SMILE (Small Incision Lenticule Extraction) - Ophthalmology

What is SMILE?

• Kanski's Clinical Ophthalmology, 10th ed., p. 305

Indications

• Myopia (typically -1.00 D to -10.00 D)
• Myopic astigmatism (up to ~-5.00 D of cylinder in FDA-approved platforms)
• Age ≥ 22 years with stable refraction (no significant change in ≥1 year)
• Adequate corneal thickness (minimum residual stromal bed after lenticule removal)
• Patients with large pupils prone to glare/halos - SMILE induces fewer higher-order aberrations (HOAs) than LASIK
• Contact sport athletes or military/law enforcement (no flap = no flap dislodgement risk)
• Patients with dry eye concerns (faster neural recovery than LASIK)

Absolute and Relative Contraindications

Surgical Technique - Step by Step

Preoperative

• Corneal topography, pachymetry (thickness), pupillometry
• Discontinue soft contact lenses ≥1 week (rigid gas-permeable ≥3 weeks) before preop mapping
• Topical anaesthetic drops instilled

Intraoperative Steps

1. Docking - The patient fixates on a target light; the VisuMax laser cone is gently applanated to the cornea. Suction is applied to stabilize the eye. No high IOP spike (unlike LASIK microkeratome).
2. Femtosecond laser lenticule creation - The laser fires two curved lamellar cuts within the stroma:
   • Posterior lenticule interface (deeper cut)
   • Anterior lenticule interface (shallower cut, creating the "cap")
   • A small side-cut incision (~2-4 mm, usually at 120° or superior position) connects the two planes to allow instrument access
3. Lenticule dissection - A small spatula/instrument is inserted through the incision. The lenticule is separated by blunt dissection:
   • Anterior plane first (between cap and lenticule)
   • Posterior plane second (between lenticule and residual bed)
4. Lenticule extraction - The freed lenticule is grasped with forceps and removed through the incision. The surgeon confirms the lenticule is intact.
5. Irrigation - The interface is washed to remove debris. No sutures required.

Postoperative Medications

• Topical steroid (dexamethasone 0.1%) - several times daily
• Topical fluoroquinolone (moxifloxacin 0.5%) - several times daily
• Lubricating eye drops - for 1-2 weeks minimum
• Follow-up at 1 day, 1 week, 1 month, 3 months

Advantages Over LASIK

Disadvantages / Limitations vs. LASIK

• Steeper learning curve - lenticule dissection requires more surgical skill
• Slower visual recovery in the first 1 month (resolves by 3 months)
• Higher intraoperative discomfort during tissue manipulation
• No hyperopia correction (LASIK treats hyperopia)
• Retreatment is complex - options include PRK + MMC on the cap, thin-flap LASIK through the cap, or the CIRCLE approach (converting the cap to a full LASIK flap)
• Cannot use wavefront-guided or topography-guided ablation in the same way LASIK can (though newer SMILE platforms are addressing this)

Outcomes

• ~99% achieve ≥20/40 UCVA at 6 months
• ~88% achieve 20/20 UCVA at 6 months

Complications

Intraoperative

• Lenticule creation errors: incomplete cut, opaque bubble layer (OBL), suction loss
• Dissection errors: lenticule tear (partial/complete), tissue fragmentation - can require conversion to PRK or LASIK
• Extraction errors: residual lenticule fragments left behind

Postoperative

• Refractive: under-correction, over-correction, regression, residual astigmatism
• Interface issues: diffuse lamellar keratitis (DLK), interface debris, epithelial ingrowth (rare without flap)
• Dry eye (less than LASIK, but present)
• Halos and glare (especially early; typically resolves)
• Ectasia (rare, from insufficient residual stromal bed - same risk factor as LASIK)
• Infection/keratitis (rare)

SMILE vs. Other Refractive Procedures at a Glance

Retinoscopy

Definition

The Retinoscope

• An illumination source (halogen bulb)
• A condensing lens that can be moved to switch between plane mirror and concave mirror modes (most modern instruments work in plane mirror mode by default)

Optical Principles

Key rule:

• With motion → add plus (+) lenses to neutralize
• Against motion → add minus (-) lenses to neutralize
• No motion (neutral reflex) → the far point is at the examiner's eye = neutralization achieved

Working Distance Allowance

• At 67 cm → subtract -1.50 D (since 1/0.67 = 1.5 D)
• At 50 cm → subtract -2.00 D (since 1/0.5 = 2.0 D)
• At 1 m → subtract -1.00 D

Types of Retinoscopy

1. Static Retinoscopy (most common)

• Patient fixates on a distant target (6 m / 20 ft) to relax accommodation
• Examiner neutralizes the reflex meridian by meridian
• Gives the objective refractive error

2. Dynamic Retinoscopy

• Patient fixates on a near target held at the working distance
• Used to assess accommodative response
• The adequacy of cycloplegia can also be checked by comparing distance vs near retinoscopy readings - Kanski's Clinical Ophthalmology, 10th ed.

