Day 5 (2): Refraction Flashcards
(45 cards)
What does refraction mean?
- Determination of the refractive status (and errors) of the eye.
- Selecting a lens of sufficient power with a secondary focal plane coinciding with the far point plane of the eye.
Goals:
1. Comfortable and satisfied patient
2. Best-corrected binocular vision
3. Relaxed accommodation
What are the different kinds of refraction?
- Objective/Manifest Refraction
- uses a Retinoscope or Autorefractometer - Subjective Refraction
- determine by subjective means the combination of lenses that would provide the patient with the best corrected VA
- provide patients with two different powered lenses at a time and ask which of the two results to better vision
- uses trial lens set or lenses mounted to a phoropter - Cycloplegic/Wet Refraction
- using cycloplegic drugs to paralyze the ciliary muscles and relax accommodation (e.g. Tropicamide/Cyclopentolate/Atropine)
- can be used in both subjective and objective
What are the steps in refraction?
- Get the best UNcorrected visual acuity.
- Determine the error of refraction (myopia, hyperopia, astigmatism)
- Get the pupillary distance
- Do objective refraction
- Confirm with subjective refraction
- Measure reading add or bifocals
- done in pts with presbyopia or > 40 years old - Writing the correct glasses prescription.
What is the purpose of Pinhole Testing?
To determine if blurring of vision is due to an error of refraction
VA improves if pinhole: (+) Error of Refraction
VA does not improve: Look for other causes
What is the pupillary distance?
Distance between the center of one pupil to the center of the other pupil.
Relevance: when creating prescription glasses, the distance between the optical centers of the frame/lens should approximate the pupillary diameter
- if not: (+) prismatic effect –> diplopia, BOV, straining, headache
Distance PD: pt looking straight ahead and fixating on an object at a distance
Near PD: pt looking at a close object
What is retinoscopy?
- An OBJECTIVE technique for determining the refractive power of an eye
- Shooting light into the eye and observing the red reflex and how it moves with respect to the movement of the light
- Locates the far point of the eye
Who?
1. Pediatric patients
2. Patients who can’t talk
What are noted:
1. spherical refractive error
2. (+/-) regular or irregular astigmatism
3. (+/-) media opacities or optical defects
What are the components of the retinoscope?
Components:
1. Illumination System
- light bulb filament
- mirror
- sleeve: adjusts separation of filament from mirror
2. Observation System
- peephole
What are conjugate points?
- Locations or points in space that are OBJECT and IMAGE of one another
- If points A and B are conjugate points: for an IMAGE to form at point B, the OBJECT should be located at point A.
What is a Far Point?
- Point in space conjugate to the retina when the eye is not accommodating
- Simply put: for an image to form exactly at the retina, object should be located at the far point
- Refractive status of the eye is a function of the far point location
- A property of an imaging system only (NOT of a lens)
What is the Primary Focal Point?
- OBJECT location from which incident light rays would EXIT the lens (and enter the eye) with zero vergence (PARALLEL)
- IMAGE location: INFINITY
PLUS Lens:
- real point outside of the eye
MINUS Lens:
- imaginary point within the eye
What is the Secondary Focal Point?
- IMAGE location when light rays with zero vergence (PARALLEL) ENTER a given lens (or the eye)
- IMAGE location: INTERSECTION POINT of the refracted light rays
PLUS Lens:
- real point inside of the eye
MINUS Lens:
- imaginary point outside the eye
What is the difference between far point, primary and secondary focal point?
Far Point
- Location of an OBJECT in space so the the IMAGE formed FALLS ON THE RETINA
- Inherent to an optical system (eye) NOT a lens
- Object: Far Point
- Image: Retina
Primary Focal Point
- Location of an OBJECT such that incident rays exit the lens PARALLEL
- Inherent to a lens
- Object: Primary Focal Point
- Image: Infinity
Secondary Focal Point
- Where object from optical INFINITY focuses and forms an IMAGE in the eye
- Inherent to a lens
- Object: Infinity
- Image: Secondary Focal Point (Intersection of refracted rays)
Where is the far point and the secondary focal point of an emmetropic eye?
