Case 3 - Pathophysiology Flashcards
what is strabismus
a squint is a misalignment of the visual axis of the eyes
what does the superior rectus muscle do
abduction and elevation
what does the lateral rectus do
abduction
what does the inferior rectus do
abduction and depression
what does the inferior oblique do
adduction and elevation
what does the medial rectus do
adduction
what does the superior oblique do
adduction and depression
what is a manifest squint
present all the time and is referred to as a ‘tropia’
what is a latent squint
present on dissociation of the eyes and is referred to as a ‘phoria’
what is exptropia squint
divergent squint (affected eye looks outwards)
what is a esotropia squint
convergent squint (affected eye looks inwards)
what is hypertrophia
upwards vertical squint (affected eye looks up)
what is hypotropia
downwards vertical squint (affected eye looks down)
what is a paralytic squint
are due to paralysis of one or more of the extra-ocular miscues. the angle of deviation varies according to the direction of gaze and the squint is greatest when looking in the direction of the action of the paralysed muscle
what are some of the causes of paralytic squints
- trauma
- diabetes
- hypertension
- acoustic neuroma
- glioma
- sarcoidosis
- vasculitis
- raised inter cranial pressure
what is a non paralytic squint
in a non-paralytic squint there is full ocular movement, hence the angle of deviation is the same in all directions
what are causes of a non-paralytic squint
high refractive error
cataracts
retinoblastoma
what is visual acuity
in children squinting affects normal visual development because the visual cortex receives a misaligned image from one eye. this causes it to suppress the visual information from the affected eye, leading to a reduction in the visual acuity known as amblyopia.
when can amblyopia only be corrected
before the age of 7
what is the alignment of the visual axes test
use a pen touch to assess the corneal reflections. there will be deviation in a squinting eye
what is the cover/uncover test
a. cover the squinting eye. the unaffected eye will not deviate
b. cover the unaffected eye. the squinting eye will move to take up fiction
c. since eye movements are equal and opposite, then as the squint moves to take up fixation, the unaffected eye also moves
d. remove the cover from the unaffected eye. the unaffected eye will resume fixation and the squint will return to its original position
what is the alternate cover test
move the cover rapidly between the two eyes. this dissociates the eyes and will show if there is a latent squint
how to determine if there is a paralytic squint
assess ocular movements
what is fundoscopy
assess for cataracts, retinoblastoma, or papilloedema suggesting a raised inner cranial pressure
how many hours a day should a patch be worn
6 hours
what is hypermetropia
common problem with the eyes focusing that can affect your vision at all distances, but especially close up.
this is because of a focusing problem. usually light comes in through the lens and focuses on the retina at the back of the eye. in hypermetropia the light is focused too far back in the eye, behind the retina, which causes things to look blurred close up.
many very young children may have mild hypertrophic that gets better by itself as they grow older. the percentage of people with hypermetropia increases with age
what are the two parts of the eye that focus images
- the cornea is the clear dome shaped front surface of the eye
- the lens is the clear structure about the size and shape of a M&M
what are the cornea and lens shaped like in a normal eye
each of these focusing elements has a perfectly smooth curvature like the surface of a marble. a cornea and lens with such curvature bend (refract) all incoming light to make a sharply focused image directly on the retina, at the back of your eye
what is a refractive error
if your cornea or lens isn’t evenly and smoothly curved, light rays aren’t refracted properly, and you have a refractive error.
fairsightedness occurs when your eyeball is shorter than normal or your cornea is curved too little. the effect is the opposite of nearsightedness
what is aphakia
when the cornea is flattened
is hypermetropia usually genetic
yes. babies and young children may suffer from hypermetropia but it usually corrects itself
why is a lazy eye developed as a result
this is because the eye with the weakest vision is ignored by the brain. if this is not corrected in young children, there is a risk that the weaker eye will never see as the other eye, where the squint happens
what is amblyopia
is a condition whereby there’s a reduction in visual acuity due to a problem with focusing in early childhood. this results in reduced acuity as the brain is not stimulated to develop correctly.
this is not necessarily an inherent problem with the eye itself. it affects about 1-5% of the population
what is the most common cause of amblyopia
strabismus. in this condition, the two eyes do not align correctly, and the brain surpasses images received from one of the eyes to avoid a perception of diplopia. the results is that vision in the suppressed eye does not develop properly
how do you treat strabismus
- treated wither patches or eye drops. both of these methods obscure the vision of the good eye and force the brain to process the image from the affected eye.
