RBV2 answers

• Describe Mohindra’s retinoscopy technique. (5 marks)

  Make sure that the room is completely dark. Dim the light that is on the retinoscope and encourage the child to look straight ahead at the light.

  ·         Occlude the eye that you are not currently assessing.

  ·         Working distance will be shorter, at about 50cm.

  ·         Instead of making a working distance allowance of +2.00D, use +1.25D, i.e. assume that the patient is accommodating by 0.75D
• List three situations in which a cycloplegic refraction would be indicated. (3 marks)

  ·         If there is a presentation of a strabismus or a decompensated phoria

  ·         If there is a family history of a high prescription.

  ·         If the patient has unequal or reduced Vas.
• State two of the most common causes of sight loss in the UK (2 marks)

  Age-related macular degeneration (ARMD)

  Diabetic Retinopathy
• Diverging light

  • All light rays going towards the patient's eye appear to originate from this point
  • The aperture stop of the illumination system is the pupil of the patient
• Converging light

  • Results when the light source is outside the focal distance of the collimator lens
  • The change occurs because light rays are crossing at this point
  • This crossing means that the light patch on the patient's retina is moving in the opposite direction from the Ret rotation
• Assuming that the prescription is RE +1.75DS, explain how the movement of the red reflex has changed from (i) (no lens) to (ii) +1.00DS to (iii) +1.50DS. With the aid of diagrams showing the optics of retinoscopy, explain why the movement has changed in the way it did. (5 marks)

  i. a slow with movement. ii. a faster with movement. iii. wide beam, fast, looks almost neutral
• What happens to a with movement when the collar is up?

  is against
• What happens when the collar is up with an against movement?

  Movement is with
• A 70-year-old patient attends your practice, their working distance is 40cm. List the steps used to calculate the tentative near addition for this patient. (3 marks) 

  ·         Take the working distance and divide by 100 (100/40=2.5) = 2.50D

  ·         Take 0.25D from the total as this is allowance for depth of focus.

  ·         The tentative add would then equal +2.25D
• What are the step in a near add method assuming the patients working distance is 40cm and aged 45-60?

  o   Assuming the px working distance is 40cm. the table works well for patients 60 years and under.

  o   If the working distance is much shorter or longer than 40cm, add or subtract a small amount (0.25D) to these values (depending on occupation and hobbies)

  o   e.g. px aged 50 years, working distance of 60cm: +1.50-0.25=+1.25D tentative add.

  o   e.g. px aged 50 years, working distance of 20cm: +1.50+0.25=+1.75D tentative add.
• What is the age versus tentative add values?

  45 = +1.00D, 50= +1.50D, 55= +1.75D, 60= +2.00D
• What is the tentative add procedure with a known working distance and the patient is over 60?

  §  Working distance 36cm (100/36=2.78) =2.75D

  §  Deduct 0.25D as allowance for depth of focus.

  §  Tentative add = +2.50D
• How can you base your tentative near add on the patient's symptoms and existing reading correction?

  o   e.g. px is 60 years old and wears +2.00 add.

  o   Current NVA R N6 L N6

  o   Patient likely requires another +0.50DS R+L to improve VA to N4 (to improve by 2 lines on the reading chart) (+0.25 improves by one line) (add +0.25 less)
• What is the tentative near add procedure when assuming the patient can use a certain proportion of their measured amplitude of accommodation?

  o   Tentative add = working distance (in D) – 1/2 x accommodation amplitude.

  o   (NB some texts suggest using 2/3 rather than 1/2)

  o   This method takes longer in practice and is less popular.
• Explain what is meant by the term 'prism adaptation'. (2 marks)

  The fusional vergence demand by the use of prisms, the tonic vergence level will shift in the direction of the prism to relieve the stress upon the phasic (fusional) vergence controller.
• Briefly describe the time course of prism adaptation in a binocularly normal observer. (2 marks)

  Most recovery occurred within the first 3 minutes following the removal of the prism. This recovery time has been shown to be linearly related to both the size and duration of the prism stimulus – adaptation recovery from prism stimulus worn for several hours can take longer.

   
• Describe how you can detect the presence of a prism adaptation in a clinical setting. (2 marks)

  ·         Using the fixation disparity test (Mallet). The protocol should be outlined.

  ·         Prism adaptation will occur relatively quickly.

  ·         Measure fixation disparity.

  ·         Add prism to be prescribed to trial frame.