Far Point: a single point located at INFINITY
- so far removed from point of origin that vergence is 0
- parallel rays enter the eye
- can see 20/20 vision or better AT DISTANCE without correction
Secondary Focal Point: RETINA
Where is the far point and the secondary focal point of a myopic eye?
Reasons:
1. Cornea or lens too curved (too much converging power)
2. Axial Myopia: globe is too long for converging power of the eye
- more common reason
Far Point: ANTERIOR to CORNEAL PLANE (front of eye)
- to be conjugate with retina, excess convergence should be offset by an equivalent amount of divergence of incident rays
- when light rays enter the myopic eye, they are still divergent enough to counteract the convergence thus focusing on the retina
- reason why myopic eyes see better at near
Note: difference in the amount of myopia is reflected in the DISTANCE between the retina and the far point
- LOW myopia: less convergence/shorter globe –> LESS compensatory divergence needed –> FARTHER far point
- HIGH myopia: more convergence/longer globe –> MORE compensatory divergence needed –> CLOSER far point
Secondary Focal Point: ANTERIOR to RETINA
- parallel incident rays coming from point at infinity meets at the vitreous or in front of the retina
- after intersecting in front of the retina, light rays diverge and forms a BLUR CIRCLE on the retina
Where is the far point and the secondary focal point of a hyperopic eye?
Reasons:
1. Cornea or lens too flat (too little converging power)
2. Axial Hyperopia: globe is too short for converging power of eye
Far Point: BEHIND the CORNEAL PLANE (within the eye)
- to be conjugate to the retina, the far point must contribute convergence to the decreased converging power of the eye
- however, converging light does NOT naturally occur in nature
- thus, far point is imaginary
Secondary Focal Point: POSTERIOR to RETINA
- imaginary point behind retina where light rays would have intersected if it had not run into the retina
- rays from infinity NEVER intersect and form a blur circle over the retina
Why are the terms nearsightedness and farsightedness confusing?
- Vision is clear if an object is located at its far point.
- The refractive status of an eye is a function of the location of the far point.
- Emmetropic: at distance (far point: INFINITY)
- Myopic: at near (far point: ANTERIOR TO CORNEAL PLANE)
- Hyperopic: none (far point: imaginary point BEHIND THE EYE and not within the light of sight) –> out of focus at every distance
What does AGAINST motion in the retinoscope mean?
- Red reflex moves OPPOSITE the light source
- Seen in: High MYOPIC eyes (greater than the working distance)
- Far point: between peephole and the patient
- FP2: anterior to retina
- Instead, blur circle is formed on the retina (light not as bright and intense)
Correction: Add MINUS lens of increasing power
- to diverge incident light rays to compensate for excess converging power of eye and focus exactly on retina
- convergence of light is gradually moved back posteriorly to the retina to achieve neutrality
- if overpowered lens is added: light is focused to an imaginary point behind the retina –> WITH motion
What does WITH motion in the retinoscope mean?
- Red reflex moves WITH (in the same direction as) the light source
- Seen in
1. HYPEROPIC eyes
2. Emmetropic eyes
3. Low myopic eyes - Far point: behind the examiner
- FP2: imaginary point behind the retina
- Instead, blur circle is formed on the retina (light not as bright and intense)
Correction: Add PLUS lenses of increasing power
- to converge incident light rays to compensate for the decreased convergent power of the eye
- the gradually increasing power of the placed lenses slowly focus the light anteriorly into the retina to achieve neutrality
- if overpowered lens is added: light is focused in front of the retina –> AGAINST motion
When is NEUTRALITY achieved in retinoscopy?