- drops used are dilation drops. the dilation of the pupil causes the vision to become very blurry and as such the inout from that eye is ignored by the brain.
- drops reduce the incidence of bullying at school, but can be difficult to administer.
what are the dilation drops usually used
atropine
how long should treatment be used for
4-6 hours a day so not to impact the development of the good eye. over patching can cause reverse amblyopia
how is a refractive error corrected
with glasses
how are congenital cataracts corrected
- treated surgically
- these cases account for less than 3% of the total number of cases of amblyopia
- cataracts are diagnosed simply by checking the red reflex
what are congenital cataracts usually causes by
intrauterine infection, such as rubella
what is a bitemporal hemianopia almost always causes by
damage to the optic chasm, and can occur from the direct or indirect effects of a variety of lesions, including tumours, aneurysms and inflammatory and ischaemic disease
what does bi temporal visual defects describe
the ocular defect that leads to impaired peripheral vision in the outer temporal halves of the visual field of each eye.
where does cranial nerve II run
runs along the midline of the ventral surface of the brain and conveys visual information from the retina of each eye to the corresponding region of the primary visual cortex. the right hand of the visual field of both eyes is processed by the left half of the retina while the right hand of the retina processes the left half of the visual field.
what do the retinal ganglion cells do
project myelinated axons, carrying CNII sensory afferent fibres through the optic chasm, where optic nerve fibres from the nasal half of each retina decussate to the contralateral side of the brain for processing
what does this decussation allow for
greater organisation of visual processing by directions all optic nerve fibres relaying information form the left visual field of hboth eyes to the right hemisphere of the brain and conversely, all fibres carrying the right visual field information to the left hemisphere.
what is the optic chiasm positioning
is along the midline of the ventral subarachnoid space of the Brain, inferior to the hypothalamus and anterior communication artery, and superior to the pituitary gland.
where is the pituitary gland situated
within the sella turcicia
what happens after the nerve passes through the optic chasm
the optic nerve becomes the optic tract that synapses to the lateral geniculate nucleus of the thalamus and subsequently projects optic radiations to the primary visual cortex of the occipital lobe.
where does the optic tract also project to
the superior colliculus, pretectal nuclei, and suprachiasmatic nuclei. this part of the optic pathway serve the important light reflex
what does bi temporal hemianopia affect and not affect
impairs bilateral peripheral vision however the central field of vision from 110-120 degrees remains intact. this range of vision still allows for generally normal function, yet other complications regarding the underlying cause may require more unique medical attention
what would a suprasellar extension of a pituitary macro adenoma lea to
compression of the optic nerve fibres decussating at the optic chasm. impingement of these nerves prevents visual information from the temporal; visual fields of each eye from reaching the processing centres in the brain, leading to peripheral vision loss
what is relative afferent pupillary defect
is a condition in which pupils respond differently to light stimuli shone in one eye at a time due to unilateral or asymmetrical disease of the retina or optic nerve
what is the swinging flashlight test
the most basic eye exams that a doctor will perform. the doctor will ask the patient to look ahead then shines a penlight first toward one eye, and then swing to the other, alternating quickly to observe patients pupils responses to the light. in case both pupils do not show a similar response to the light stimuli, shone in none eye at a time, the patient will be diagnosed with RAPD pupils
where is a RAPD seen
in unilateral or bilateral asymmetric lesions of the prechiasmal optic nerve starting from the retina but can occur anywhere in the afferent pupillary pathway including the optic tract and the pretectal afferent fibres in the dorsal midbrain
what does the pathologic respond that characterises the RAPD include
- the light reaction causes pupil restriction in both eyes when the light shines in the normal eye,
- dilation of the pupils in both eyes when the light stimulus is rapidly transferred from the normal eye to the pathologic eye.
