What is the purpose of the autorefractor before a manifest refraction?

The autorefractor provides an objective, automated estimate of the patient's refractive error before the physician performs the subjective refraction. This objective starting point typically reduces the time required for subjective refraction by giving the examiner a close approximation of the final prescription. The examiner then refines this estimate through subjective testing (asking the patient which lenses look better). In adults, autorefractor readings are usually a reliable starting point; in young children or patients with poor fixation, the autorefractor may be less reliable.

Why is vertex distance important in phoropter setup?

Vertex distance is the distance from the back surface of the refraction lens to the front of the cornea. For prescriptions above plus or minus 4 diopters, changes in vertex distance change the effective power of the lens reaching the eye. During refraction, the phoropter should be positioned at the standard vertex distance of 12 to 14 mm. If the final prescription is significantly high-powered, the optical lab will adjust the calculated lens power to account for the difference between the refraction vertex distance and the final spectacle vertex distance to ensure the patient gets the correct effective power in their glasses.

What is cycloplegia and which drops cause it?

Cycloplegia is the paralysis of the ciliary muscle using anticholinergic (parasympatholytic) drops, which eliminates the eye's ability to accommodate. Without accommodation, the lens remains in its relaxed, flattest state, allowing the true refractive error to be measured without the confounding effect of accommodative tone. Common cycloplegic agents and their durations are: cyclopentolate 1% (onset in 20 to 30 minutes, lasts 12 to 24 hours), tropicamide 1% (milder cycloplegia, onset 20 minutes, lasts 4 to 6 hours, not a full cycloplegic), and atropine 1% (onset over several days, lasts up to 2 weeks; strongest cycloplegic, used for young children and high hyperopia).

What instructions should be given to a patient before sitting at the phoropter?

Before beginning phoropter refraction, explain to the patient: they will look through a large instrument with many lenses at a chart; the examiner will present lens combinations and ask which looks clearer (A or B, or 1 or 2); there are no wrong answers, and the patient should say 'about the same' if the two options look equally clear rather than guessing; they should keep both eyes open unless told to cover one; they should not squint (squinting changes the effective refractive power); and if nothing looks clear, they should say so rather than guessing. A relaxed, well-instructed patient produces more reliable refraction results.

What does 'PD' mean in the context of phoropter setup?

PD stands for pupillary distance, the horizontal measurement between the centers of the two pupils. When setting up the phoropter, the interpupillary distance control must be adjusted to match the patient's measured PD. This aligns the optical centers of the phoropter lenses with the patient's pupils, ensuring the patient is looking through the center of each lens rather than off-center. If the PD is set incorrectly, the patient looks through an off-center portion of the lens, which induces prismatic effect and distorts the refraction result.

Phoropter & Refraction Assistance for the CPO Exam

Refraction is the process of determining the lens prescription that provides a patient with the clearest vision. As a CPO, you prepare the patient and instruments for refraction, assist the physician or optometrist, and help the patient understand the process. Understanding the phoropter and refraction fundamentals is key CPO exam content.

The Phoropter

The phoropter (also called a refractor head) is a large optical instrument containing hundreds of lenses of varying powers mounted in rotating discs. When positioned in front of the patient's eyes, the examiner can quickly present different lens combinations to determine which provides the best visual acuity. The phoropter typically includes:

Sphere lenses (for myopia and hyperopia correction)
Cylinder lenses with adjustable axis (for astigmatism correction)
Prism lenses (for binocular vision testing)
Auxiliary lenses (pinholes, red/green filters, Maddox rod, polarized filters)
A pupillary distance adjustment to center the lenses over each pupil

Preparing the Phoropter for the Patient

Before the patient sits at the phoropter, the CPO typically:

Sets the phoropter to the patient's last recorded prescription (or to plano if no prescription on file). This gives the examiner a starting point.
Adjusts the interpupillary distance (PD) setting to match the patient's measured PD.
Adjusts the height of the phoropter so the patient can look comfortably through the optical centers.
Levels the phoropter horizontally.
Sets the vertex distance to approximately 12 to 14 mm from the back of the phoropter lens to the front of the cornea.
Ensures the patient is seated comfortably and can see the eye chart clearly (uncovered eye only for monocular testing).

The phoropter PD setting must match the patient's actual PD. If the PD is set incorrectly, the optical centers of the phoropter lenses will not align with the patient's pupils, inducing prism and potentially leading to an incorrect refraction result.

The Autorefractor

Before the physician begins the refraction, a CPO typically obtains an autorefractor reading. The autorefractor uses infrared light reflected from the retina to automatically calculate an objective estimate of the patient's refractive error, providing a starting prescription (sphere, cylinder, and axis) that the examiner can use as a baseline. This speeds up the refraction significantly.

Autorefractor technique:

Adjust the chin rest so the patient's eyes align with the instrument's markers.
Instruct the patient to keep both eyes open and look at the target inside the instrument (usually a hot air balloon or house scene).
Allow the instrument to autofocus and capture readings; take at least three measurements per eye.
Print or transfer the readings to the patient's chart for the examiner's reference.

Manifest vs. Cycloplegic Refraction

A manifest (dry) refraction is performed without drops, relying on the patient's natural accommodative state. This is the standard refraction for adults and older children.

A cycloplegic refraction uses drops (cyclopentolate or atropine) to paralyze the ciliary muscle and eliminate accommodation, revealing the true refractive state of the eye without accommodative tone. It is used in young children, in patients with high hyperopia who may be accommodating away their hyperopia, and when the manifest refraction seems inconsistent or the patient cannot relax accommodation.

After cycloplegic drops are instilled, the patient must wait for the drops to take full effect before the cycloplegic refraction begins: 20 to 30 minutes for cyclopentolate 1% and 40 to 60 minutes for atropine 1%. Beginning the refraction before the drops have fully cyclopleged the accommodation will give an inaccurate result.

The CPO's Role During Refraction

During the refraction itself, the CPO may:

Ensure the patient is comfortable and properly positioned.
Verify the chart is at the correct testing distance (typically 20 feet or 6 meters for distance).
Explain the task to the patient ("We're going to show you different lens combinations and ask which looks clearer. There are no wrong answers.").
Assist with near vision testing by providing a near card at the appropriate distance.
Record results in the chart as directed by the examiner.

Key Takeaways

The phoropter contains sphere, cylinder, prism, and auxiliary lenses for subjective refraction.
Set the phoropter to the patient's last Rx, correct PD, appropriate height, and 12 to 14 mm vertex distance before the exam.
The autorefractor provides an objective starting estimate of the patient's Rx before subjective refraction.
Manifest refraction uses no drops; cycloplegic refraction paralyzes accommodation to reveal true refractive error.
Cycloplegic drops require adequate waiting time (20 to 60 minutes depending on the agent) before cycloplegic refraction begins.