Ocular anatomy and dysfunction Flashcards
(39 cards)
Wilbrand’s knee
Where fibres from the inferior nasal retina of one eye loop back up into the other optic nerve after reaching the optic chiasm for up to 3mm before continuing down the optic tract.
It’s existence in normal people has been questioned
Visual pathway anatomy
Retina–> optic nerve –> join at the optic chiasm where the fibres from the nasal retina of each eye cross–> continue in optic tracts accounting for the contra-lateral half of visual space –> these terminate at the lateral geniculate body/nucleus –> optic radiations connect this to the primary visual cortex
General rules for localising visual field defects
Lesions anterior to the chiasm produces unilateral defects
Posterior to the chiasm –> contra-lateral homonymous defects
Lesions at the chiasm produce bitemporal defects
Generally more congruous the defect in each eye the further back the lesion
Total monocular blindness indicates a lesion where?
Unilateral optic nerve compression/lesions
Bitemporal homonymous hemianopia indicates a lesion where?
Midline sagittal chiasm lesion/compression - full coronal lesion of the chiasm would produce total blindness
Mostly neoplastic-> pituitary expansion->increasing compression
Initially incomplete and asymmetric, becoming more complete with increasing compression
Monocular Nasal hemianopia indicates a lesion where?
Unilateral perichiasmal lesion - damaging the temporal retina fibres of that eye
Homonymous hemianopia
Contralateral lesion of the optic tract - Or - lesion to the visual cortex (occipital lobe) –> posterior to the chiasm
Loss effecting the same half of the visual field in both eyes
The more symmetrical the defects in each eye the closer to the occipital lobe the lesion
Optic radiations
Visual information from the LGN to the primary visual cortex
Nasal retinal fibres decussate at the chiasm so that each tract and radiation deal with the contralateral half of visual space
Divide into Meyer’s loop (more vulnerable) and superior optic radiation (Baum’s loop)
Meyer’s loop
Half of the optic radiation on each side where fibres from the inferior contralateral retina travel laterally through the temporal lobe to the primary visual cortex
Carry information on superior contralateral visual space
Damage produces a homonymous contralateral superior quadrantanopia
Baum’s loop
The superior part of the optic radiation–> LGN up through the parietal in the retrolenticular limb of the internal capsule
Carry information from the contralateral superior retina on contralateral inferior visual space
Damage causes contralateral homonymous inferior quadrantanopia
Superior homonymous quadrantanopia indicates a lesion where?
Meyer’s loop contralaterally
Inferior homonymous quadrantanopia indicates a lesion where?
Baum’s loop contralaterally
Bitemporal superior quadrantanopia
Indicates a compression of chiasm –> often first sign of pituitary tumour
Defects can often be asymmetric and respect the vertical midline but not the horizontal -> progress to hemianopia as tumour grows
May produce eventual optic neuropathy
Central visual field defects
Most commonly due to AMD –> monocular
May also be - optic nerve disease –> bilateral, progressive and sub-acute, can be toxic/metabolic, nutritional or genetic
Usually have associated colour vision and acuity defect
Can also be due to trauma –> eg laser burn
Diplopia
Double vision –> occurs when the eyes become mis-aligned
Normal at extremes of vision
Can be vertical, horizontal or oblique
Causes of Diplopia
Physical displacement of the globe –> tumours, trauma or infection/inflammation (orbital cellulitis)
Cranial nerve palsies –> 3rd,4th and 6th (extra-ocular muscles)
Muscle fatigue –> myasthenia gravis/decompensation of squint
Thyroid eye disease
For Diplopia in children consider
Birth history
Any history of amblyopia and patching treatment
Any family history of strabismus
Amblyopia
loss of visual acuity due to problems in the visual cortex
Often occurs in children as the visual cortex is still developing, if one eye is used less due to illness or weakness the other is used more and becomes stronger
Treated by patching the good eye to force the other to catch up
Strabismus
A misalignment of one of the eyes from fixating straight ahead
Often congenital –>Will cause Diplopia acutely but when young the brain will adapt to misalignment allowing single vision
Can be surgically corrected but if done after childhood the brain is unable to adapt to new alignment
Abnormal head posture
Indicates possible new onset Diplopia
Often due to muscle problem limiting movement of one eye
This produces a Diplopia which can be corrected by changing head position
Position may indicate something about the nature of the problem
Monocular Diplopia
Double vision which persists when one eye is covered
Indicates a refractive error problem which is uneven across the lens/cornea - corneal scarring, cataract, untreated refractive error
Usually described as ‘ghosting’
Binocular Diplopia
Indicates the eyes are misaligned or become so in certain gaze positions
May be due to muscle palsies
Must investigate –> acuity, any abnormal head posture, eye movements and refraction
4th nerve (trochlear) palsy and Diplopia
Supplies the superior oblique –>rotates the eye down and in
Patients complain of Diplopia when reading or going down stairs
Diplopia may be vertical, diagonal or rarely torsional
May tilt head away from the lesion, and eye will deviate upwards as it moves toward the nose
4th nerve (trochlear) palsy - causes
May be congenital or acquired and bilateral or unilateral
Acquired may be due to diabetes or hypertension, or demyelination (MS)
Other causes : giant cell arteritis, tumours, aneurysms
Most common cranial nerve to be injured by trauma
