things to memorise Flashcards
(23 cards)
prism adaptation
Prism adaptation: when the fusional vergency demand is altered with the use of prisms, the tonic vergence level will shift in the direction of the prism to relieve stress off the phasic (fusional) vergence controller.
time course of prism adaptation in a binocularly normal observer
The rate of adaptation is seen to be 3min for a 6-diopter prism (base in and out).
Depending on the base of the prism and the fixational distance, the rate of adaptation changes: -
* At a 4cm fixation distance, the rate of adaptation was faster for base
OUT
* At a 40cm fixation distance, the rate of adaptation was faster for base IN
How to detect the presence of prism adaptation in clinical setting
Use a Mallet Unit
Place a prism in front of the patient (e.g., 6 prism dioptres base IN)
There will be a fast initial convergence response to maintain the single vision
When you cover one eye, the eye will relax back into its original position - will appear as a 6-diptre exophoria
Measure the fixation disparity and add the prism to be prescribed into the trial frame
Leave the prism in place for 10 minutes- it is critical that the patient continue. to look directly through the prism
- The output of the tonic/slow component of the binocular system build up contraction and the fusion free position of the eye will return to the original position- overcomes the prism
Reassess the fixation disparity: if the disparity returns to its original position, the adaptation has occurred, and the prism will have no therapeutic effect to the patient
Short notes on accomodation microflucuations
The accommodation system fluctuates rapidly by very small amounts
The magnitude of these fluctuations is 0.25D for near and a frequency of 2Hz
The magnitude reduces for distant objects and when looking out into
bright empty fields
If the accommodation level that is needed changes, the micro-fluctuations in one direction will make things better whereas if they were to fluctuate in the other direction- they would get worse
The micro-fluctuations are said to be smaller with smaller pupil diameters- may be due to the increase in the depth of focus
May be a possible ‘hunting mechanisms’ - reducing the pupil size would be expected to result in an increase in fluctuation for a detectable change
Short notes on sympathetic innervation to the ciliary muscles
Starts at the hypothalamus
The fibres travel down the spine to the cilio-spinal centre of Budge
The fibres then synapse at the superior cervical ganglion
The nerve fibres then travel to the ciliary body
The sympathetic innervation does not produce an opposing action to the parasympathetic innervation it is only there to alleviate stress off the muscle
Pathway of CN6
- Quite low down in the midbrain
- Exits the midbrain at the junction between the pons and medulla
- Passes along the temporal bone
- Passes forward and runs over the groove (petrous wing)
- Travels through the cavernous sinus
- Straight into superior orbital fissure
- Innervates the lateral rectus
Describe the principles underlying the design of clinical tests for stereoscopic acuity.
In stereoscopic acuity tests, we measure the stereoacuity which is the minimum depth perception between 2 objects which can just be perceived.
Depth can be presented using separate flat pattens to both eyes but there will be a degree of disparity in each pattern.
The patterns must be similar and have a difference of disparity that is still within the ‘panum’s region’ to allow for the brain to fuse the 2 images together.
patterns are presented haploscopically. This is when the RE will see the top images and the LE sees the lower image- which given rise to the perception of depth.
State the tests you would you choose for your practice and explain why (stereo acuity)
Frisby Stereo Test (if there is concern that the stereoacuity may be problematic)
* Has a real depth element e.g. 2 images overlapping one another
* Can achieve a wide range of depth by using different plates at different distances
* Good option for children + don’t need to use lenses
* Can alter the distance to achieve a ‘finer’ level of depth
* Can measure a wide range of values
* Both distance and near versions are available
Red and Green Dot Test (to check if stereoacuity is normal)
* Random dot pattern creates depth
* The 2nd butterfly can only be seen if the brain can fuse the red and green dots together whist wearing red and green filter lenses
* Cannot carry out this test without binocular vision since there is no monocular cues
* Large disparity therefore the binocular vision doesn’t have to be acute
* Can be used in pre-verbal children e.g. pointing
Mohindras ret technique
Ensure that the room is dark
Dim the retinocope adn encourage the patient to focus on the ret target
occlude the eye that is not being examined
perform ret at a distance of 50cm
use less working distance to accomodate for opx accomodation
3 situations where cycloplegic refraction would be used.
When dry ret is sig more plus than sunjective
VA in 1 or both eyes is quiet lower than the acrage age of the PX
there is a esotropia or a convergence excessive esotropia
surgical and pathoogical causes of anisometropia
pathological; unilateral or bilateral cataracts or unilateral keratoconus
Define a horopter
Horopter = when fixatong on an object, there is a locus of corresponding points in a plane which are seen singularly
physiological basis of a horopter in out bionocular vision system
- The visual system is developed so that the fibres from the nasal retina of each eye cross over the optic chiasm to join the temporal fibres of the contralateral eye.
