The Visual Pathway and Eye Muscles Flashcards
What is your visual field?
Everything you see with one eye including the periphery
How is visual field tested?
Confrontation test (outpatient screening) or automated perimetry
How are images of objects seen in field of vision?
Upside down and inverted
Left half of visual field seen on right part of eye and right half seen on left side
Where do all fibres of the eye pass through?
Through optic nerve into optic chiasma
What happens to the fibres at the optic chiasma?
Medial nasal fibres cross to opposite side
What fibres does the optic tract contain?
Fibres from lateral temporal half of ipsilateral eye and crossed over nasal fibres from contralateral eye
Corresponds to all fibres from opposite half of visual field
Where do the fibres from the optic nerve synapse?
At LGB - lateral geniculate body of the thalamus
What happens after lateral geniculate body?
Optic radiation passes behind internal capsule to reach primary visual cortex in occipital lobe - area 17
What happens is right optic nerve is damaged?
Blindness in one eye - right
What happens if optic chiasma is disrupted in the middle?
Bitemporal hemianopia - lateral half of eye on both side are blind
What happens if right optic tract is damaged?
Homonymous hemianopia - left visual field is affected
What happens if optic radiation is damaged?
Contralateral homonymous hemianopia - if right optic tract then left side of both eyes have blindness
When do recti muscles arise from?
Apex of orbit from annular fibrous ring
Where do recti and oblique muscles insert?
Recti - sclera anteriorly
Obliques - posteriorly
What is the origin of superior and inferior oblique muscles?
SO - lesser wing of sphenoid
IO - medial part of orbital floor
What are the terms for eye movement?
Elevation, depression, levoversion, dextroversion, dextrodepression, dextroelevation, levodepression and levoelevation
Intorsion and extorsion
What does RADSIN stand for?
Recti Adduct and Superior Intortors
SR and IR adduct and SO and IO abduct
SR and SO intorsion
What are the actions of individual EOMs influenced by?
Muscles attached along orbital axis not optical so pull eyeball on angle
Oblique muscles are attached at posterior part of sclera - pull posterior part up and down and anterior moves in opposite direction
What happens to the obliques when eye is adducted?
Elevate/ Depress
What happens to the superior and inferior obliques when eye is abducted?
Elevate/ depress
What is strabismus?
Squint which is misalignment of the eyes
Can be esotropia (convergent) or exotropia (divergent)
What are the functional consequences of a squint?
Amblyopia and diplopia
What is amblyopia?
Lazy eye - brain supresses image of one eye leading to poor vision in eye without pathology - correctable in early years by eye patch to strengthen
What is diplopia?
Double vision
Occurs in squints due to nerve palsies
Describe the intrinsic muscles of the eye
Ciliaris muscle in ciliary body
Constrictor pupillae in iris at pupillary border
Dilator pupillae radially running muscle in iris
Ciliaris and constrictor are parasympathetic - CNIII
Dilator is sympathetic
What does increased illumination cause?
Parasympathetic - both pupils constrict
What does decreased illumination cause?
Sympathetic - both pupils dilate
Describe the pupillary reflex
In dimly lit room - pen torch in front of one eye to check both are constricting
Swing light to both sides and both should remain constricted
Describe the pathway of the light reflex - afferent limb
Fibres to activate pupillary reflex don’t go to LGB but leave optic tract to go to midbrain where IIIn nucleus is situated
Part of IIIn nucleus is Edinger-Westphal nucleus for parasympathetic fibres
Go to EWN of both sides
Describe the pathway of the light reflex - efferent limb
From EWN - preganglionic parasympathetic fibres pass through IIIn into orbit
Then parasympathetic fibres synapse in ciliary region
Postganglionic fibres go through short ciliary nerves to constrictor pupillae
What is anisocoria?
Pupils are different sizes - ex. Horner’s syndrome or result from injury
Pupils can look normal but act abnormally to light
What are common causes of absent or abnormal pupillary reflex?
Diseases of the retina - detachment or degeneration
Diseases of optic nerve - optic neuritis
Diseases of III CN - efferent limb
What do you check is patient has IIIn palsy?
Check pupillary reflex - if absent then suspect a cerebral artery aneurysm which is an emergency
Medical cause like diabetes usually has no effect on parasympathetic fibres
Describe Horner’s Syndrome
Anisocoria due to damage of sympathetic innervation of the pupil
Symptoms - ptosis on affected side, miosis, anhidrosis (loss of sweating on affected side)
What can be causes of Horner’s syndrome?
Disruption - thoracolumbar outflow, sympathetic chain and cervical ganglion, postganglionic sympathetic fibres travel along blood vessels in head and neck
