Advanced Anatomy & Refraction

Binocular Vision Requirements:

For comfortable single binocular vision, the eyes must:

1. Have similar visual acuity

2. Be properly aligned (no significant strabismus)

3. Have adequate fusional vergence reserves

4. Have similar image sizes (no significant aniseikonia)

Vergence System:

- Convergence — Eyes turn inward for near viewing

- Divergence — Eyes turn outward for distance viewing

- Fusional Vergence — The motor response that maintains single vision despite small misalignments

Types of Vergence:

- Tonic vergence — Baseline resting position

- Accommodative vergence — Convergence driven by accommodation (AC/A ratio)

- Fusional (disparity) vergence — Response to retinal disparity

- Proximal vergence — Response to awareness of nearness

Convergence Insufficiency (CI):

- Difficulty maintaining convergence at near

- Symptoms: eye strain, headaches, double vision at near, losing place while reading

- Common in students and computer users

- Treated with vision therapy, base-out prism, or both

AC/A Ratio:

The accommodative convergence to accommodation ratio. Normal is 3-5:1 (3-5 prism diopters of convergence per 1 diopter of accommodation). Abnormal AC/A ratios can cause near or distance phorias.

Definition:

Low vision = best-corrected visual acuity of 20/70 or worse in the better eye, or significant visual field loss. Legal blindness = 20/200 or worse, or visual field ≤20°.

Acuity Notation Conversions:

Magnification Calculation:

M = F/4 (for a simple magnifier at focal distance, 25cm reference)

- A +10.00 D magnifier = 10/4 = 2.5x magnification

- A +40.00 D magnifier = 40/4 = 10x magnification

Low Vision Devices:

- Hand magnifiers — Portable, variable working distance. Good for spot reading.

- Stand magnifiers — Fixed distance, stable. Ideal for patients with tremors.

- Spectacle-mounted magnifiers — Hands-free but fixed working distance.

- Bioptic telescopes — Mounted in spectacles for distance tasks (signs, faces).

- Electronic magnifiers (CCTV/video) — Adjustable magnification up to 60x+, contrast enhancement, various color modes.

- Tablet/phone apps — Camera-based magnification, text-to-speech.

Eccentric Viewing Training:

AMD patients lose central vision. They can be trained to use a preferred retinal locus (PRL) — a healthier area of retina adjacent to the damaged macula.

Age-Related Macular Degeneration (AMD):

- Dry AMD (90% of cases) — Drusen deposits, gradual central vision loss. No cure; nutritional supplements (AREDS2) may slow progression.

- Wet AMD (10%) — Abnormal blood vessel growth (choroidal neovascularization). Treated with anti-VEGF injections. Rapid vision loss if untreated.

- Low vision aids and magnification are the primary optician-relevant interventions.

Diabetic Retinopathy:

- Caused by damage to retinal blood vessels from poorly controlled blood sugar

- Non-proliferative (NPDR) — Microaneurysms, hemorrhages, exudates

- Proliferative (PDR) — Neovascularization, risk of vitreous hemorrhage and retinal detachment

- Annual dilated exams are critical for diabetic patients

Glaucoma:

- Open-angle (most common) — Gradual, painless IOP increase. Peripheral visual field loss progressing to tunnel vision.

- Angle-closure — Sudden onset: pain, redness, halos, nausea. Medical emergency.

- Treated with drops, laser, or surgery to reduce IOP.

Cataracts:

- Clouding of the crystalline lens. Most common cause of reversible vision loss.

- Types: Nuclear (central, yellowing), Cortical (spoke-like opacities), Posterior subcapsular (glare, near vision issues).

- Treated with surgical lens removal and IOL implantation. Medicare covers one pair of post-cataract glasses.

Keratoconus:

- Progressive corneal thinning and cone-shaped protrusion

- Causes irregular astigmatism not fully correctable with spectacles

- Managed with rigid gas permeable contacts, scleral lenses, or corneal crosslinking

- Opticians should refer for specialty fitting

Critical Period of Visual Development:

The visual system develops most rapidly from birth to age 7-8. Any disruption during this period can cause amblyopia (permanent vision reduction).

Causes of Amblyopia:

- Strabismic — Eye misalignment causes the brain to suppress one eye

- Refractive (anisometropic) — Significant Rx difference between eyes

- Deprivation — Obstruction of vision (ptosis, congenital cataract)

Treatment:

- Correct refractive error first (spectacles)

- Patching — Cover the better eye to force use of the amblyopic eye

- Atropine penalization — Blur the better eye with atropine drops

- Most effective before age 7-8; some benefit possible up to age 17

- Compliance is crucial and can be challenging with young children

Pediatric Dispensing Considerations:

- Polycarbonate or Trivex REQUIRED for impact safety

- Proper fit is critical — children are active; cable temples recommended for toddlers

- Frame size must be checked regularly as children grow

- Spring hinges increase durability

- Bright colors and character themes improve compliance