Retinoscopy (Autosaved) PDF

Preview

Summary

Retinoscopy Roles of optometrists:  Measure vision and refractive error  Prescribe glasses/contact lenses  Checks health of the eyes  Treat minor eye conditions  Low vision assessments  Specialist clinics  TeachingRefractive states of the eye  Emmetropia – distant object is in sharp focus on...

Description

Retinoscopy Roles of optometrists:  Measure vision and refractive error  Prescribe glasses/contact lenses  Checks health of the eyes  Treat minor eye conditions  Low vision assessments  Specialist clinics  Teaching Refractive states of the eye  Emmetropia – distant object is in sharp focus on the retina with the lens of the eye in a relaxed state, without any correction needed.  Myopia – focused in front of the retina.  Hyperopia – focused behind the retina.  Astigmatism – if the eye is shaped more like a rugby ball than a football – results in two points of focus – different degrees of myopia or hyperopia present. Only one part of the object is in focus at one time. Retinoscopy – objective technique to determine the refractive error of the eye. 1. Light is shone into the eye through the pupil. 2. Light is reflected from the anterior eye (beam). 3. Light is reflected from the fundus (back of eye) – called reflex. 4. Optometrist sweeps/oscillates the retinoscope from side to side and observes movement of the reflex compared to the beam. Quick, easy, reliably accurate and requires minimal co-operation from the patient. Vergence slide – vergence control should be in the down position. Spot retinoscope  Spot allows to see both principle meridians at the same time.  Easier for children and uncooperative pxs. Streak retinoscope  Better at detecting and correcting small amounts of astigmatism.  Axis easier to determine more accurately, especially in high cylinders.  When the ret beam is horizontal – sweep the ret vertically.  When the ret beam is vertical – sweep the ret horizontally.

 Beam is always perpendicular to the meridian being assessed.  Set up at same visual axis (eye level; on axis).  Distance from ret to the px’s eye = working distance – ret usually held 66cm from the eye (arm length).  Ret result corrects this divergence and is more positive – more plus or convex lens power is needed than it would be for a distant target – need to remove this extra positive power – not needed when the eye views a distant target. Interpreting the reflex  Ret reflex movement may be with or against the movement of beam.  WITH = eye is either hyperopic or has low myopia (less than divergence due to working distance) – to correct this, positive lens is needed – cause convergence of the light as it passes through the eye.  Against = eye is myopic – need negative lenses.  When there is no movement and the whole pupil reflects light as the beam is moved – ret is complete – no movement seen – reversal.  Dull and slow movement = large refractive error present – add high positive lens.  Bright and fast = small refractive error present – add low powered lens.  Bright and no movement = reversal – no lens required – subtract working distance. Accuracy  Practitioner will need to wear their own specs/ contact lenses.  Right eye for right eye, vice versa.  Ensure working distance remain constant – too close can result in with movement – more divergent light, focused behind retina.  Make sweeping movements of the beam from one side of the centre of the pupil to the other – limit number of sweeps.  Make large changes in lens power if reflex is dull and difficult to see (more positive or negative sphere).  Smaller changes as reflex becomes brighter.

 Once at reversal: o Add +0.25DS – reversal should change to against. o Add -0.25DS – reversal should change to with. o Move closer – change to with. o Move further away – change to against. Astigmatism –  Visual defect in which the unequal (or nonuniform) curvature of one or more refractive surfaces of the eye (usually the cornea), prevents light rays from focusing clearly at one point on the retina.  There are two focal points – each formed by one of the two different curvatures of the cornea and/or lens of the eye – the two curvatures always perpendicular to each other. Spherical ametropia  One power in all meridian  Power 1 = power 2  Need spherical lenses. Astigmatic ametropia  Different powers in the 2 principal meridians  Use spherical and cylindrical lenses (power occurs in only one of the two main meridia of the lens – in the perpendicular meridian, there is no power at all). Principal meridian almost always 90o to each other. Retinoscopy technique: 1. Determine principle meridia  Begin with beam at 90, then rotate beam to 180, 45 and 135. 2. Interpret the reflex  Direction/orientation; speed; brightness.  As each principal meridia has a different power – reflex seem in each meridian will be of different speed and shape.  If the astigmatism is high, the reflex will be a more distinct shape – better defined than low astigmatism. 3. Add correcting lenses

High degree of astigmatism

Low degree of astigmatism

 Begin with sphere – correct meridian with the slowest with or fastest against movement (highest +ve power; lowest –ve power).  Keep sphere in place and move on to the other meridian.  Use a minus cylinder to correct the against movement – cylindrical lens has a marked axis, perpendicular to this is the power – need to place the axis perpendicular to the meridian in which the against movement is found. 4. Record results  Subtract the working distance from sphere.

