4 Chapter 4: Starting Lens Determination: Habitual, Retinoscopy and Autorefraction

4A. Starting Lens Determination

[1.] What is the starting lens?

It is your best ESTIMATE of what the patient’s spectacle prescription is before beginning subjective refraction.

[2.] Why do we need a starting lens?

Ignoring the multitude of lenses within a phoropter/refractor and only considering reasonable possibilities, 99% of the population has between +4.00 and -10.00 D of spherical power, 180 degrees of rotation and between 0 and -3.00 diopters of astigmatism.  This means there are almost 242,000 possible lens combinations.  It is logistically impossible and a giant waste of time to attempt all those possibilities.

With a good starting lens, and good visual acuities, we can limit that huge possible range down to more manageable number.  In most cases, you can limit the options down to 5-10 (sometime 1-2).  This greatly limits testing time.

In some extreme situations where the patient is none responsive, infants, stroke patients, etc, your starting lens will be also your prescribing lens.

[3.] Methods of determining starting lens

There are three primary methods for determining starting lenses, they all have their unique strengths and weaknesses.

1. Habitual spectacle prescription

2. Retinoscopy

3. Autorefraction

[4.] Habitual Spectacle Prescription

Habitual spectacle prescription means exactly as it sounds, what the patient has been historically wearing.  Often times, this is the first choice of practicing optometrists choose as it is closest to what the doctor will prescribe in the end.  This was the prescription used for the entering VAs so it is the closest to the predicted refractive change.  For patients being followed for years in the same practice, this is the best option typically.   The biggest drawback to using the habitual prescription as your starting lens is when the habitual is very old (>2-3 years) or there has been a large decrease in visual acuity.

[5.] Retinoscopy

Retinoscopy allows a doctor to accurately asses the refractive error.  It is often the go to choice for refractions involving patient that cannot respond (children, stroke patients, etc.) It’s biggest weakness is that it requires significant skill to perform correctly and accurately.  For this reason, in V551 through your third year in clinic, we will use a retinoscope to determine our starting lens for refraction. 

Often, clinicians will refer to retinoscopy as an objective technique as it removes the patient’s bias and subjectivity from the process.  This is true, but the doctor’s subjective decision and bias are still included in the process.  Due to this, retinoscopy is not absolutely objective.   

[6.] Autorefraction

Autorefraction is fairly accurate at determination of astigmatism axis and power.  It often times does not control accommodation well in patients with accommodation so sphere power can lead to significant over minusing. It is often used in busy practices as a technician can perform it quickly to allow the doctor to focus on patient care.   

Autorefraction is the only truly OBJECTIVE technique.  Neither the examiner nor the patient are using subjective judgement to determine the end results. 

Below is an example printout from an autorefractor.  Most follow some similar structure.  

[7.] Putting it all together

Which ever starting lens is used, it is a first best estimate of what to use for subjective refraction.  Personally, I always choose to use their habitual lens as the starting lens in MOST cases.  It was what acuity was taken in.  It is where the patient is adapted to and what they are having complaints with.  That means your predicted changes from acuities most apply to it. 

4B. Retinoscopy Introduction

[1.] Working Distance

Working Distance: The working distance is the the length your eye is from the model/person’s eye.  This distance alters the power of the ret reflex and needs to be accounted for in the final prescription.  Typically, most people either have a working distance of 66 cm or 50 cm depending upon the length of their arms.  66 cm is for longer arms, 50 cm is for shorter arms.  These distances are set to given dioptric amounts.  66 cm converts to 1.50 diopters and 50 cm to 2.00 diopters.  This makes it easy to subtract out the working distance to reach the final prescription.  The working distance always adds more PLUS.  To correct in the final rx, subtract out your working distance.

Examples:

[2.] Working Distance Induces Negative Power

The working distance in retinoscopy is the distance the doctor’s eye is from the patient’s eye typically in centimeters.    Because light diverges from a source, the working distance induces negative power i.e. reduces the power to the retina.  The amount of power reduced is the inverse of the distance.  As an example, a 50 cm working distance would result in 2.00 diopters less power.  Look at the figure below.

In the first of these three examples, there is an emmetropic eye and the doctor is working at 50 cm.  Again, 1/0.50 m = 2.00 D.  The eye has 2.00 diopter less power so the image becomes focused behind the retina.  In the second example, it has an even shorter working distance of 25 cm.  1/0.25 m = 4.00 D.  The eye has a 4.00 diopters LESS power so the image is focused even further behind the retina.  In the third example, the normal 50 cm working distance is back in a myopic eye.  Myopic eyes have TOO MUCH power.  So, the reduced power of the working distance combined with a myopic eye results in light being perfectly focused on the retina.  This person would be a -2.00 D myope.

[2.] Normal working distances

Most people work with either a 66 cm or 50 cm working distance because these create clean/simple dioptric values to add and subtract.  Using a 66 cm distance results in a power of 1.50 D (1/0.66 m = 1.50).  Using a 50 cm distance results in a power of 2.00 D (1/0.50 m = 2.00).  The choice of which distance to use is dependent upon the doctor’s comfort and arm length.  Both are totally acceptable.  

[3.] Using working distance to calculate refractive error

In our myope example above, we showed that you can vary the working distance to determine a person’s prescription for emmetropic and myopic refractive errors (and astigmatism that is focused in front of the retina.)  Hyperopes need MORE power.  Due to the doctor (and the retinoscope) being in real space, they can never add power through the working distance.  So, it is impossible to determine a hyperope’s prescription from varying the working distance.  