3. Cycloplegic Retinoscopy

• Performed after cycloplegic drops are instilled to paralyze accommodation
• Essential in children, first eye exams, strabismus, amblyopia, and anisometropia
• Cyclopentolate 1% is the standard agent (0.5% under 6 months); maximal cycloplegia in 30 minutes
• Atropine 1% used for stronger cycloplegia (high hyperopia, heavily pigmented irides); twice daily for 1-3 days prior
• Kanski's Clinical Ophthalmology, 10th ed.

4. Mohindra (Near) Retinoscopy

• Used when cycloplegia is contraindicated or unavailable
• Retinoscope light acts as near fixation stimulus
• Final value = objective finding minus 1.25 D (accounts for working distance + residual accommodation at near)

Technique - Step by Step

1. Setup: Dim the room. Patient looks at a large distant target (6/60 or larger). Set phoropter/trial frame to plano or best estimate.
2. Examiner position: Sit/stand at chosen working distance (~67 cm). Align with patient's visual axis.
3. Sweep the streak: Orient streak vertically → sweep horizontally (examining horizontal meridian). Note reflex direction and quality.
4. Assess each principal meridian:
   • Observe reflex motion (with / against / no motion)
   • Note width, brightness, and speed of the reflex:
     • Narrow, bright, fast reflex = close to neutralization
     • Wide, dull, slow reflex = far from neutralization
5. Neutralize each meridian:
   • Add + or - lenses until no motion is seen in that meridian
   • Rotate streak to 90° and neutralize the perpendicular meridian
6. Astigmatism: If the two meridians neutralize at different powers → cylindrical correction needed
   • The axis is set parallel to the streak orientation at neutralization
   • A break in the reflex (the streak inside the pupil is angled differently from the external streak) indicates the axis of astigmatism
7. Subtract working distance allowance to obtain net refraction

Reflex Characteristics Summary

Keratoconus: Retinoscopy shows an irregular scissoring reflex; direct ophthalmoscopy from 50 cm shows an oil droplet reflex. - Kanski's Clinical Ophthalmology, 10th ed., p. 224

Astigmatism in Retinoscopy

• Two principal meridians have different neutralization points
• Identified by:
  • Break: streak inside pupil is not aligned with external streak
  • Tilt: streak appears tilted at oblique meridians
• Correct using:
  • Spherical lens to neutralize the first meridian
  • Cylindrical lens (axis parallel to the streak at that meridian) to neutralize the second
• The difference between the two meridional powers = the cylinder power

Clinical Uses Beyond Refraction

• Keratoconus screening: scissor reflex
• Cataract assessment: reduced/absent red reflex
• Amblyopia workup: objective refraction in pre-verbal children
• Strabismus: cycloplegic retinoscopy to detect latent hyperopia driving accommodative esotropia
• Accommodative dysfunction: dynamic retinoscopy reveals lag/lead of accommodation
• Post-refractive surgery monitoring: irregular reflexes in ectasia

Advantages

• Completely objective - no patient cooperation needed
• Works in infants, cognitively impaired, non-verbal patients
• Portable - can be used in field, mission trips, screening camps (with loose lenses/skiascopy bar)
• Provides qualitative corneal information (irregular reflexes)
• Can assess accommodation objectively

Q13: Enumerate Types of Keratorefractive Surgery

Classification of Keratorefractive Surgeries

TYPE 1 - Incisional Procedures

RK was the dominant refractive procedure before the laser era. It has largely been replaced by laser procedures but is still asked about in exams.

TYPE 2 - Laser (Photoablation) Procedures

TYPE 3 - Non-laser Thermal / Other Procedures

Simple Exam-Ready Classification (3 types)

1. INCISIONAL   → Radial Keratotomy, Astigmatic Keratotomy
2. LASER        → PRK, LASIK, LASEK, SMILE
3. THERMAL      → CK, LTK

1. Subtractive  → Laser procedures (remove tissue)
2. Incisional   → RK, AK (cut/weaken tissue)
3. Additive     → Epikeratophakia, INTACS (add tissue)

Key Comparison Table