Objective vs. Subjective Refraction
Objective refraction determines a patient's refractive error without relying on their verbal responses. This is the foundation of the refraction process and is especially valuable for young children, cognitively impaired patients, or anyone who cannot reliably respond to subjective questions. Two main techniques exist: retinoscopy and autorefraction. As a CPO, you will routinely perform autorefraction and may observe or assist with retinoscopy.
Retinoscopy
Retinoscopy is a technique performed by the physician or skilled clinician using a streak retinoscope, an instrument that projects a streak of light into the patient's eye. By observing the movement of the light reflex in the pupil as the streak is swept across different meridians, the examiner can determine the direction and amount of refractive error.
With vs. Against Movement
- With movement: the reflex moves in the same direction as the streak sweep, indicating undercorrected hyperopia or insufficient plus power. Add plus lens power to neutralize.
- Against movement: the reflex moves opposite to the streak sweep, indicating myopia or excess plus power. Add minus lens power to neutralize.
- Neutralization: when the reflex fills the entire pupil with no discernible movement, the working distance correction has neutralized the refractive error.
Autorefraction
An autorefractor is an automated instrument that measures refractive error objectively using infrared light. The patient rests their chin in the instrument and fixates on an internal target (often a balloon or house image on a road) while the device analyzes the reflected light pattern from the retina.
How to Perform Autorefraction
- Position the patient comfortably at the chin and forehead rest
- Align the instrument using the joystick until the pupil is centered
- Ask the patient to keep both eyes open and blink normally
- Capture 3-5 measurements per eye; the instrument averages them
- Print or record the result: sphere, cylinder, axis, and pupil size
Interpreting the Autorefractor Printout
The autorefractor printout typically shows:
- S: sphere power (D)
- C: cylinder power (D)
- A: axis (degrees)
- PD: pupillary distance (mm)
- SE: spherical equivalent (sphere + half the cylinder)
The result is a starting point for subjective refraction, not a final prescription. Autorefraction can vary by ±0.50 D from the true refraction. It should always be refined subjectively.
Keratometry Data
Many autorefractors also measure keratometry (corneal curvature) and report it alongside the refraction data. The K readings (flat and steep meridians) are useful for contact lens fitting and surgical planning.
Key Takeaways
- Objective refraction (retinoscopy or autorefraction) does not require patient responses
- Retinoscopy: with movement = add plus; against movement = add minus; neutralization = endpoint
- Subtract working distance lens (−1.50 at 67 cm, −1.00 at 1 m) from retinoscopy findings
- Autorefraction: 3-5 measurements averaged per eye; instruct patient not to squint
- Autorefraction is a starting point for subjective refraction, never a final prescription