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Ophthalmology > Day 6 (3): Basics of Excimer Laser Refractive Surgery > Flashcards

Day 6 (3): Basics of Excimer Laser Refractive Surgery Flashcards

1
Q

What is Refractive Surgery?

A
  • OPTIONAL eye surgery to improve the refractive state of the eye
  • Surgery on a NORMAL eye
  • Most common methods today use excimer lasers to reshape the curvature of the cornea
  • Goal:
    1. Decrease or eliminate dependency of ametropic patients on glasses or contact lenses
    2. Patient satisfaction, not necessarily 20/20 vision
  • Advantages:
    1. Quick
    2. Minimally invasive
    3. Precise
    4. Minimal downtime
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2
Q

What are examples of refractive surgery procedures?

A
  1. Photorefractive Keratectomy (PRK)
  2. Laser-Assisted Sub-Epithelial Keratectomy/Laser Epithelial Keratomileusis (LASEK)
  3. Laser-Assisted In-Situ Keratomileusis
  4. Astigmatic Keratotomy/Limbal Relaxing Incisions
  5. Intracorneal Ring Segments
  6. Orthokeratology
  7. Phakic IOLs
  8. Multifocal or Accommodating IOLs
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3
Q

What is an excimer laser?

A
  • Laser with short wavelengths in the ultraviolet range (150 - 300 nm)
  • Excited dimer: formed by the reaction between a halide and an inert gas (e.g. Argon-Fluoride)
  • Electrical energy is used to stimulate Argon to form dimers with Fluoride gas, releasing light as a byproduct with wavelength of 193 nm and 6.4 eV per photon.
  • Photons formed are able to break C-N or C-C bonds of collagen thus it’s utility in reshaping the cornea
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4
Q

How does laser vision correction work?

A
  • Excimer laser is used to sculpt and recontour the corneal surface for correction of refractive errors.
  1. Myopia: excess converging power due to more curved cornea
    - flatten the cornea by removing the central cornea tissue
  2. Hyperopia: lack of converging power due to flatter cornea
    - steepened by removing donut-shaped mid-peripheral tissue
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5
Q

What is the Munnerlyn’s Formula?

A

Ablation depth (um) = [(diameter in mm)^2 x D]/3

  • Used to calculate the ablation depth in MYOPIC corrections
  • Thickness of the tissue ablated (um) is equal to the square of the diameter of the optical zone (mm) multiplied by the dioptric correction (spherical equivalent) and divided by three.
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6
Q

What are the common reasons for undergoing refractive surgery?

A
  1. Other refractive corrections are inconvenient and associated with more risks

SCL: giant papillary conjunctivitis, corneal hypoxia, infiltrates, pannus, infections

Spectacles: poor peripheral vision esp. with higher grades

  1. Pt just do not want to wear corrective lenses anymore
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7
Q

Why is it important to set expectations when undergoing refractive surgery?

A

Unfulfilled patient expectations: leading cause of dissatisfaction

  • avoid unrealistic expectations: guaranteed 20/20 vision, perfect vision, clear distance and near even though > 40 yo with presbyopia
  • 20/20 is not equal to 20/happy
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8
Q

What are the pertinent non-ocular findings to elicit in the history?

A
  1. Profession and visual requirements
  2. Daily and recreational activities
  3. Comorbidities:
    - connective tissue disease: prone to dry eyes and poor healing
    - immunocompromised state: prone to infection
  4. Medication intake:
    - systemic steroids: prone to infection, posterior subcapsular cataract
    - chemotherapeutics: prone to infection
    - isotretinoin: prone to dry eye disease and poor wound healing
    - sumatriptan/HRT: prone to delayed epithelial healing in PRK
    - antihistamines: prone to dry eye disease
  5. Pregnancy or lactation status
    - pregnancy may cause mild corneal edema, increase in corneal thickness and transient changes in refraction
    - reexamine after 3 months
  6. Uncontrolled diabetes: affects stability of refraction
  7. Presence of cardiac pacemakers and implantable defibrillators
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9
Q

What are the pertinent ocular findings to elicit in the history?

A

History of:

  1. Dry eyes
  2. Blepharitis
  3. Recurrent corneal erosions
  4. Retinal tear/detachment
  5. Glaucoma
  6. Ocular medications
  7. Refraction stability: stable within 0.5 D for 1 year or more
  8. History of previous corrections (spectacles, RGP, SCL)
SCL: discontinue for 3 - 7 days
RGP: prone to corneal warpage
- discontinue for 6 - 8 weeks
- 1 week/year of wear
- do serial corneal topography and refraction until cornea is stable
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10
Q

What is presbyopia?