Typically, we use lenses to neutralize the retinoscopy reflex, but changing distance also can. 

4C. Retinoscopy Reflex

[1] Sweeping the pupil

The retinoscope is examining the prescription axis PERPENDICULAR to its orientation.  When the retinoscopy beam is VERTICAL, the beam is sweeped across the horizontal meridian.  When the retinoscopy beam is HORIZONTAL, the beam is sweeped across the vertical meridian.

[2] With and Against Motion

It is critical that you can see with and against motion.  When the sleeve is down, with motion means you need to add PLUS to the phoropter to reach neutrality.  Against motion means you need to add MINUS to the phoropter to reach neutrality. 

[3.] The reflex as it approaches neutrality

Neutrality is the end point in retinoscopy.  It is reached when the correct prescription is reached minus the working distance.  As you approach retinoscopy, the pupil becomes more filled with light.  At neutrality the entire pupil lights up.  When you are very far away from neutrality, the entire pupil lights up, but it is very dim/faint.

[4] Retinoscopy and Axis

The axis of the prescription is determined by alignment of the ret reflex.  As can be seen below, when sweeping the beam across the pupil, the reflex is not aligned to the origination of the beam.  This is called a “skew” reflex.  The beam needs to be rotated to be parallel or in align with the reflex.  After that, set the axis of the phoropter/refractor to the new axis.

[5.] Retinoscopy and Irregular Astigmatism

Diseases effecting the optics of the eye like kerataconus (a distortion of the cornea) and cataracts alter the retinoscopy reflex.  This reflex is called “scissoring”.

4D. Autorefraction

[1.] What is Autorefraction?

Think of autorefraction as “computerized retinoscopy.” It is an objective measurement where the instrument determines one’s refractive error using light reflecting off the retinal interface.

Targets: Hot air balloon, barn at the end of the farm road, sailboat on the ocean, etc.

These instruments are constantly updating as technology continues to advance. They can provide multitude of information including refraction, keratometry, glare testing (brightness acuity testing), interpupillary distance.

Measurement ranges (estimate, spherical measurement varies slightly for differently manufacturers):

Sphere: -30D to +22D

Cylinder: 0 to +/- 10D

Axis angle: 0 to 180 degrees

Minimum pupil diameter: 2mm

[2.] Theory behind Autorefractors

Virtually all modern autorefractors use Near-Infrared Radiation (NIR) to measure the refractive error of the eye.  This is done for two reasons.  First, NIR light easily reflects off the back surface of the fundus without being absorbed by pigment of the eye like melanin and xanthophil.  This allows for more over all light to reflect back than visible sources.  Second, it is effectively invisible to the visual system, which allows it to not drive accommodation.  The biggest draw back to NIR in the eye is that it is impossible to localize what structure in the retina the light is reflecting off of.  Ideally, it would be the photoreceptor layer, as that is where vision begins.  However, NIR could reflect off the ganglion cells, or other layers of the eye. 300 microns change in axial length results in 1.00 D of change in refractive error.  The thickness of the retina is about 250 microns, so there is a potential error using NIR of 0.75 D when using it.

[3.] Understanding printout

[4.] Advantages and Disadvantages

An advantage of autorefractions is that in a busy practice, it can service as your starting point for refraction. This is critical in new patient exams, where you may not know what their refractive error might be, especially if they are presenting without any habitual glasses. In these cases you can always perform retinoscopy, however, that may add significant time to your exam. Aside from providing information about one’s refractive error, autorefractors can provide keratometry readings and perform glare testing.

Disadvantage of autorefractions is that they tend to over minus patients, especially younger patients who have high magnitude of accommodation. Additionally, anything that affects the stability and quality of light rays from entering and exiting pupil can be a source of variability in autorefractor measurements. This includes:

• Eye lids (ptosis, lashes)
• Unstable tear film
• Keratoconus
• Refractive surgeries
• Media opacities (corneal, lenticular)
• Small/irregular pupils
• Accommodative spasm
• Amblyopia
• Poor fixation

[5.] Using Autorefraction in the clinic

Different clinics utilize autorefractors as it best serves them. This includes using them to get a quick estimate of one’s refractive error. This may be performed by a technician as part of pre-testing, and is made available to the optometrist/ophthalmologist at the start of the exam. Depending on the nature of the patient visit, this can then be used as a starting point for subjective refraction or for other diagnostic purposes.

Some optometrists may elect to perform autorefraction on their patients at the start of the exam (as part of pre-testing), and AGAIN after dilation. Cycloplegic autorefraction readings will yield decreased myopia, increased internal consistency between readings and high confidence values. In younger patients, this is beneficial, as it may reveal any latent hyperopia that may have been missed pre-dilation. Some doctors elect doing this, rather than performing damp/wet retinoscopy. Some may perform both.

Knowing that autorefractors tend to over minus prescriptions, consider cutting -0.50D from the autorefraction measurements when performing subjective refraction. Autorefractors tend to be accurate for astigmatism axis and power.

Clinics may also utilize the glare testing (brightness acuity testing) capabilities of the autorefractors as part of pre-testing on their elderly patients. This provides the managing optometrist more information regarding the status and impact of cataracts in their patients.