Test 17  (Refractive surgery)

1. The refractive state of the eye may be altered by:

a. paralysing the ciliary muscle

b. changing the depth of the anterior chamber

c. removing the vitreous

d. increasing the thickness of the lens

e. changing the axial length of the eye
 

Answera.T   b.T   c.F    d.T   e.T
Paralysing the ciliary muscle will reduce the plus power of the lens and make the patient less myopic or more hypermetropic.
Changing the depth of the anterior chamber will alter the effectivity of the lens and hence the refractive power.
Vitreous is made up of 99% water and its removal does not significantly affect the refractive index of the vitreous, unless the vitreous is filled with silicone oil or gas.
Increasing the lens thickness increases its refractive power.
The axial length of the eye is one of the factors that determine the ocular refractive power.

 

2. Monovision:

a. refers to the use of one eye for distant vision and the other for near

b. is mainly reserved for presbyopic patients

c. requires one eye to be made hypermetropic and the other myopic

d. reduces visual acuity

e. reduces stereopsis
 

Answera.T   b.T   c.F   d.T   e.F
Monovision uses one eye for distant vision and the 
other for near. It is reserved for presbyopic patients. 
One eye is made emmetropic and the other myopic. 
However, 50% of patients have problems with monovision. As the patient can not use both eyes, the visual acuity and the stereopsis are reduced.

3. The following are contraindication to refractive surgery:

a. age less than 21

b. keratoconus

c. discontinuation of gas permeable lenses wear for 
    less than ten weeks

d. tilted disc

e. irregular astigmatism
 

Answera.T   b.T   c.T   d.F   e.F
Before refractive surgery is carried out, it is important to make sure that the corneal topography is stable. Therefore, it is contraindicated in children and young adults whose refractive error has not stabilized and also patients who has recently worn contact lenses in which the corneal topography may be affected by warpage.
Keratoconus is associated with thin cornea and refractive surgery may accelerate the progression.
 

4. Photorefractive keratectomy:

a. uses excimer laser made up of ultraviolet radiation

b. uses thermal energy for reshaping the corneal contour 

c. corrects myopia by flattening the central cornea

d. causes more stromal scar in high myope than low
    myope

e. is less predictable for high myope than low myope
 

Answera.T   b.F   c.T    d.T   e.T
Photorefractive keratectomy uses excimer laser that 
works by breaking the molecular bonds. It reduces 
myopia by reducing the corneal curvature. It is more 
effective for low myope. Stromal scar is more common in high myope.

5. LASIK is superior to photorefractive keratectomy in that it :

a. is a less painful procedure

b. gives a faster visual rehabilitation

c. can treat a higher myopia

d. has less complication

e. has less myopic regression
 

Answera.T   b.T   c.T    d.F   e.T
As LASIK does not expose raw corneal surface 
at the end of surgery  it is less painful. It can also
treat a higher myopia, causes less scarring and has
less myopic regression. However, because a flap 
need to be created, it has a higher potential risk. 

6. LASIK:

a. is contraindicated in patients with thin cornea

b. involves the use of microtome

c. involves the creation of a complete circular flap

d. can be repeated by lifting the flap created if further refinement is necessary

e. causes less scarring than photorefractive keratectomy
 

Answera.T   b.T   c.F   d.T   e.T
Thin cornea is a contraindication to LASIK. Cutting the cornea too deep or treating a patient with thin cornea may give rise to cornea ectasia. Microkeratome is used to create a flap which has a hinge. Excimer laser is used to treat the stroma underneath the flap. It causes less scarring than photorefractive keratectomy.

 

7. The side-effect of LASIK include:

a. reduced corneal sensation

b. inaccurate biometry if the patient requires cataract 
   surgery in later life

c. tear film abnormality

d. erroneously high intraocular pressure with applanation 
    tonometer

e. glare
 

Answera.T   b.T   c.T   d.F   e.T
Reduced corneal sensation and tear film instability are side-effects of LASIK. Glare may result from scarring 
or too small an optic zone. Biometry can be inaccurate unless correction of the corneal topography is taken into account. LASIK gives rise to a thinner cornea and may give an erroneously low intraocular pressure with applanation tonometer. 

8. Radial keratotomy:
 

a. is an irreversible procedure

b. weakens the cornea

c. is used to reduce the refractive power of the cornea

d. is used to increase the refractive power of the cornea

e. flattens the central cornea
 

Answera.T   b.T   c.T   d.F   e.T
Radial keratotomy involves cutting the cornea radially and this gives rise to gaping of the wound and flattening of the cornea. It is used to treat myopia and astigmatism but not hypermetropia. The refractive power of the cornea is reduced. A cornea which has had radial keratotomy is at an increased risk of rupture during blunt trauma along the incision line.

 

9. Radial keratotomy:

a. involves incisions with depth which are at least 80% 
    of the corneal thickness

b. has a greater effect the deeper the incision

c. has a greater effect the longer the cut

d. has a greater effect the smaller the central zone

e. is more predictable than LASIK in treating myopia
 

Answera.T   b.T   c.T   d.T   e.F
The depth of radial keratotomy is typically between 80-90% in depth. Its effect is greater if the cut is deeper, longer and the central zone smaller. It is less predictable than LASIK and the refraction takes longer to stabilize.
 

10. Temporal corneal incision during phacoemulsification has 
      the following advantages:

a. easier access to the eye

b. reduced incidence of endophthalmitis

c. less surgically induced astigmatism

d. increased surgical view

e. decreased incidence of epithelial downgrowth
 

Answera.T   b.F   c.T   d.F   e.F
Temporal incision has the advantages of easier access to the eye as the brow is not interfering with manipulation of the instruments. And as the central cornea is further from the temporal limbus than the superior limbus, the induced astigmatism is less. The other advantages do not apply.
 