A
  • Decreased ability to accommodate associated with aging resulting in progressively worsening near vision
  • Important consideration in patients with myopia approaching the age of 40 (capability for near vision is decreased in a patient only accustomed to near vision)

Goal: Monovision

  • dominant eye is corrected for distance (D-D)
  • non-dominant eye is corrected for near (N-N)
  • target: - 1.50 to - 1.75 D correction
  • simulated with spectacles or SCL
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11
Q

Components of the preoperative examination prior to refractive surgery?

A
  1. Visual Acuity
    - Uncorrected VA (distance and near)
    - Best Corrected VA (distance and near)
    - Refraction (Manifest/Dry or Cycloplegic/Wet)
  2. Contrast Sensitivity
  3. Pupillary Examination
    - Photopic: under bright conditions
    - Scotopic: under dark conditions
    - (+/-) RAPD
  4. Ocular Anatomy
    - small palpebral fissures or prominent eyebrows: device might not fit leading to difficulty in flap creation
  5. Ocular Motility
    - asymptomatic tropia/phoria: may decompensate and become symptomatic post-op; simulate with SCL to test
  6. Intraocular Pressure
    - screening for glaucoma
    - flap creation: temporary increase of IOP
    - steroids post-op: increase risk of glaucoma
    - thin cornea: false low IOP (pliable cornea)
  7. Slit-Lamp Exam: ocular adnexa to anterior vitreous
    - if (+) cataract: remove first prior to refractive surgery
  8. Dilated Fundoscopy
    - optic nerve: glaucoma, optic nerve atrophy
    - retina: holes/tears/detachment, lattice degeneration
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12
Q

Scotopic vs Photopic

A

Scotopic: vision under low light conditions

  • “skotos”: darkness and “opia”: sight
  • ROD cells only; cone cells NOT in use
  • black and white vision

Photopic: vision under bright conditions

  • CONE cells used: (+) color perception
  • better visual acuity and temporal resolution
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13
Q

Define the following:

A

Guttata: droplet-like accumulations of nonbanded collagen on the posterior surface of the Descemet’s membrane

Corneal Pannus: growth of fine blood vessels onto the clear corneal surface

Superficial Punctate Keratitis: inflammation of the corneal epithelium

Epithelial BM Dystrophy: extra sheets of BM extend into the epithelium trapping maturing epithelial cells and forming cysts

Keratoconus: progressive thinning of the cornea

Staphyloma: abnormal protrusion of uveal tissue through a weak point in the globe presenting as a circumscribed outpouching

Pellucid Marginal Degeneration: crescent-shaped band of inferior corneal thinning involving approx. 20% of thickness while the central cornea remains normal

Ocular Cicatricial Pemphigoid: autoimmune conjunctivitis leading to cicatrization or scarring

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14
Q

What ancillary tests may be performed prior to refractive surgery?

A
  1. Corneal Topography
    - Irregular astigmatism: not good candidates due to unpredictable refractive outcomes and progressive ectasia after LASIK
    + keratoconus/forme fruste keratoconus
    + pellucid marginal degeneration
    + corneal warpage secondary to prolonged RGP lens use
    - Excessively steep/flat corneas: increased risk of flap contractures
  2. Pachymetry (Ultrasound, Optical, Pentacam)
    - measures central corneal thickness (“pachy”)
    - calculation of anticipated residual stromal bed thickness
    - post-op residual stromal bed: should be > 250 um
    - thin: corneal ectasia
    - thick: poor endothelial function –> Fuch’s Endothelial Dystrophy
    - contraindication: CCT < 500 um
  3. Specular Microscopy
    - measure endothelial parameters (cell density, size and shapes)
    - post-op endothelial cell count: > 1000
  4. Biometry (IOL Master/Ocuscan)
    - measure axial length
  5. Hartmann- Shack Aberrometry
    - for wavefront analysis using Zernike polynomials
    - measures both lower- and higher-order aberrations
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15
Q

Differentiate Lower-Order from Higher-Order Aberrations.

A

Lower-Order

  • myopia, hyperopia
  • correctible by lenses

Higher-Order

  • glare, flare, haloes
  • not corrected by lenses
  • corrected by LASIK or refractive surgery
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16
Q

Absolute contraindications to refractive surgery.

