Ocular anatomy and dysfunction Flashcards
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
4th nerve (trochlear) palsy - treatment
In children - treat any amblyopia and refractive error
Prisms may be useful in adapting the patient in preparation for surgery or to produce functional monopia
Surgery is used to treat Diplopia and in some cases for cosmetic reasons –>careful with adults with neurally compensated Diplopia
6th nerve (abducent) palsy - diplopia
Supplies the lateral rectus –> palsy causes a failure of abduction to the affected side –> causes horizontal Diplopia which is worse when looking to the affected side
May hold head turned to affected side to minimise diplopia
6th nerve (abducent) palsy - causes in children
May occur transiently in neonates or occur benignly 1-3 weeks post-viral
6th nerve (abducent) palsy - causes in adults
Hypertension/diabetes if >50yrs, MS, tumours, trauma, infections
Raised intracranial pressure (6th is particularly sensitive because of intracranial course)
6th nerve (abducent) palsy - treatment
Investigate for cause with MRI/blood tests and orthopic tests (Hess test to identify weak muscles)
Treat with prisms or surgery if does not resolve spontaneously or with treatment of cause
3rd nerve (oculomotor) - function
Supplies the superior, inferior and medial rectus and inferior oblique –> failure gives ‘tramp’s palsy’ as eye is ‘down and out’
Sympathetic to levator palpebrae superioris (failure>partial ptosis)
Parasympathetic to iris via cillary ganglion (failure causes fixed dilated pupil)
3rd nerve (oculomotor) palsy - causes
Diabetes and hypertension can cause a micro-vascular palsy which will spare the pupil (parasympathetic fibres are superficial and so spared from ischaemic/metabolic injuries)
Other causes–> trauma, neoplasm, migraine, giant cell arteritis
A painful unilateral 3rd nerve palsy with dilated pupil may be due to
Posterior communicating artery aneurysm
Requires an urgent neurological assessment/imaging
Optic nerve disease/swelling can cause
Reduced visual acuity
Visual field defects
Reduced colour vision/red desaturation
Loss of pupillary reflexes/RAPD
Unilateral Optic nerve swelling can be caused by
Central retinal vein or artery occlusion
Non-arteritic anterior ischaemic optic neuropathy (occlusion of short posterior cillary arteries) - Or - Arteritic anterior ischaemic optic neuropathy (temporal arteritis)
Papillitis or neuroretinitis
Papillitis or neuroretinitis
Inflammation of the optic nerve head, or nerve head and surrounding retina
Idiopathic
Bilateral Optic nerve swelling can be caused by
Raised intracranial pressure due to mass or blockage of CSF drainage –> will cause visible papilloedema on fundoscopy
Malignant hypertension —> rapidly rising hypertension, optic disc swelling indicates grade 4 hypertension
Optic disc drusen
Optic disc Drusen
Drusen are calcified deposits–> can occur at the optic disc or deeper
When they are behind the disc they can give the appearance or swelling –> can cause visual field defects, usually asymptomatic
No treatment is avaliable
Optic disc swelling - examination
Full ophthalmic, neurological and systemic history
Must rule out –> raised intracranial pressure, hypertension and giant cell arteritis
Idiopathic intracranial hypertension is associated with
Obesity Pregnancy Oral contraceptives Steroids Not Amiodarone
