Refractive

Question 0

You are considering placing a toric intraocular lens at the time of cataract surgery on a patient with the following topography refraction: -2.00 +2.25 x 90. Which of the following astigmatic lens powers of the Alcon AcrySof toric IOL would correct her astigmatism best? (Please assume that all of the astigmatism is corneal and there is no contribution from the posterior corneal curvature)

Answer 0

Remember that cylinder at the spectacle plane translates to more cylinder at the intraocular lens plane. The 3 diopter Alcon AcrySof toric IOL will only correct +2.06 D of astigmatism at the spectacle plane. This value is empirically-derived.

Note that you generally do not want to overcorrect astigmatism which can lead to flipping of the axis 90 degrees. Most refractive surgeons will never flip the axis from with-the-rule astigmatism (WTR) to against-the-rule astigmatism (ATR). Some surgeons do not mind as much switching the axis from ATR to WTR with the thinking that patients tolerate WTR astigmatism better.

This patient has WTR astigmatism (i.e. plus cylinder in 90 degree axis). Therefore, you would not want to flip his axis (e.g. with the 3.75D toric lens).

Question 1

nontuberculous mycobacteria keratitis (NTM)

Answer 1

“nontuberculous mycobacteria” = all Mycobacteriaceae species besides M. tuberculosis and M. leprae.

Ex: a post-LASIK patient presenting with a delayed (e.g. 2-10 weeks after) corneal infiltrate with feathery borders* that is recalcitrant to typical antibiotic treatment.

The main organisms: M. chelonae (majority of post-LASIK) and M. fortuitum.

*for the purposes of the boards, LASIK + delayed feathery lesion = NTM keratitis. Like fungal keratitis, NTM keratitis can also present with satellite lesions.

DDx:

1) DLK (diffuse lamellar keratitis) - different b/c early (1st 24 hours s/p procedure & only @ flap interface) & quiet AC
2) fungal keratitis
3) infectious crystalline keratopathy

Dx: acid-fast stain and the optimal culture media to use is either Lowenstein-Jensen or Middlebrook 7H9 or 7H12 media.

Rx: typically prolonged and the optimal regimen has not been deduced. However, most corneal specialists would institute a multidrug treatment consisting of fortified amikacin (the classic treatment) + clarithromycin (topical +/- oral) + a 4th-generation fluoroquinolone.

Question 2

Axial power map vs. instantaneous map

Answer 2

Axial power map: “sagittal curvature” map. less accurate when measuring the peripheral cornea. Based upon reference axis through the line of sight. Better estimate of central cornea power.

Instantaneous map: estimates K power based upon best-fit spherical approximation of the point of the cornea being measured. Better estimate of PERIPHERAL corneal power. Aka. tangential power map or meridonal power map.

Question 3

Thinnest part of the cornea

Answer 3

1.5 mm TEMPORAL to the geographic center. Best to measure cornea here for pachy (~540 um)

Question 4

where is pupil located in relation to cornea

Answer 4

located inferonasal

Question 5

What condition most resembles ICE?

Answer 5

PPMD and ICE = endothelium acts like epithelium.

PPMD = bilateral AD dystrophy.    Vesicles and bands on specular microscopy
ICE = unilateral, sporadic.  On specular microscopy = ICE endothelial cells = dark areas

corectopia, glaucoma, endothelial dysfunction.

Question 6

A=R syndrome

Answer 6

bilateral congenital condition (AD or sporadic) = correctopia and glaucoma
posterior embryotoxin

Question 7

LASIK criteria

Answer 7

0.5 diopters = limit of dioptric change in a year
HSV should be inactive for at least 1 year
VZV = relative contraindication.
21 yo = minimum age for LASIK candidate

Question 8

Macrostriae vs. Microstriae

Answer 8

Macrostriae = flap slippage/dislocation, striae involving ENTIRE flap thickness, typically assoc/w/WIDER FLAP gutter. Rx: refloat immediately +/- stretch flap

Microstriae: folds in Bowman layer, gutter is typically SYMMETRIC, more common s/p ablation of high myopes since more tissue is removed. Rx: observation.

Question 9

arcuate incisions - radial vs. tangential

Answer 9

Incisional effects
radial - flatten in both the meridian of the incision and 90 degrees away
tangential (arcuate/linear) - flatten in the meridian of the incision and steeping in the meridian 90 degrees away (COUPLING!)

Question 10

Laser types

Answer 10

Photodisruption - femtosecond laser (when you are creating flaps for LASIK or Intacts); 1053 nm infared beam
tissue changed into plasma. high pressure and temperature lead to microscopic cavities

photothermal - holmium: YAG laser
collagen shrinking from heat

Photoablation = excimer laser (UV 193)

Question 11

Lasers - broad beam vs. scanning slit.

Answer 11

Lasers
broad beam - large beam, high energy, slow repetition rates
scanning slit - narrower slit beam scanned over the surface for smoother ablation

Question 12

How long after delivery and cessation of nursing should a women wait prior to refractive surgery?