A

Ocular:

  1. Clinical Keratoconus
  2. Severe ocular surface disorders with epithelial breakdown
    - SJS, OCP, Chemical burns
  3. Exposure Keratopathy

Systemic:

  1. Autoimmune diseases
    - rheumatoid arthritis, systemic lupus erythematosus, polyarteritis nodosa, collagen vascular diseases
  2. Immunosuppressed/Immunocompromised states
  3. Other systemic illnesses that might affect wound healing
17
Q

Relative contraindications to refractive surgery.

A

Ocular:

  1. Unstable refraction: determine etiology first
  2. Irregular astigmatism: poor candidate
  3. One-eyed
  4. Active HSV keratitis
  5. Active HZ ophthalmicus
  6. Active or recurrent ocular disease
  7. Previous ocular surgeries
  8. Depressed corneal scars

Systemic:

  1. Uncontrolled diabetes mellitus: unstable refraction
  2. Clinical atopy: predisposed to poor wound healing
  3. Pregnancy/lactation: wait for 3-6 months for transient corneal edema to subside
18
Q

Components of Informed Consent

A
  1. Risks
  2. Benefits
  3. Complications
  4. Alternative treatments
19
Q

Important issues to discuss with the patient regarding refractive surgery.

A
  1. Expected UCVA: 20/20 vs 20/happy
  2. Possible over/undercorrection
  3. Issue of presbyopia: reading adds or monovision
  4. Possible higher order aberrations: glares, haloes, decreased contrast sensitivity
  5. Risk of corneal infections
  6. Possible complications:
    - Diffuse Lamellar Keratitis: inflammatory infiltrates beneath the corneal flap interface
    - Epithelial ingrowth
    - Regression
20
Q

Components of patient preparation prior to refractive surgery

A
  1. Discontinue contact lens wear
    - SCL: 3 - 7 days prior
    - RGP: 6 - 8 weeks prior
    + 1 week per year of wear
    + serial topography and refraction to confirm stability of refraction
    + option: switch to SCL then spectacles 1 week prior
  2. Prophylactic antibiotics: optional
  3. Dry eyes
    - treat first with lubricant, steroid or cyclosporine eye drops
    - done to determine ACTUAL refractive error
21
Q

What is Laser In-Situ Keratomileusis?

A
  • Involves creating a corneal flap using a microkeratome or a femtosecond laser, reshaping the cornea by ablating the underlying stromal bed using an excimer laser and then replacing the flap.
  • Variations:
  1. Wavefront-Guided LASIK
    - the excimer laser ablates a sophisticated pattern based on measurements from a wavefront aberrometer
    - achieve a more optically perfect ablation based on all of the optical aberrations measured not just sph and cyl refractive errors
  2. Femtosecond LASIK
    - infrared range
    - corneal incision created by delivering laser pulses at a predetermined depth in the cornea
    - cause microphotodisruption of collagen and formation of an expanding bubble of CO2 gas and water that cleave the tissue, creating a plane of separation
    - advantages:
    + safer
    + more predictable and uniform flap thickness
    + precise flap dimension and location
    + decreased risk of flap-related complications
    + better predictability of refractive outcomes
    + lower enhancement rates
    - disadvantage: transient light sensitivity syndrome with 1st generation femtosecond lasers
22
Q

Consideration in flap creation, size and thickness

A

+ Cut with either mechanical microkeratomes or femtosecond lasers:

  1. Mechanical microkeratomes
    - cut depths: 120 - 180 µm
    - trend: thinner flaps with the newer models (more precise)
  2. Femtosecond lasers
    - cut depths: 100 - 120 µm
    - more precise and uniform flap thickness

+ Considerations:

  1. Type of refractive error
    - hyperopia: needs larger flaps
    - (+) higher order aberrations: use wavefront-guided LASIK
  2. Anatomy: corneal diameter and curvature, palpebral fissure
  3. Microkeratome suction ring dimensions
  4. Surgeon’s preferences: flap centered on pupil or off-center
  5. Flap thickness: 140 - 160 um
    - residual posterior stromal thickness: at least 250 μm
    - others: stromal bed at least half of the original corneal thickness
    - thinner flaps: preserve greater stromal bed thickness and reduce the risk of post-LASIK ectasia
    - thicker flaps: less prone to complications (dislocation, striae, buttonholes, incomplete flaps)
    - measured by intraoperative ultrasound pachymetry or AS-OCT
    - thickness can deviate significantly from the nominal setting and routine measurement can evaluate actual thickness obtained
  6. Hinge location
    - NASAL: corneal nerves partially intact in the hinge even if temporal side is cut; less denervation and less dry eye symptoms
    - SUPERIOR: less prone to flap displacement due to effect of gravity; prone to DED because of denervation from cutting of nerves from both sides
  7. IOP determination: 65 mmHg
    - confirm with Barraquer applanation tonometer or palpation
    - avoid pseudosuction
    - low IOP: thin or incomplete flap
23
Q