The Knife-edge Test Lens testing (tests the quality of lenses)  In paraxial optics, an ideal image is formed and all rays come to a single (stigmatic) focus – knife edge moved across the focus would immediately cut off all light. Ideal (paraxial) image formation  All rays from one object point pass through one image point – stigmatic imagery.  Object and image are geometrically similar.  Eye is a positive optical system – forms a real image on the retina.  Analogy of the optic of the eye – simple positive thin lens, illustrated in the diagram as ideal image formation – all rays end up at one focus.  Real image formation – errors at the focus formed – not all rays at the same point. Defocus (ametropia)  Very few optical systems exhibit ideal image formation.  Largest problem affecting image quality – error of focus (defocus) – for eyes, it’s called ametropia.  The effect of higher order aberrations is smaller. At focus: raising knife-edge and lowering it – the bright light disappears quickly, light that is left is due to reflections – fairly instantaneous flash and disappearance of the beam – expected as all the light is focused at one point. Inside focus: knife-edge moved up and down across the focus – as it moves up, there’s a shadow – cuts off the beam starting at the bottom – same direction as the knife-edge is moving – with movement. Outside focus: as the knife-edge is moved up, the shadow comes down, as its moved down, the shadow moves up – against movement.

 Rays from a point source of light – form a single focus – ideal image formation.  Knife edge moved inside the focus (WITH) – all of the light that passes through the aperture of the lens reaches the focus and beyond – lens appears fully illuminated (white disc – indicating all light passes through). Upper position – some of the rays of light are cut off – these rays started from the lower part of the lens, lower part of the lens goes into shadow, upper part still illuminated. Shadow is moving up to cover the pupil at the same time that the knife-edge is moving up = with movement.  Knife edge at focus - lower position – all the light passes through, aperture of lens is fully illuminated (white disc). Upper position – as the focus is crossed – all the light that passes to the aperture of lens is going to get cut off – knife-edge is cutting off that focus – go from lens aperture that’s fully illuminated to one that’s in complete shadow = reversal.  Knife edge outside focus – lower position – all the light filling the aperture of the lens goes through, aperture fully illuminated (white disc). Upper position – as the knife edge moved up, the rays are cut off – rays originated from upper half of the aperture of the lens, crossed over at the focus, that’s why although the knife edge below the optical axis, it cuts off the rays from upper half of the lens – that’s why this goes into shadow first – as knife edge moves up, the shadow moves down from the top of the lens aperture = against movement.

Optics of Retinoscope Principles of retinoscopy  Illuminate fundus – light is reflected back out of the eye – get image of fundus formed at subjects far point.  Relax formed – bright glow seen in pupil.  Sweep the ret – mirror rotation.  Observations – brightness/ direction/speed – allows neutralisation of refractive error.  Reversal obtained – far point (artificial) with the working distance lens in place at the retinoscope itself.  Correction determined – subtract the working distance. Self-luminous retinoscope Streak and spot retinoscope  Difference between is the type of bulb – personal choice.  Streak – advantage in astigmatism eyes – allows indication of meridian.  Spherical ametropia means the refractive error is the same in all meridians. Retinoscope optics  Light source – small bright bulb – produces a uniform source of light to make the reflex look uniform.  Condensing lens – collects light – that would otherwise be sent out the sides and potentially be lost. Rays of light starting from the light source diverging – condensing lens pulls them in to make them less diverging – to focus more light onto the mirror.  Mirror – angled at 45o – directs the light from the handle of the ret towards the eye being examined. Can either have a hole (sight-hole) or be semi-silvered – most common.  Aperture – allows to switch between two different sized apertures – smaller one gives greater sensitivity near to reversal but reduce some light.