Answer 12

A: 3 months to make sure it is a stable K, and also b/c you have to give steroids

Question 13

K astigmatism

Answer 13

tight suture: steepens corneal curvature in the axis of the suture and flattens the cornea in the opposite axis

Ex: suture placed at 180 degrees, the axis of the astigmatism is 90 degrees while the power of the astigmatism is 180 degrees. Thus, a positive cylinder lens would need to be oriented at 180 degrees to negate the induced astigmatism in this patient. (+ cylinder tells you which suture you need to remove to decrease the patient’s astigmatism).

Optometrists use negative cylinder in their refraction notation because that is how cylinder is actually ground into lenses (i.e. on the back surface).

Ex: loose suture- allows wound gape and induction of astigmatism in the axis 90 degrees away. Rarely does this occur with phaco wounds which are small. But this can occur with extracapsular cataract wounds or other large wounds such as corneal transplants.

Question 14

A-scan U/S assumptions.

Answer 14

A-scan ultrasound assumes that the eye is filled with aqueous and vitreous. It uses known values for ultrasound speed through aqueous and vitreous to provide data on axial length.

When the eye is filled with other substances (e.g. oil and gas), the readings will be very inaccurate. Most A-scan units can be switched to a silicone oil mode.

Newer biometry units, such as the IOL Master, are optical and have the patient look at a fixation target. This improves the percentage of times the fovea is centered. Dense posterior subcapsular cataracts sometimes limit earlier versions of the IOL Master from obtaining accurate measurements

Question 15

Posterior staphylomas

Answer 15

=focal areas of scleral thinning with outpouching of that area
Most often located in the peripapillary region, adjacent to, but not centered at, the macula.

Ex: If the examiner thinks the fovea is at the center of the staphyloma, the measured axial length will be longer than the true axial length. This error will result in the surgeon picking an intraocular lens that is too weak. Thus, the patient will have a hyperopic refractive surprise. TOO WEAK LENS = HYPEROPIC SURPRISE

Question 16

instantaneous (tangential) vs. axial (saggital) power map

Answer 16

instantaneous map: estimates K power based upon a best-fit spherical approximation at the point of the cornea being measured. The instantaneous map is a BETTER estimation of PERIPHERAL corneal power. The instantaneous power map is also known as the tangential power map or meridional power map.

axial power map: known as a “sagittal curvature” map. The axial power map is less accurate when measuring the peripheral cornea. The axial power map is based upon a reference axis through the line of sight and gives a better estimation of central corneal power.

MNEMONIC ASS - axial/sagittal curvature/line of SIGHT

Question 17

SRK formula

Answer 17

The SRK formula is:

P = A - 2.5L - 0.9K

(where P = the power of the lens for emmetropia; A = the “A” constant of the lens; L = axial length of the eye; and K = average dioptric power of the cornea)

Question 18

IOL calcs s/p refractive surgery

Answer 18

historic method formula is: post-operative K = (pre-operative K + pre-op spherical equivalent refraction) - post-op spherical equivalent refraction.

contact lens method calculates the true power of the cornea = (dioptric power of the base curve of the contact lens + power of the contact lens + over-refraction) - the spherical equivalent of the manifest refraction without the contact lens.

topography method takes the corneal power from topography and then reduces it by a specified amount.

Commonly, post-refractive patients have an OVERESTIMATION of corneal power by topography which leads to a HYPEROPIC SURPRISE after cataract surgery. This can easily be remembered because if the surprise was typically a myopic surprise, people would not be as concerned about the calculations as they are.

Question 19

Refractive stability?

Answer 19

Stable refraction: no more than 0.5 D change in sphere or cylinder in at least the past year.

When unclear whether pt’s refraction is stable, best to recheck the refraction in 6-12 mo.

Question 20

Hartmann Shack aberrometer

Answer 20

measures the wavefront error of the ENTIRE visual system. From tear film to the retina

Question 21

Munnerlyn formula

Answer 21

ablation depth in um approximately equal to (=)

(diopters of myopia)*(optical zone diameter/3)^2

Question 22

As amount of ablation increases, so too does the…

Answer 22

optical zone increase

As optical zone DECREASES, you have glare/halos/potential for regression increase

Question 23

Adequate suction signs

Answer 23

pupillary dilation, pt has blacked out VA, IOP > 60 mm Hg

Question 24

Epikeratoplasty

Answer 24

aka epikeratophakia

creation of a 360 degree partial-thickness incision in which to place the lenticule that reshaped the cornea.

Question 25

Conduction keratoplasty

Answer 25

radio waves to heat the mid-peripheral cornea to cause scarring, which secondarily steepens the central cornea

Steeper central cornea corrects low degress of hyperopia (or in emmetropic pts, induces myopia to correct presbyopia in NONDOMINANT eye)

regression: within months to a year

Question 26

coupling

Answer 26

when 1 meridian flattened from an astigmatic incision, an amount of steepening occurs in the meridian 90 degrees away

Question 27

PRK for strabimus?

Answer 27

Hyperopic PRK to reduce accommodative convergence may improve esotropia

Question 28

Cross-linking

Answer 28

riboflavin (vitamin B3) and UVA
may have mild hyeropic effect

w/riboflavin, ~ 95% of UVA irradiance absorbed anteriorly in 1st 300 um. (So most studies require 400 um K thickness s/p epithelial removal)