Steps in doing LASIK

A
  1. Check excimer laser, suction ring, microkeratome and blade (or femtosecond laser settings)
  2. Confirm correct treatment data are entered into the laser computer
  3. Cover fellow eye and anesthetize the operative eye
  4. Place eyelid speculum
  5. Mark the cornea to aid in postoperative flap alignment
    - (+) astigmatism >/= 0.50 D: mark axis while sitting up
  6. Placement of suction ring to achieve fixation
  7. Creation of corneal flap: microkeratome or femtosecond laser
  8. Flap is reflected away from the cut surface
  9. Excimer laser ablation: centered on the pupil or corneal vertex
    - may use eye-tracker and iris registration technology to ensure
    well-centered laser treatment
  10. Irrigation to remove debris
  11. Replacement of flap: align corneal markings and gutters
  12. Drying of flap
24
Q

LASIK post-operative care

A
  1. Patching for 15 minutes immediately post-op
  2. Check for flap displacement or striae: repair ASAP
  3. Mild discomfort may last for 4 to 6 hours
    - keep eyes closed and rest or take a nap
  4. Avoid rubbing of eyes after the surgery
  5. Steroid and antibiotic drops QID x 4 to 10 days (ave: 1 week)
  6. Preservative-free lubricants every 3 - 4 h or PRN x 1 - 6 mos
    - depending on dry eye symptoms and corneal punctate staining
  7. Follow-up: 1 day post-op –> 1 week –> 1 month –> 3 months
    - -> 6 months –> 12 months
  8. Refractive stabilization for myopes
    - up to 3 months depending on the amount of treatment done
25
Q

Common postoperative LASIK complications

A
  • Serious adverse complications leading to significant permanent visual loss occur rarely

Flap complications:

  • if flap is irregular, incomplete, or buttonholed, laser treatment CANNOT be safely performed in the same session
  • secondary LASIK or PRK procedure may be performed in some cases after a healing period
  • extremely rare when using a femtosecond laser
  • common after trauma
  1. Free Cap: hinge of the corneal flap detaches
    - most common cause: use of a microkeratome on flat corneas (predisposed to small flap diameter)
  2. Displaced/Slipped Flap
  3. Incomplete Flap: flap creation ends before reaching the intended hinge position
    - flap not large enough to allow for laser ablation
  4. Buttonholed Flap: caused by an abnormal lamellar cut
    - performing excimer laser ablation can result in an irregular corneal surface and flap-stromal bed contour mismatch
  5. Wrinkled Flap
  6. Striae/folds: micro (no intervention) vs macro

Interface complications between flap and stromal bed:

  1. Interface debris
  2. Epithelial ingrowth: between flap and corneal bed
  3. Persistent epithelial defect (abrasions, erosions)
  4. Diffuse lamellar keratitis (with or without scarring)
  5. Infection

Subjective/Functional:

  1. Dry eyes: most common complication; normalizes after 6 months
  2. Higher-order aberrations:
    - nighttime starbursts: concentric rays or fine filaments radiating from bright lights
  3. Decreased contrast sensitivity

Long-term complications:

  1. Post-LASIK ectasia: too much tissue ablated leading to alteration of corneal biomechanical stability
  2. Ischemic Optic Neuropathy
  3. Steroid-induced complications (stromal keratitis or glaucoma due to IOP elevation, posterior subcapsular cataracts, HSV keratitis)
26
Q

Advantages and disadvantages of LASIK.

A

Advantages:

  1. Little to no post-operative pain and discomfort
  2. Quick visual recovery: as early as 1st day post-op
  3. Less risk of steroid-induced complications: shortened use

Disadvantages:

  1. Risk of flap complications especially following trauma
  2. Higher risk of dry eyes: denervation due to cutting of corneal nerves when creating the flap
  3. Higher risk of ectasia: flap creation involves cutting into and ablating the deeper layers of the cornea which can weaken corneal biomechanical stability esp. in thin corneas
  4. Higher-order aberrations: due to the flap
27
Q

What is Photorefractive Keratectomy?

