Ophthalmology Notes @ OphthalNotes.blogspot.com

Ophthalmology Notes @ OphthalNotes.blogspot.com
A comprehensive collection of ophthalmology revision notes that cover a broad range of topics.
Showing posts with label REFRACTION. Show all posts
Showing posts with label REFRACTION. Show all posts

REFRACTION NOTES

REFRACTION

JACKSON’S  CROSS-CYLINDER
  • It is a combination of two cylinders of equal strength but opposite power with their axis at right angles to each other.
  • Commonly used are  +/-0.25 and +/-0.50
  • REFINEMENT OF AXIS-
- Always done first.
-Cross-cylinder is held with its handle along the axis of cyl in the trial frame [i.e at 45deg] first with -0.5D and then with +0.5D
-If the patient reports no change in V/A bet the two positions-axis of the cyl in the trial frame is correct.
-If the visual improvement is found in one of the pos,a ` ‘plus’correcting cylinder shud be rotated in the direction of the plus cyl componentof the cross-cyl & vice versa.

REFINEMENT OF CYLINDER POWER-
-Cross-cyl of +/-0.25 is placed with its axis along the axis of the cross-cyl in the trial frame first with the same sign & then with the opp sign
-In the first pos the cyl correction is enhanced by o.25D & in the second it is diminisned by the same amount.
-When the V/A does not improve in any of the pos,the power is correct
If V/A improves in any of the pos,a corresponding correction shud be made.                                       

                        TRANSPOSITION
Q.1.Perform the toric transposition-


+3.00/-1.00 × 180 to the base curve of +6D


Step 1-first transpose the cylinder, so that it is the same sign as base curve.—
+ 2.00 /+ 1.00 ×90


Step 2-The required spherical is obtained by subtracting BC power from spherical power.-
S = S - BC
+ 2 – [+6] = - 4DS


Step 3-specify axis of the BC.This is 90 deg. To axis of reqd cyl.
+ 6D × 180


Step4-Add the reqd cyl to BC power
C = C + BC
+1D + 6D= +7d ×90


Complete toric formula-
-4DS /+6D ×180 /+7D ×90


Q 2. -2.00 /+1.5 ×90 to BC of -6D


Step 1- -0.5 /- 1.5 × 180 →  cyl transposition


Step 2- -0.5 – [-6]= -0.5 + 6= 5.5DS→ reqd spherical


Step 3- -6D ×90→ axis of BC


Step 4- -1.5 + [-6]= -1.5-6= -7.5 ×180→ reqd cyl
Complete toric formula-
5.5DS /-6D ×90 /-7.5 ×180


        TORIC LENSES [spherocylindrical]
  • These are curved lenses where one surface is spherical & the other is toroidal.
  • Toric lenses are used wen a cyl is also present in the prescription.
  • Spherical power is ground on the anterior  surface & the posterior surface is made toroidal.
  • Such lenses do not  produce a single defined image bcos the principle meridians form separate line foci at right angles to each other
  • Toric lens =  spherical power / cylindrical power
  • Spherical equivalent = Sphere + ½  cylinder
                           PRISMS
USES-
DIAGNOSTIC PRISMS-
1.  PBCT- For phorias & tropias
2.  Prism bar reflex test [Krimski’s test]
3.  Maddox double prism test – for cyclophorias
4.  Prism vergence test-
*   For convergence- base-out prism-N- 50 PD [ < 20PD-convergence insuficiency]
*   For divergence- base –in-prism-N-5-7PD [< 5PD- divergence insufficiency]
5.  4 Dioptre prism test
6   Diplopia test for ARC
7.  Detection of malingering
    -If a prism is placed in front of the seeing eye,the eye will move    to regain fixation


THERAPEUTIC PRISMS-
  1. T/T of phorias
Exercising prism- base towards deviation
Relieving  prism-base away from deviation
  1. To relieve diplopia
  2. T/T  of ARC
  3. T/T of Eccentric fixation
  4. T/T of microtropia [< 2deg]


OPTICAL INSTRUMENTS-
  1. SL
  2. Applanation tono
  3. Keratometer
  4. Ophthalmoscope
  5. synoptophore
  6. Binocular loupe
  7. Prism bar
  8. phorometers
  9. Fundus camera
  10. Exophthalmometer


                             


REFRACTIVE SURGERY

                     LASIK-
LASER IN –SITU KERATOMILEUSIS
DEF-
Combines the precision of excimer laser photoablation with the advantages of an intrastromal procedure that maintains the integrity of Bowman’s layer & overlying epith.


Can correct –Myopia upto -13D
  • Astigmatism upto -6D


PRE-REQUISITES-
  1. Adequate globe exposure
  2. Corneal thickness- 450μ
  3. Flap thickness- 180μ
  4. Stromal bed thickness- 250μ
  5. IOP – 65 mm Hg


TECH-
  • Pilocarpine 1%-miosis→ to aid centration
  • Topical anaesthesia
  • Cleaning & draping
  • Exposure- wire speculum
  • Corneal marking-with gentian violet
  • marker-inner ring [3mm]-coincides with the pupil-centration
  • outer ring-[10.5mm]-aids placement of suction ring
  • pararadial lines-join the 2 rings-for correct alignment
  • Fixation of suction ring-fixed on the sclera with decentration towards the hinge.IOP is raised to 65 mmHg.checked with a Barraquer tono
  • Corneal flap-
-Cor is moisetened with BSS
-Microkeratome head is inserted in yhe track on the suction ring
-Forward foot pedal is pressed-micro moves forward & cuts the flap
-Reverse foot pedal is pressed-mic returns to its original pos leaving behind a good hinged flap
*  Stromal ablation-
-Flap is retracted along the hinge & stromal bed is dried
-CUT WET & ABLATE DRY
*  Ablation-Done within 30 sec
*Irrigation done
*  Excess fld removed with a cellulose sponge
*  Flap is reposited


COMPLICATIONS-
INTRAOPERATIVE
A] During flap preparation-
1. Flap of variable & suboptimal thickness & diameter
2.Tear / hole
3.Cor perforation
4.Free flap instead of hinged flap
5.Loss of flap
6.Interface debris


B]  During laser ablation –
1.  Decentration of ablation-Glare & monocular diplopia
2.Ablation of hinge- prismatic effect


C] During flap reposition-
1.  Incorrect placement
2.  Wrinkling on reposition


POSTOPERATIVE-
  1. SANDS OF SAHARA SYN-
Intrastromal / intralamellar keratitis
  1. Epithelial ingrowths
  2. Central islands-Localised cor elevation d/t intrastromal ablation
  3. Undercorrection / Overcorrection
  4. Induced astigmatism
  5. Microstriae –d/t gross misalignment of flap- “ Thumbprint” sign
  6. Infection-nontuberculous mycobacteria
  7. Dry- eye as cor Ns are severed during flap creation
  8. Ectasia if residual stromal bed is not 250 micron


                                LASEK

  • Lasaer subepithelial keratomileusis
  • Also k/as epilasek
  • Combines LASIK + PRK