8.4 Near Add Determination, Vertex Recording & Refraction in Difficult Situations

Determining the near add at the phoropter

The distance refraction must be finalised first — a near add is always added to a correct distance correction, never used to compensate for an incorrect one.

1. Start from the age expectation (roughly +1.00 D at 45, +1.50 D at 50, +2.00 D at 55, +2.50 D at 60 and above).
2. Check against the half-amplitude rule: measure the amplitude of accommodation, leave half in reserve, and prescribe the difference between the working distance demand and the usable half.
3. Verify with the near range of clear vision: with the trial add in place, ask the patient to move the near card in and out and report where print first blurs each way. The working distance should sit near the middle of that range.
4. Refine with a binocular cross cylinder at near if the instrument has one, or simply by adding and subtracting 0.25 D and asking for a preference on near print.
5. Confirm the actual task. A patient who reads music at 70 cm, works a bench at 35 cm or uses two monitors at different distances may need a different add, a trifocal, a progressive or dedicated occupational spectacles.

Red flags at the near add step:

• Add above +3.00 D in a routine presbyope — suspect an incorrect distance sphere (usually under-plussed) or an unmet low vision need.
• Unequal adds between the eyes — almost always an error in the distance refraction or the amplitude measurement.
• A 30-year-old requiring a near add — investigate accommodative insufficiency, convergence insufficiency, medication effects, or systemic disease rather than simply prescribing.

Measuring and recording vertex distance

Vertex distance is measured from the back surface of the lens to the front of the cornea, conventionally 12 to 14 mm in spectacles.

Method with a distometer: the patient closes their eyes, the fixed foot of the instrument rests on the closed lid, the moving arm contacts the back of the lens, and the scale reads the vertex distance with an allowance for lid thickness. Alternatively, view the patient from the side through the slit lamp or use a millimetre rule against a profile view.

When to record it: whenever any meridian exceeds ±4.00 D. A high-power refraction performed at a phoropter's vertex and then dispensed in a frame sitting at a different distance will be the wrong power, and the laboratory cannot correct for a distance it was never told.

Related fitting parameters to record for high powers: pantoscopic tilt (conventionally 8 to 12 degrees), face-form wrap, and the optical centre height, because each alters the effective correction.

Refraction in difficult situations

Keratoconus and irregular corneas. Expect scissoring on retinoscopy and unstable, unreliable autorefraction. Spectacle refraction often plateaus well short of 20/20. The productive step is an over-refraction across a rigid or scleral trial lens, because the tear lens behind the rigid lens neutralises the irregular anterior surface. Record spectacle best-corrected acuity and the rigid-lens acuity separately, since the difference is exactly the argument for a specialty lens fit.

Post-refractive-surgery corneas. LASIK, PRK and SMILE create a multifocal cornea with a flatter central zone and a transition ring. Autorefractors average across that profile and can be badly wrong. Use retinoscopy and careful subjective refinement; topography helps explain the residual symptoms. Patients often report good chart acuity but poor night vision and glare, which is an aberration problem rather than a refractive one.

Dense media opacity. Retinoscopy may be impossible. Use the potential acuity meter and the patient's spectacle history, and record explicitly that a reliable refraction could not be obtained.

Nystagmus. Full occlusion typically worsens the nystagmus and underestimates acuity. Fog the fellow eye with about +5.00 D instead of occluding it, allow the patient's null-point head posture, and record both monocular and binocular results. A trial frame is better than a phoropter because the patient can adopt their head posture freely.

Low vision. Use large-step bracketing — compare ±0.50 D or ±1.00 D rather than ±0.25 D, because a patient at 20/200 cannot discriminate quarter-dioptre steps. Use a trial frame, a bright well-contrasted target, and a shortened test distance recorded explicitly.

Children and non-communicative patients. Cycloplegic retinoscopy is the reference method. For a child, use the loose lens (lens rack) technique with a trial frame rather than a phoropter, keep the interaction brief and use a fixation target that holds attention.

Malingering or functional visual loss. Techniques include refracting with a plano trial lens described as powerful, fogging one eye while claiming to test the other, and checking whether stereoacuity and the optokinetic response are consistent with the claimed acuity. The technologist documents objective observations and leaves interpretation to the clinician.

Phoropter versus trial frame

Rule of thumb: any change greater than about 0.75 D of sphere, any cylinder axis change over 10 degrees in a meaningful cylinder, or any first-time cylinder should be trial framed and walked before it is dispensed. Five minutes at the trial frame prevents a remake.

Final documentation checklist

• Manifest or cycloplegic, with agent, concentration and time if cycloplegic
• Sphere, cylinder, axis for each eye in a stated convention
• Best-corrected acuity for each eye and binocularly
• Near add and near acuity, with the test distance
• Vertex distance for any meridian over ±4.00 D
• Interpupillary distance, distance and near
• Any prism, with base direction
• Notes on reliability, fixation and cooperation