A
  • Correction of refractive errors using an excimer laser to ablate the anterior stroma and reshape the cornea WITHOUT creating a corneal flap

Candidates:

  1. Narrow palpebral fissures: difficulty creating flap
  2. Prominent brows and deep-set eyes: difficulty creating flap
  3. Thin corneas: residual stromal bed may be too thin
  4. (+) Epithelial BM Dystrophy
  5. (+) Irregular astigmatism/topography: not amenable to LASIK
28
Q

Steps in doing PRK

A
  1. Anesthetize operative eye
  2. Insert ocular speculum
  3. Instruct patient to focus on a target light
  4. Epithelial debridement via:
    - manual scraping with blunt knife
    - 20% alcohol
    - Amoils epithelial scrubber
  5. Excimer laser ablation of anterior corneal stroma
  6. Epithelial layer left to heal spontaneously in 3-4 days

Post-operative Care

  1. Bandage contact lens:
    - protect the cornea as epithelium grows back
    - helps decrease discomfort
  2. Steroid with antibiotic drops: prevent infection, scar formation and corneal haze; prolonged use over months
  3. Preservative-free lubricants: decrease discomfort from dry eye
  4. Topical NSAID drops: decrease post-op pain during first 3 days
  5. Oral NSAIDs if needed
29
Q

Advantages, disadvantages and complications of PRK.

A

Advantages:

  1. No risk of flap complications
  2. Less dry eye symptoms: no denervation due to nerve dissection
  3. Less risk of ectasia: ablation is more superficial and greater portion of cornea is left untouched
  4. Avoids higher-order aberrations caused by LASIK flap
  5. Extra margin of safety in patients with unusually-shaped corneas: more of the cornea is left untouched

Disadvantages:

  1. Prominent post-operative pain: mild to moderate FB sensation
  2. Longer visual recovery: 7 - 10 days or longer
  3. Higher risk of steroid-induced complications: prolonged use

Complications:

  1. Tearing, light sensitivity and haze
    - worst on the 2nd and 3rd post-op day but gradually improves as the epithelium grows back
  2. Persistent epithelial defects
  3. Infection
  4. Steroid-induced complications (elevated IOP, posterior subcapsular cataracts, HSV keratitis)
30
Q

What is an Enhancement procedure?

A
  • retreatment procedure for correction of residual refractive error or if the outcome of the initial procedure does not meet expectations
  • done after stabilization ~ 3 months post-op
  • either:
    + LASIK: lifting the created flap and reablating the stromal bed
    + PRK: removal of epithelium and reablating the stromal bed
  • do preoperative OCT/intraoperative ultrasound pachymetry to ensure preservation of at least 250 μm of residual bed thickness
31
Q

PRK vs LASIK vs LASEK vs Epi-LASIK

A

Similarities:

  1. ALL reshape the cornea to correct refractive errors
  2. ALL use excimer laser to ablate the anterior stromal bed

LASIK: LASer In-situ Keratomileusis
- (+) corneal (epithelium + anterior stroma) flap of PRECISE dimensions

PRK: Photorefractive Keratectomy

  • (-) corneal flap
  • epithelium NOT preserved: mechanically removed

LASEK: LASer Epithelial Keratomileusis

  • more superficial LASIK
  • (+) epithelial flap created by soaking epithelium in a 20% ALCOHOL SOLUTION
  • flap repositioned after ablation
  • epithelium NOT VIABLE
  • alcohol can damage limbal stem cells
  • not done anymore: epithelial layer heals faster if flap is removed

Epi-LASIK:

  • more superficial LASIK
  • (+) epithelial flap created by scraping epithelium with an EPI-KERATOME
  • flap repositioned after ablation
  • epithelium remains PARTIALLY VIABLE
  • limbal stem cells undamaged
  • not done anymore: epithelial layer heals faster if flap is removed
32
Q

What is ReLEx Smile procedure?

A
  • Uses a femtosecond laser to cut a high-precision, thin, circular disc of stromal tissue that is removed via a small incision in the cornea

Advantages:

  1. Similar safety profile to PRK: if LASIK is contraindicated
  2. BUT with quicker healing time and vision recovery
  3. Less risk of ectasia: anterior corneal surface untouched

Disadvantage: visual acuity is not as crisp as LASIK

Ophthalmology (44 decks)

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