Subjective Refraction Assistance: Phoropter Setup, Fogging, and JCC

What Is Subjective Refraction?

Subjective refraction is the process of fine-tuning a patient's prescription based on their verbal responses. It builds on the objective starting point from retinoscopy or autorefraction and refines sphere power, cylinder power, cylinder axis, and binocular balance. Although the doctor typically performs this procedure, ophthalmic assistants must understand each step to properly set up equipment, assist during the exam, and troubleshoot common issues.

Phoropter Setup

Proper phoropter setup is the foundation of an accurate refraction. Before the doctor begins:

• Level the phoropter so the apertures are aligned with the patient's eyes
• Adjust the pupillary distance (PD) to match the patient's measured interpupillary distance
• Set the vertex distance so the back surface of the phoropter lenses sits approximately 12-14 mm from the cornea
• Ensure the patient can see the acuity chart without tilting their head or straining
• Dial in the starting prescription from the objective refraction, previous glasses, or autorefraction results

A poorly positioned phoropter introduces errors before the refraction even begins. If the PD is off, the patient looks through induced prism. If the vertex distance is wrong, high-powered prescriptions will be inaccurate.

Fogging Technique

Fogging is used to relax accommodation and find the maximum plus (or least minus) sphere power that gives the best acuity. The technique works by adding extra plus power to blur the patient's distance vision, which forces the ciliary muscle to relax because accommodation would only make the blur worse.

The process:

1. Start with the objective refraction in the phoropter
2. Add +1.00 to +2.00 D of extra plus power to fog the patient (they should see approximately 20/60 to 20/80)
3. Slowly reduce plus power in 0.25 D steps, asking the patient to report when each line becomes clearer
4. Stop at the maximum plus power that achieves best corrected acuity

Jackson Cross Cylinder (JCC) Refinement

The Jackson Cross Cylinder is a lens with equal but opposite cylinder powers on perpendicular axes. It is used to refine both the axis and power of the cylindrical component of the prescription.

Refining Cylinder Axis

1. Place the JCC handle aligned with the current cylinder axis
2. Flip the JCC and ask "Which is better, one or two?"
3. Rotate the axis toward the red (minus) dot of the preferred position
4. Continue flipping and adjusting until the patient reports no difference between the two positions

Refining Cylinder Power

1. Align the JCC so its axis markings overlap the current cylinder axis
2. Flip and ask "Which is better, one or two?"
3. If the patient prefers the position where the minus axis of the JCC aligns with the cylinder axis, add more minus cylinder
4. If the opposite, reduce cylinder power
5. Continue until the patient reports equality

Remember to always refine axis before power, because an incorrect axis makes power refinement unreliable.

Binocular Balance

Binocular balance ensures both eyes are equally relaxed in terms of accommodation when viewing together. Without this step, one eye may be slightly over-plused or over-minused relative to the other, causing discomfort.

Common techniques include:

• Prism dissociation: Base-up prism before one eye separates the images, and both eyes read the same line. Plus power is adjusted until both eyes report equal blur.
• Alternating occlusion: The examiner alternates between eyes, comparing clarity and adjusting plus/minus until balance is achieved.

After binocular balance, the examiner typically performs a final binocular check at distance to ensure the prescription is comfortable with both eyes open.

Key Takeaways

• Proper phoropter setup (PD, vertex distance, leveling) prevents systematic errors before refraction begins
• Fogging relaxes accommodation by adding plus power, then reducing it to find maximum plus to best acuity
• The JCC refines cylinder axis first, then cylinder power, using a flip comparison technique
• Binocular balance ensures equal accommodative effort between the two eyes
• Each step builds on the previous one: setup, sphere, cylinder axis, cylinder power, then balance