by inferior retinal fibres
- Inferior visual fields are detected by superior retinal fibres
Quadrantanopia’s
- Affects left/right superior quadrant fibres and inferior quadrant fibres
Homonomous inferior quadrantanopia (left)
Lesion on right superior optic radiation (parietal lobe)
- Superior ipsilateral temporal fibre affected
o Loss of inferior nasal visual field
- Superior contralateral nasal fibre affected
o Loss of inferior temporal visual fields
Homonomous superior quadrantanopia (left)
Homonomous superior quadrantanopia (right)
- Inferior temporal fibres on ipsilateral side affected
o Loss of superior nasal visual field
- Inferior nasal fibre on contralateral side is affected
o Loss of superior temporal visual field
Homonomous superior quadrantanopia (left)
Homonomous superior quadrantanopia (right)
- Inferior temporal fibres on ipsilateral side affected
o Loss of superior nasal visual field
- Inferior nasal fibre on contralateral side is affected
o Loss of superior temporal visual field
What if superior and inferior radiations are affected? E.g. stroke
- Superior and inferior ipsilateral temporal fibres affected
- Superior and inferior nasal fibres contralateral affected
- Homonomous hemianopia
macula sparing
Macula sparing
Occipital lobe has dual blood supply
- Middle Cerebral Artery (occipital pole)
- Posterior Cerebral Artery
In stroke affecting the posterior cerebral artery…
- Most occipital lobe will be lost
- However middle cerebral supplies the occipital pole (represents the macula)
- Therefore macular function (central vision) will be saved
pupillary light reflex
Afferent (sensory arm)- optic nerve
Efferent arm (parasympathetic)- oculomotor
- Shine light in eye (e.g. left)
- Optic nerve senses light
- Optic nerve reaches the chiasm and becomes the optic tract
- When it is in the brainstem the optic tract transmits info to the Edinger-Westphal nucleus on both sides of the nucleus
- Parasympathetics will leave the Edinger Westphal nucleus carrying this info on both oculomotor nerves CN III to both eyes
o Both pupils should constrict (sphincter pupillae) in exactly the same time and amount
RAPD
Relative Afferent Pupillary Defect (RAPD, Marcus Gunn Pupil)
An RAPD is a defect in the direct response. It is due to damage in optic nerve or severe retinal disease.
It is important to be able to differentiate whether a patient is complaining of decreased vision from an ocular problem such as cataract or from a defect of the optic nerve. If an optic nerve lesion is present the affected pupil will not constrict to light when light is shone in the that pupil during the swinging flashlight test. However, it will constrict if light is shone in the other eye (consensual response). The swinging flashlight test is helpful in separating these two etiologies as only patients with optic nerve damage will have a positive RAPD.
Swinging Flashlight Test:
Swing a light back and forth in front of the two pupils and compare the reaction to stimulation in both eyes.
When light reaches a pupil there should be a normal direct and consensual response.
An RAPD is diagnosed by observing paradoxical dilatation when light is directly shone in the affected pupil after being shown in the healthy pupild to be from a pathologic process
Accommodation reflex
- Required for near vision
- 3 aspects (3 C’s)
o Convergence (medial rectus)
o Pupillary constriction (constrictor pupillae)
o Convexity (becomes thicker) of the lens to increase refractive power (ciliary muscle) - Cerebral cortex must be involved because its relating to image analysis
o Therefore the reflex follows the visual pathway via the lateral geniculate nucleus to the visual cortex
o Midbrain receives info from visual cortex
o Sends off information via cranial nerve 3 nuclei (MR) and the Edwinger Westphal nuclei (EWN) (pupillary constriction)