 Condensing lens produces a virtual image of this real light source. In this pic, the real light source – it’s virtual image is called the effective source – effective positon of the source if the condensing lens is taken out. There is an image of the effective source in the mirror at immediate source – it is okay to assume to consider the illumination from the ret as just coming from the immediate source and diverging from that point – controlled by the vergence slide. Mirror rotation  Diagram – the immediate source from the ret and the light diverging from that (dashed line = mirror). Straight ahead diagram -Takes the immediate source on the optical axis – and looks at where it forms an image.  Sweeping vertically along a 90o meridian – rotate the mirror in the upward position as the ret is tilted – light is diverted in the upward direction going into the eye – as a result the immediate source moves down (arrow LHS) – illuminated patch on the fundus, which forms the reflex moves up.  As the mirror rotates up – immediate source moves down – blur patch on fundus moves up.

Reflex Movement in Spherical Ametropia Reversal  Bright flashy reflex = reversal.  Single surface model of the eye with positive power – working distance lens in front (red).  Ametropic eye (corrected eye) – light from the fundus is conjugate with the far point – at the sight hole of the ret.  Straight ahead position – both the point source of light at the back of the eye (generating the orange glow) is on the optical axis.  All the light passes through the sight hole in the ret – orange glow in the pupil – as the ret is swept upwards, point of illumination on the fundus is moves upwards – can see the point where two dash rays come from tracing the light back out of the eye, has moved off the optical axis, upwards. Trace the rays back through the reduced eye – form a focus off the axis – outside the sight hole – get an instant flash off. Summary  Practitioner views the glow in the pupil of the subject – reversal occurs when artificial far point coincides with sight hole – as a result reflex instantly disappears. Myopia  Straight ahead position – myopic eye, without correction and with the WD in place – artificial far point is going to be between the practitioner and the reduced eye itself. All the rays that pass through the pupil of the eye – are going to pass through the sight hole – orange glow in the pupil.  When the ret is swept along vertical meridian –as a result of mirror movement, the patch of illumination on the back of eye is going to move up. Rays from back of eye being – reflected out again from the fundus – fill the pupil, come through the sight-hole – cross over – form an image at the far point plane – by the time they reach sight-hole they spread out again. Sight-hole is obstructing some of the rays and letting some

through – obstructing the rays that originate from the top half of the pupil of the eye – shadow appears in the top part of the pupil – AGAINST movement. Summary  Sight-hole vignettes rays from upper part of pupil – in the straight-ahead position, orange glow in the pupil – as we sweep the ret up, the shadow region comes down from the top of pupil. Hypermetropia > 1.5D  Straight ahead position – light emanating from the fundus – creating the reflex, passes through components – comes from the point behind the px’s eye – artificial far point – if hypermetropia is strong enough, it will overcome the convergence added by the working distance lens.  When the ret is swept along vertical meridian – mirror will be moved up on the sight-hole – point of illumination that emanates at the start from the back of eye, moves up away from the optical axis – trace the rays back through the eye, some of the rays are obstructed by the sight hole – these rays emanate from the far point plane – from the bottom part of the pupil that are cut off by the sight hole. As we sweep up, the shadow comes up from the bottom, of the pupil = WITH movement. Summary  Sight-hole vignettes rays from lower part of pupil – in the straight-ahead position, orange glow in the pupil – as we sweep the ret up, the shadow region comes up from the bottom part of the pupil. Hypermetropia < 1.5D  This hypermetropia is sufficient to overcome the power of the WD lens.  Weaker degree of ametropia – the WD lens provides some convergence which the hypermetropia eye can’t overcome – far point behind the practitioner.  When the ret is swept along vertical meridian – point of illumination at the back of the eye from the fundus also moves

up from the optical axis – trace the rays back – some cut out by the finite size of the sight-hole – rays that emanate from lower part of pupil.  Sight-hole vignettes rays from lower part of pupil – lower part in shadow. Straight ahead position – everything lined up –circular glow in the centre of pupil. Tilt the ret up – shadow from bottom part of pupil or orange glow moving off the top part of the pupil. Summary With working distance:  Myopia = against movement.  Hypermetropia = with movement. Without working distance  Strong myopia (more than -1.50D) = against movement.  Weak myopia (less than -1.50D) = with movement.  Hypermetropia = with movement.