- Peephole is AT the neutral or far point and is conjugate with the retina
- NO movement of red reflex and an intense and bright reflection filling the entire pupil
- Light converges exactly at a single point in the macula
- Far point: neutral point (where peephole is)
- FP2: at a single point on the retina
What parameters are noted in the red reflex seen in retinoscopy?
- Movement
- WITH motion: same direction as light source
- AGAINST motion opposite direction as light source - Speed
- NO movement: target end-point
- Slow: high grade EoR
- Fast: low grade EoR - Brilliance: reflected light intensity
- Dull: high grade EoR
- Bright: low grade EoR - Width of red reflex
- Narrow: high grade EoR
- Broad: low grade EoR
Low grade: close to neutrality, adjust power of lenses in smaller intervals
High grade: far from neutrality, initially adjust in larger intervals
What are the steps in performing retinoscopy?
- Determine working distance (0.67 m ~ 1.5 D)
- Determine type of error of refraction
- WITH motion: hyperope, emmetrope, low myope
- AGAINST motion: high myope - Determine axis of principal meridians
- Neutralize meridian 1
- Neutralize meridian 2
- Determine spherical power
- Determine cylinder axis
- Determine cylinder power
- Subtract working distance power from the gross sphere power
Shortcut:
1. Need to start with a WITH motion. If on initial test you see AGAINST motion, add MINUS lens of increasing power until you see WITH motion.
- to focus the light rays behind the retina
2. Add power in increments of 1 D (MORE PLUS or LESS MINUS) in each meridian until the reflex is neutralized in both meridians.
- to focus light rays gradually forward into the retina
3. Subtract the working distance power (1.5 D) FROM THE SPHERE POWER once gross refractive power is obtained.
What is the average working distance and why it is subtracted from the final refraction?
67 cm: average arms length
- equivalent to around 1.5 D power
- a 1.5 D lens will put the far point at 67 cm from the pt’s eye
- may input working distance power on the optical system or manually subtract from the gross neutralizing power
At this distance, if eye is:
emmetropic: neutral point (neutrality achieved)
myopic: (+) against motion
hyperopic: (+) with motion
- In doing retinoscopy, the goal is to know the error of refraction of the eye and make it emmetropic with a far point at INFINITY:
1/Infinity + D = V –> 0 + D = V –> D = V (the vergence of light contributed by lens is the vergence of light exiting to hit the retina)
- This is not possible because testing is done at a WORKING DISTANCE
- This provides some amount of divergence to the incoming light rays (U is NOT 0 –> - 1/n + D = V)
- Thus the NEUTRALIZED POWER is in reality HIGHER than the TRUE POWER of the eye
- To maintain V as a constant and make sure light still falls on the retina, the overpowered lens needed to compensate for the divergence of light rays at the working distance must be subtracted by - 1.5 D to arrive at the true power of the eye
How is the axis of the principal meridians determined using a retinoscope?
Once intercept/light source is shone on the eye, note the following parameters of the red reflex that’s reflected back:
1. Break
2. Thickness
3. Intensity
4. Skew: is streak parallel to red reflex?
Goal: BRIGHT, WIDE red reflex close to filling the pupil oriented PARALLEL and moving in the SAME direction as the streak
Meridian 1: tested meridian
Meridian 2: 90 degrees from the tested meridian
Reminders when neutralizing the meridians.
- The light streak tests the power of the meridian it sweeps across
- i.e. the meridian perpendicular to the streak axis
- vertical streak (90 degrees axis) moving from L to R: tests the power of the 180 degree meridian
- horizontal streak (180 degrees axis) moving up or down: tests the power of the 90 degree meridian - For practicality: first start with a vertical streak to test the 180 degree meridian
- Once done with meridian 1, rotate streak by 90 degrees and test meridian 2 with a horizontal streak moving up and down.
- To avoid confusion, always start with a WITH motion. If on first testing you see AGAINST motion, use a MINUS lens to move light behind the retina and gradually work your way up with less minus or more plus lenses until neutralization is achieved.
