Exam revision Flashcards
(30 cards)
6th nerve pathway
starts at abdusence -> exits brainstem, between pons and medulla -> enters subarachnois space and runs along dorellos canal -> passes over tip of temporal bone -> enters cavernous sinus and runs along internal carotid artery -> enters lateral aspect of superior orbital fissue and passes anteriorly to innervate the lateral rectus
4th nerve pathway
starts at trochlear in midbrain -> passes through inferior colliculus and travels along cavernous sinus -> enters superior orbital fissure before passing over anulus of zinn adn then attaches to sup oblique
3rd nerve pathway
originates in oculomotor nucleus -> located within midbrain of the brainstem -> emerges from anterior aspect of mbrain -> passes inferiorly to posterior cerebellar artery and then superiorly into cerebellar artery
The nerve then pierces the dura mater and enters the lateral aspect of thecavernous sinus. Within the cavernous sinus, it receives sympathetic branches from the internal carotid plexus.These fibres do not combine with the oculomotor nerve – they merely travel within its sheath.
Tonic accomodation
baseline accomodation level maintained by ciliary muscles in absence of visual stimuli such as complete darkness or featureless feilds (white room)
key points
- resting rate of accomodatin
- occurs without any consciousness, blueor vergence stimuli
- measures in dark or infared accomodation
Typical values
- 1.00 - 1.50 D
- varies with age
clinical relavence
- influences refraction in younger px
- can cause pseudomyopia - over accomdation at retest
- important to consider cyclo-refraction to eliminate tonic acomodation
Sympathetic innervation of ciliary muscles
origin - preganglionic fibres arise at cileospinal center of buldge in spinal cord
Pathway - fibres ascend via sympathetic chain to superior cervical ganglion -> synapse. Post ganglionic cell fibres fravel acoss intracarotid plexus, passes. through cavernous sinus and enters via long ciliary plexus
target - innervates ciliary muscles (maintain longtitudinal fibres)
function - cause inhibition or relaxtion of thre ciliary muscles - leads to passive increased tension avoiding distance vision, modulatory role, not a primary driver of accomodation.
Clinical points - Sympathetic disfunction ay cause subtle disfunction with dilation or focus, no major accomodation loss in parasypathetic relaxed.
Headache questions
L- Location
O - onset
F- frequency
T- time
C - containment
M - meds
structural changes in lens for presbyopia
- loss of elasticity of zonular fibres
- increased lens density and opaque colour therefore decreased light transmission
-thickening of lens capsule, threrefore reducing the ability of zonular fibres
Mohidras ret procedure
-when performing Mohindras ret start by placing the near target onto the retinoscope as well as altering your working distance by 1.5D due to the smaller working distance of 0.5m used to perform this technique. You would then get your patient to focus on the near target while performing ret as normal, this is done in dim lighting as opposed to dark lighting. Throughout ret continue to make sure that the patient continues to focus on the near target for accurate results
Mohindras ret purpose
peds or px with limited cooperation
any time when cyclo cant be used as this is a no cyclo techneque that can still relax accomodation
sutiations where cyclo are needed
peds refraction - children have strong accomodation responses therefore by paralising accomodation more accurate refraction can be found
Px with hyperoipa will accom too much to compensate for refractive error, once again cyclo paralises accom
Px who are unable to participate in subjective refraction
TIB steps
- present a single target to both eyes, ensuring that the images are initially blurry for each eye
- gradual defocus 1 eye by altering the lens for that eye
- ask the px to adjust the defocus until they see a single clear image
- verifiy both eyes that have equal accomodation and convergence, which should be indicated by the pxs ability to report clear single vision without strain.
Wavefront analyser
consists of
- light source to project light into the eye
- wave front sensor - measures the diatance in the light reflected from the retina
- display system - shows the measure of distortion indicating the quality of the eyes images
- the sensor detects the wavefront aberrations and helps quantifiy optical imperfections in the eyes, such as astigmatism or higher order abberations
collumnar arrangement of primary visual cortex
V1 is organsied invertical columns of cells, each column processes a specific feature of visual input. i.e. orientation, motion or colour
Columns work together to intergrate visual info across the visual field, allowing for depth of perception and pattern recognition.
Simple cells detect edge orientation, while complex cells process more complex stimuli such as motion
cells within each area is also tuned to respond to stimuli from the corresponnding retinal area
Prism adaptation
refers to the process by which the brain adapts to the optical shift induced by the prism glasses
this addaptation reduces the visual discomfort and double vision caused by the misalignment of the visual feild defect
time course in a binocular normal observer
occrs over mins to hours of prism wear
Brain gradually compensates for the optical change / shift, alligning the visual input from both eyes
How to detect prism adaptation in a clinical setting
- re-evaluating eye alignment - after px has wore the prism for set peroids of time by the use of cover test or maddox rod
- measuring symptoms - a decrease in symptoms such as diplopia after prism wear suggests adaptation
- px feedback- ask px if their perception of double vision has gone.
How to reduce binoular fusion affecting the value of heterophoria measured
preventing binocular fusion (via mono occlusion) will allow for latent misalignment of the eyes to be manifested - heterophoria
prism cover test - measures the amount of misalignment when Bv is disrupted
alternating cover test - assesses the amount of phoria by measuring how much the eyes deviate when 1 eye is covered
maddox rod - detects the degree of heterophoria by placing a vertical or horozontal line infornt of the eye while other eye percieves a white soft.
Changes made to ST for Cataratcs and AMD
Help px into room
dont push px as quick on visions
bigger lenses for BVS and crosscyl
trail frame and no foropter
Symptoms of presbyopia
difficulty reading small print
Eye strain and fatugue reading up close for long peroids of time
methods to calculate working distance
Age table - 45- 1.00 , 50 - 1.50, 55 - 1.75, 60 - 2.00
WD/100 - 0.25 for depth of focus
place current reading add and then perform BVS at near.
MTF
quantifiyes a lenses abiltiy to reproduce contrast senitivity at different spatial frequencies
- increases MTF indicates better optical performance
Contract sensitivity function
how well a person an detect differences in contract sensitivity frequencies
- ability of the visual system to percieve objects and patterns at different levels of cotrast both light and dark
Key points:-
1. spatial frequencies - level of detail in visual pattern, typically measured in cycles per degree. low frequency - large coarse pattern, high frequency - high Cours pattern
- Contrast - diff in either luminance or colour between an object and background, typically expressed as a ratio or a percentage
- CSF curve - plot of contrast sentivitities across diff spatial frequencies, shurve shows: high sensitivity ot low spatial frequencies, peak at mid range spatial frequencies and decline at higher spatial frequencies.
CSF diagram -
- x axis - spatial frequency (cycles/deg)
- y axis - contrast sensitiity (limitless)
- highest sensitivity - occurs around midrange frequencies (3-6 cycles per deg)
- letter acuity - typically 6 cycles/deg or slightly higher where contrast sensitivity is optimal for visual tasks like reading.
Opthalmoscope as optometer
refractive error can be gauged by observing reflection in opthalmlscope, if focused straight away px is emmetropic
If blurred px has a refractive error
Fixation disparity curve
used to assess allignement of eyes and ability to fuse images into a single image.
- normal curve (flat) - both eyes are aligned and fusion is stable across the visual distances
- exofixation disparity curve - eyes slightly divergent, indicating that they are under a small degree of xop
- exofixation disparity curve - eyes slightly convergent, signs of SOP
- High amp curve - larger diff is observed, often indicates excessive eyes misalignement and diplopia