  ·         Leave in place for 10 minutes (Px should always look through prism)

  Reassess fixation disparity (If the disparity has returned to the original value, then adaptation has occurred, prism will have no therapeutic benefit)
• Prism adaptation in binocularly normal observer

  • Faster rate of adaptation for base-out prism at 4m fixation distance
  • Faster rate of adaptation for base-in prism at 40cm fixation distance
• Prism adaptation

  1. Patient must overcome unwanted prismatic effect

  2. To maintain clear single binocular vision
• Prism adaptation takes finite time, 3 minutes to adapt to 2 dioptres of vertical prism
• Patient would require maintaining fixation at near for several minutes before adaptation could reduce the stress upon the fusion mechanism
• What effect does age have upon prism adaptation? (2 marks)

  ·         It has been shown that prism adaptation is slower and of lower final magnitude than that which is found in younger subjects.

  ·         It would appear that there is a tendency for the output of the tonic vergence controller to reduce with increasing age.
• Write short notes on tonic accommodation.

  ·         This is the level adopted by the accommodation and vergence systems when retinal image quality no longer depends on the level of either the accommodation or vergence systems.

  ·         Veridically, it represents the accommodation or vergence response level in the absence of the other three components mentioned above (reflex, accommodative-vergence, proximal or psychic)
• Write short notes on sympathetic innervation to the ciliary muscle. (5 marks)

  ·         Arises in the hypothalamus and nerve fibres from here travel down the spinal cord to the ciliary centre of Budge.

  ·         The fibres continue to synapse in the superior cervical ganglion from where a nerve travels to the ciliary body.

   
• Write short notes on accommodation microfluctuations. (5 marks)

  ·         we aren’t normally aware of the rapid fluctuation of accommodation.

  ·         Micro-fluctuations for near objects: the claim that there may be a feed-back loop to keep the object in focus.

  ·         These fluctuations significantly decreased for distant objects and are significantly reduced when viewing an empty field.

  ·         If the accommodation level need changes, then fluctuating in one direction will make things better and fluctuating in the other will make things worse.
• Write short notes on saccadic eye movements. (5 marks)

  ·         The stimulus for a saccade is normally voluntary and is determined by the subject wishing to place a particular object on the fovea. Saccadic eye movements have a typical latency of 200ms and the time taken to complete the movement is dependent on its angular extent.
• Smooth pursuit eye movements

  Eye movements used to generate smooth tracking movements which closely follow the velocity of a moving target
• Smooth pursuit eye movements

  1. Stimulus is a moving target

  2. Non-visual stimuli also have an influence

  3. Latency is around 100ms
• Smooth pursuit eye movements

  • Maintain the object of attention on the foveae during movement of the object
  • Produce a smooth conjugate version movement
  • Can precisely match the speed of the object up to about 30 degrees/sec
• Describe the characteristics of saccadic eye movements. (5 marks)

  ·         conjugacy

  o   both eyes move in the same direction and by same amount.

  ·         Latency

  o   The time elapsing between the appearance of aa stimulus and the onset of the eye movement response, this type is typically 150-200ms.

  ·         Size and velocity

  o   How fast and by how much the eyes move.

  ·         Accuracy

  o   Saccades are inaccurate in two ways.

  §  Pulse dysmetria – the pulse size is inappropriate for target position.

  §  Post saccadic drift – eyes continue moving after original movement has stopped.
• Explain the phenomenon of physiological diplopia.

  The images of objects placed appreciably in front or behind the horopter, will fall on disparate (noncorresponding) points, be localised in different visual directions, and will therefore appear double
• Describe the principles underlying the design of clinical tests for stereoscopic acuity (6 marks)

  ·         Stereoscopic depth is only a relative perception, one object being perceived as nearer or further away than another.

  The essential requirement in the measurement of stereoacuity is the removal of all monocular clues. If this requirement is fulfilled, judgements of depth will be made purely on the basis of the disparity between the two images.
• What is the tinnitus (housefly) test.

  Can assess if there is basic stereoacuity. Useful fr children to try and grab the wing of the fly. Grab above the page, basic stereoacuity is present if not it is a good indicator that it may not be there or have very little. This is due to it containing very large disparities. Reaction can also be noted.
• Describe the Frisby stereotest.

  Real depth to assess stereoacuity at a near distance à The depth is created by using plastic sheets of different thicknesses. It gives the clinician a wide range of stereoacuity levels due to the variability – the different thicknesses and it can be performed at different distances.
• Describe the three-needle test.

  The advantage of this type of test is that it is amenable to statistical treatment and is therefore both accurate and reliable.

   
• Lateral separation of eyes

  • Each eye gets a different view of an object
• Visual cortex can analyse a small disparity between two images

  To produce the sensation of depth
• Horopter

  Objects whose images fall on the corresponding points of the two retinas
• Panum's region of binocular single vision

  Region in front and behind the longitudinal horopter where objects are seen singularly
• Objects placed within Panum's region

  Images fall on slightly disparate points, but are seen singly and are located in front or behind the fixation point