11. The following are true about astigmatic keratotomy:

a. the incision is placed perpendicular to the steepest 
    meridian

b. causes coupling

c. the nearer the incision to the centre the greater 
    the reduction of astigmatism

d. if astigmatic keratotomy were performed for a phakic 
    patient, the site of incision should be based on the 
    corneal topography

e. astigmatic keratotomy can be carried out either as a 
    transverse or arcuate incision
 

Answer
a.T   b.T   c.T   d.F   e.T
Astigmatic keratotomy is placed perpendicular to the steepest meridian. Coupling is a problem and it is defined as steepening of cornea at 90 degrees perpendicular to the meridian that has been flattened by the incision though not to the same degree. The nearer the incision to the central zone, the greater the effect. However, it carries the risk of glare and irregular astigmatism if the incision is made too near the centre of the optical zone. It is generally advised that the incision is made 6-7mm from the centre of the cornea. 
Astigmatism in a phakic eye may be due to cornea and/or lens. Corneal topography only provide information on corneal astigmatism. It is important to obtain the refraction result to understand the total astigmatism. 

12. The advantages of arcuate incision over transverse incision in 
      astigmatic keratotomy include:

a. more uniform relaxation of the cornea

b. cutting a more uniform thickness of the cornea

c. the entire length is at the same distance from the 
    calculated central zone

d. actual incision length is about 10% longer than 
    a linear incision spanning the same number of degrees

e. reduced risk of post-operative keratitis
 

Answer
a.T   b.T   c.T   d.T   e.F

Both transverse and arcuate keratotomy can be used

in astigmatic keratotomy. The advantages of arcuate incision over the transverse incision include:
more uniform relaxation of the cornea
cutting through a more uniform thickness of the cornea
the entire length is at the same distance from the calculated central zone
also the acute incision length is about 10% longer than a linear incision spanning the same number of degrees.
Keratitis is a complication but is not related to the type of incision performed.
 

13. When a tight suture is placed radially in the cornea, the following 
       may occur:

a. the cornea adjacent to it is flattened

b. the curvature of the cornea along that meridian is 
    increased

c. the curvature of the cornea at 90⁰ to that meridian is 
   decreased

d. the effect on corneal curvature is increased if the suture 
    is deep

e. the effect on corneal curvature is increased if the suture 
    is nearer the centre of the cornea
 

Answer
a.T   b.T   c.T   d.T   e.T
A tight suture flattens the cornea adjacent to it but increases the corneal curvature along that meridian. The curvature of the cornea at 90 degrees from it is decreased due to coupling. A deeper suture and one nearer the centre of the cornea has more effect on the corneal curvature.

14. The following are true about intrastromal corneal ring:
 

a. it can be used to treat myopia

b. it can be used to treat hypermetropia

c. it is made up of PMMA

d. it is a reversible procedure

e. it involves the use of keratome
 

Answer
a.T   b.F   c.T   d.T   e.F

Instrastromal corneal ring is used to flatten the cornea 

and thereby reduces its refractive power. It is used in the treatment of myopia. The ring is made up of PMMA. The ring is inserted following the creating of a tunnel. It can be reversed by removing the ring. 
 

15. The following involves the use of donor cornea:

a. epikeratophakia

b. keratomileusis

c. keratophakia

d. lamellar keratoplasty

e. penetrating keratoplasty
 

Answer
a.T   b.F   c.T   d.T   e.T

Epikeratophakia involves the suturing of a pre-shaped

donor corneal stroma tot he surface of the host cornea
Keratomileusis involves the use of microkeratome to remove a lamella of anterior corneal stroma which is reshaped on a cryolathe and replaced.
Keratophakia involves the use of keratome to lift a lamella of anterior stroma and this is replaced over a shaped lenticule of donor corneal stroma.
Lamellar and penetrating keratoplasty are partial thickness and full thickness corneal grafts respectively.
 

16. In a patient who has had vitreoretinal surgery:

a. buckling causes a hypermetropic shift

b. buckling induces astigmatism

c. cataract formation is a common complication with 
    macular hole surgery

d. injection of air into the vitreal cavity of a phakic patient 
    causes a myopic shift

e. injection of air into the vitreal cavity of an aphakic 
    patient causes a a hypermetropic shift 
 

Answera.F  b.T  c.T   d.T  e.T
Buckling increases the length of the eye and hence
a myopic shift. Buckling if placed asymmetrically 
can induce astigmatism. Gas used in macular hole
often induces cataract. Air or gas injected into the 
eye increases the refractive power of the back surface 
of a phakic eye and causes a myopic shift. In aphakic eye, the air increases the diverging power of the back surface of cornea and causes a hypermetropic shift.

 

17. The effect of silicone oil in an eye include:

a. reducing the hypermetropic power of an aphakic eye

b. causes a myopic shift in a phakic eye

c. increases the speed of ultrasound passing through it

d. band keratopathy

e. glaucoma
 

Answera.T  b.T  c.F  d.T  e.T
Silicone oil has a higher refractive index than the crystalline lens and causes the posterior surface of the lens to become a diverging rather than a 
converging interface. As a result, a hypermetropic shift occurs. In an aphakic eye, the silicone oil acts as a converging interface and causes a myopic shift. Hence, the hypermetropia caused by aphakia is reduced by silicone oil. Silicone oil reduces the speed of ultrasound and will therefore gives a erroneously longer axial length. Band keratopathy and glaucoma are known complications of silicone oil.

 

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