the RAPD is a critically impotent sign in patients with unexplained visual loss because it is an objective findings of afferent pupillary disfunction
visual field defects diagram
what is the visual pathway
is the route by which retinal stimuli are transferred to the occipital cortex of the brain. it encompasses the retina, optic nerve, optic chasm, optic radiations and the visual centre of the occipital lobe
what is the visual fields
- the visual field is the entire area that can be seen by a patient without movement of their head and with their eyes on one point
- the image of an object in the visual field is inverted upside-down and flipped left-to-right on the retina
- confrontational assessment of the visual field is an important part of the cranial nerve examination. it can be more accurelty assessed using perimetry studies.
summary table of visual field defects
what are the rods and cones in the retina stimulated by
photons of light entering the eye
what is the light sensitive surface membrane proteins in the cells
rhodopsin
what is rhodopsin stimulated to do
propagate second messenger responses which convert light energy into electrical signals. the photoreceptors synapse with retinal bipolar cells, which in turn transmit these signals to retinal ganglion cells
where do the retinal ganglion cells converge
at the optic disc forming the optic nerve
where does the optic nerve exit the eye
travels through a defect in the lamina cribrosa of the sclera.
what is the optic nerve covered by
meninges of the central nervous system
where is the optic chasm located
directly above the sella turcica of the sphenoid bone
what happens at the chiasm
fibres from the nasal aspect of each retina cross over the contralateral optic tract, while fibres form the temporal retina remain on their respective sites
Therefore where do the left sided post chiasmal fibres go
pertain to the right side of the visual field and vice versa
the optic tracts extend from the chiasm to where
the thalamus
what happens at the extension from the chiasm to the thalamus
here, the sensory nerves from the eye synapse with the second order sensory neurones at the lateral geniculate nucleus in the thalamus
where do the sensory nerves radiate
radiate dorsally to the calcarine sulcus of the occipital lobe
where do optic radiations loop
either through the parietal lobe or through the temporal lobe
what is the loop though the temporal lobe also called
Meyer’s Loop
what do the radiations travelling through the parietal lobe correspond to
the upper hand of the retina/lower visual field
what do the radiations travelling though Meyer’s Loop in the temporal lobe correspond with
the bottom half of the retina/upper visual field
where do the optic radiations terminate
in the calcarine sulcus of the occipital lobe where the cortical visual field is situated
what is the calcarine sulcus responsible for
retinal image processing. here images from both the eyes are finally collated and a final image s formed. this image is inverted, and as a consequence of this, the brain has to re-invert the image so that information is correctly oriented in space
from the occipital visual centre, where are signals sent
to the frontal, parietal and temporal lobes to further make sense of the input information
what is the rule about pre-chiasmal lesions
they will all result in an ipsilateral monocular visual field defects
what do post chiasmal lesions result in
homonymous visual field defects of the contralateral side
diagram showing different visual losses
what does the visual pathway comprise
the retina. optic nerve, optic chiasm, optic radiatoons and the visual centre in the occipital lobe
what do optic nerve lesions tend to cause
ipsilateral monocular blindess
what do lesions affecting the calcarine sulcus of the occipital lobe tend to cause
homonymous hemianopia with sparing of the macula
how does headaches relate to poor vision
one of the most frequent causes of headaches associated with eye issues is eye strain. overusing the muscles involved in vision focus can lead to eye strain and subsequently headaches. any type of activity that causes you to focus your eyes for an extended period of time can lead to eye strain
what is stereo acuity
the threshold of detection of a difference in depth on the basis of binocular disparity is known as stern acuity.
under the best conditions, a depth interval of 4mm can be detected at a distance of what
5m
what is the test used for stereo acuity
random dot E test