- This means that the fibres from each retina which look at the same area in the visual field, will pass back to same region is the visual cortex
- The corresponding cells are binocular and therefore respond best when there is similar
- input for both eyes
- Therefore, if an object is place at such a point that the image falls on corresponding points- the same cell will be stimulated and will gave us single vision
reading adds
- Take note of the patients habitual working distance (in this case its 40cm)
- Ensure the room lighting is like the patient’s normal environment
- Workout the tentative add (since the patient is 53 years, the add would be +1.50D)
- Combine the distance correction with the tentative add and place this in front of each eye
- Ask the patient to hold the reading card at their habitual working distance determine the smallest len that can be read
- Add a -0.25D lens and ask, ‘does this make the print cleared?’ o If the answer is yes or the same, given the -0.25D (continue to do this until the patient says the lens makes it worse)
* If the answer is no, stop and move on - Add a +0.25D and ask, ‘does this make the print clearer?’
* If the answer is no, stop and move on
* If the answer is yes, give the +0.25D (continue to do this until the patient says the lens makes it worse or no longer improves) - Record the final near add
oi
Determine the range of clear vision
* Ask the patient to move closer until the print starts to become blurred - Determine if this range is sufficient for the patient
AMD BVS
When doing bvs, make sure PX can read one full line, allow PX to adopt their habitual head and eye position.
* Rather than moving in 0.25D when doing bvs, use 1.00D steps and same for cross cyl, use 0.75D or 1.00D and a larger axis when rotating.
* Use pinhole to make sure you’ve not missed a large refraction. +1.00 and Duochrome are often inappropriate are often inappropriate.
* Take into account PX usual working distance and find out particular task that they do
How can the direct ophthalmoscope be used as optometer? Sketch a ray-diagram for the case of an emmetropic patient (assuming no accommodation) to justify your answer.
The principle is that the observer judges whether an object imaged on the patient’s retina is in focus or not The optometer includes: an illumination system, observation system, mirror/prism, and coupling
The illumination system is the divergent beam whereas the observation system is the direct ophthalmoscope
The observer focuses on the patient’s retinal blood vessels
The power of the lens when the fundus is in focus = the examiners and patients prescription + the examiners and patients accommodation
List the main problems that limit the accuracy of refractive error measurement using a direct ophthalmoscope?
There is no control over the accommodation
Focuses on the macula
There is a large depth of focus
There is poor intensity of the image and of the reflections
There is a corneal reflex
List 5 components of accommodation.
Tonic accommodation
Convergence accommodation
Proximal accommodation
Reflex accommodation
Voluntary accommodation
List 3 anomalous myopias
Night myopia
Empty-field myopia
Instrumental myopia
State which of the 5 components of accommodation is responsible for each of the 3 anomalous myopias. Explain your answer.
Tonic accommodation is responsible for ‘empty field myopia’ e.g., pilots in a cloudy white environment.
Since there is nothing to focus on, the accommodation response does not return back to 0, but instead it falls back to the base line level or resting state (roughly 1.00-1.50D more myopic) -
Tonic accommodation is also responsible for ‘night myopia’
Since the level of luminance is low, the accommodation system does not have any target to focus. Therefore, the system returns to the base line or resting level (roughly -0.60 to -2.00D more myopia)
Proximal accommodation is responsible for ‘instrument myopia’ as a consequence of the object’s physical proximity. Tonic accommodation can also cause ‘instrument myopia’ as a consequence of small exit pupils
saccadic eye movements
Are ‘refixational movements’
They are fast and accurate
Have a voluntary and reflex component
Is a conjugate eye movement: both eyes move in the same direct at the same time
Latency = the time delay between the appearance of a stimulus and onset is roughly 150-200ms
The movement is ‘pre-programmed meaning that the eyes cannot stop moving until it reaches the target
Saccades can be accurate in 2 ways:
* Dysmetria =is when the target is undershot by 10% then is fixes by a corrective saccade
* -Post-saccadic drift = is when the eye tends to continue to move after the original movement has stopped
Smooth pursuit eye movements
Are very accurate - if the motion is predictable, the accuracy increases
There is a small-time delay
Is a conjugate eye movement: both eyes move in the same direct at the same time
Stimulus = is the movement of the image across the retina
Latency = roughly 100-150ms
Velocity = 40-100 degrees-2 (during the first 20ms)
Explain how measurements of fusional reserve are used to assess heterophoria control. Illustrate your answer with examples.
If we know the magnitude of the relevant fusion reserves, we can assess the proportional of the total fusional vergence which is being used to correct the phoria.
* In esophoria, the positive fusional vergence is used to correct the deviation. The positive fusional reserves represent the fusional. convergence (that still remain) which can be exerted in order to obtain single vision
* In exophoria, the negative fusional vergence is used to correct the deviation. The negative fusional reserves represent the fusional divergence (that still remain) which is in reserve after the phoria has been overcome
For example, if the patient has a distance heterophoria of 6 XOP and the positive fusional reserve at distance is 18 prism dioptres. So, to calculate the total positive fusional vergence = 6 + 18 = 24 prism dioptres. Therefore, this means than ¼ of the positive fusional convergence is being used to overcome the heterophoria
Sheard’s Criteria states that the fusional reserve should be at least twice the fusional demand for ocular comfort
* In esophoria, the positive fusional reserve should be 2x the size
* In exophoria, the negative fusional reserve should 2x the size
For example, if a patient has an esophoria of 4 prism dioptres (at distance) and a negative fusional reserve of 6 prism dioptre. By adding a 1 prism dioptre base OUT, the esophoria will change to 3 prism dioptres and the negative fusional reserves changes to 7 prism dioptres.