Speed of the Reflex

Reflex speed  W = working distance (-1.50D)  X = immediate source location in Dioptres – can be changed with vergence slide.  K = subject’s ametropia – as ametropia is reduced, the speed of reflex changes. 1. Beam coming out of ret = collimated: o Red curves – speed is positive (with) for hypermetropia and negative (against) for myopia. o If there is extreme amount ametropia – 5.00D or more, there isn’t a lot of change in speed. o Speed exponentially increases as corrected closer to reversal. 2. Vergence slide moved down – diverging beam: o Blue curves – moderate to high degree of ametropia – very little change in the speed of reflex – speed of reflex rapidly increases closer to reflex than it did for the red curves – get a more sensitive measure of reflex – much closer to Y axis. 3. Vergence slide moved up – converging beam: o Green curves – in the hypermetropic region – negative speed (against movement) and myopic region – with movement. o Converging light will swap the reflex movements.

Analysis of Retinoscopy Reflexes: Astigmatism

Clues to Astigmatism  Reflex becomes elliptical in shape – ‘band’ reflex in spot retinoscopy.  Eccentric movement or twist of the reflex – differences in movement between the two principle meridians.  Reflex speed will be different along the principle meridians. Consider astigmatism as two different degrees of spherical ametropia along the two principle meridians – two different speeds/movements. The reflex in mixed astigmatism  Myopic meridian at 90o – as we sweep along 90 = against movement. Far point with the WD in place is in between the observer and the eye (RHS).  Rays that are reflected from the retina forms the reflex – light fills the pupil of the eye – by the time it comes back to the observer and the sight-hole, some rays fall outside the bottom of the sight-hole – happens when the ret is swept up along the 90 meridian – point on retina moves up – shaded set of rays vignette – originate from the top part of pupil – shadow moves down across from the top, coming downwards – therefore get against movement. As we sweep up, the shadow comes down.  Hypermetropic meridian at 180o – location of the far point with the WD in place behind the ret.  Rays originate from the right of the fundus – as we swept the ret to the right – come back to the point on the far point plane now to the left. Some rays fall outside the sight-hole – it’s the set of rays that originate from the lefthand side of the pupil – falling outside the ret – therefore the LHS part of the pupil that goes into pupil as we sweep right.  As we sweep right, the shadow comes in from the LHS of the pupil – moves across the pupil – same direction as we move our ret = with movement. Band Reflex

 Correcting astigmatism – correct most positive or least negative first.  Assume the hypermetropic meridian has been bought to reversal – vertical meridian has become more myopic as the positive spheres are used to correct the hypermetropic meridian – far point with the WD has moved closer to the eye (for the 90 meridian).  Rays from the 180 meridian are now focusing down on the sight-hole – get a fast flash reflex.  The 90 meridian – rays of light are focusing closer to the eye – some are falling outside sight-hole. Reflex is the glow seen in the pupil of the eye (ellipse on the RHS) – top and bottom of that reflex are in shadow, those rays are vignetted – end up with elliptical shaped reflex – horizontally arranged.  When one meridian is close to reversal – other is far from reversal – causes vignetting – creating band reflex  astigmatism for streak ret.  -7.50/-1.00 x100  no band reflex is seen due to the dominance of the spherical component. The band reflex can be masked by spherical ametropia. It may be necessary to reduce the level of sphere component before the effects of astigmatism are seen. Determining Principle Meridian  Sweeping down 90 meridian – band reflex siting above pupil – as we move down, there’s a with movement.  Sweeping down 110 meridian – get orange glow on top of the pupil, middle and bottom half of pupil. It becomes difficult if there’s a strong amount of astigmatism to be very precise about the meridian – reflex appearance can be same when sweeping down meridians close together.  -0.50/-3.50 x20  appearance of the reflex (seen in the pupil) appears the same along both the 90 and 110 meridians. A strong band reflex can ask the axis of principle meridian.

Movement of reflex

 Start with an eye with compound myopic astigmatism – both meridian myopic, one closer to reversal than the other.  Horizontal closer to reversal than vertical – both will give against movement. Horizontal meridian will give brighter and faster reflex compared to vertical meridian – get slower, duller against movement.  Sweep ret along 45 meridian – split this movement – one along vertical meridian and further movement along horizontal. As we sweep along the green dotted line = slow against movement. As we move along the red dotted movement = fast against movement. Combine the two components for the overall movement for the reflex – add the two arrows together = produces the resultant movement of reflex = called vector addition. Stre...