Ординатура / Офтальмология / Английские материалы / Small Incision Cataract Surgery (Manual Phaco)_Singh_2002

INTRODUCTION

Hypertension should be well-controlled during cataract surgery like any other surgery. Several studies have documented that patients with hypertension have higher risk of suffering from major cardiac complications during or shortly after non-cardiac operations than the patients who have always been normotensive. However, most of this increase is because of IHD (ischaemic heart disease), chronic heart disease left ventricular dysfunction, renal failure or other abnormalities that often occur in the patients of hypertension.

Blood pressure should be well-controlled prior to elective surgery and anti-hypertensive medications should be continued throughout the preoperative period. If there is a period in which the patient is unable to receive oral medication, topical or intravenous equivalents should be substituted. Rapid withdrawal of beta-blocking medications is associated with adverse effect on heart rate and blood pressure and may precipitate myocardial ischaemia.

Definition of Blood Pressure

Definition of hypertension is difficult and by necessity is arbitrary and there is no real separation between

normotension and hypertension. The higher the blood pressures the higher the risk of stroke and coronary events. By JNC VI (1997) and WHO (ISH 1999), the high blood pressure has been classified in the following categories depending upon the level of diastolic as well as systolic blood pressure based on average of more than two readings taken at each of two or more visits.

Among patients who are treated for hypertension, preoperative evaluation should include review of present medications and any history of intolerance to previous anti-hypertensive medications, assessment of adequacy of anti-hypertensive therapy and for evidence of target organ damage or associated cardiovascular pathological conditions.

Before going for cataract surgery one must assess the effect of hypertension on retina as it carries prognostic significance and should carry out the following minimum investigations to assess the target organ damage.

•ECG–left ventricular hypertrophy, ischaemic heart disease.

•X-ray chest–heart failure.

•Blood urea and serum creatinine—Nephropathy

•Urine examination

•Clinical examination–heart failure, stroke, TIA (Transient Ischaemic Attack) and peripheral vascular disease.

Management

In patients with mild to moderate hypertension diastolic blood pressure <110 mm of Hg and systolic blood pressure < 180 mm Hg, and in non-cardiac surgeries are generally well-tolerated. However, severe hypertension (Diastolic blood pressure > 110 mm Hg) should be wellcontrolled prior to cataract surgery. Patient with severe hypertension in the immediate preoperative period are at increased risk for perioperative MI and congestive heart failure It is neither mandatory nor desirable to delay cataract operation (Non-cardiac operation) for weeks or months that may be required to achieve ideal blood pressure control in stable patients who have mild to moderate hypertension but who have no hypertensive end-organ damage.

Patient on anti-hypertensive therapy are at increased risk of perioperative hypotension also.

If surgery is urgent then preoperative blood pressure control can be achieved rapidly with the use of intravenous beta-blockers, calcium blockers, nitroglycerin or nitroprusside. Sublingual nifedipine should not be used as it can precipitate myocardial ischaemia or myocardial infarction.

In management of hypertension lifestyle modification should be practiced for mild to moderate hypertension, e.g.:

•Lose weight if overweight.

•Limit alcohol intake.

•Increase aerobic exercises.

•Reduce sodium intake (< 100 m mol/day).

•Maintain potassium intake (90 m mol/day).

•Maintain calcium and magnesium intake.

•Stop smoking and reduce saturated fats.

The drug therapy should be started with diuretics and

beta-blockers in uncomplicated case. The isolated systolic hypertension in elderly, which is very common in the

patient of cataract, can be treated with long acting calcium channel blockers. The hypertensive diabetic patients going for cataract surgery should be controlled on ACE inhibitors. A hypertensive patient with angina, the drug of choice is beta-blockers. Thus, the drug therapy should be individualised according to the presence of concomitant disease.

CONCLUSION

Blood pressure should be well-controlled prior to cataract surgery and anti-hypertensive medication should be continued through out the perioperative period. In postoperative period the blood pressure should be carefully monitored as some patients on anti-hypertensive therapy may have hypotension. Mild to moderate hypertension in the absence of significant coronary or myocardial dysfunction does not add significantly to the cardiovascular risk of cataract (Non-cardiac) surgery.

SUGGESTED READING

1.Elliott HL, Connel JMC, GT Mcinner: The year in Hypertension 2000.

2.Eugene Brawnwaid, Douglas P Ziges, Peter Libby: Heart Disease: A Textbook of Cardiovascular Medicine, (6th ed): 2001.

3.Goldman L, Caldera DL: Risks of general anaesthesia and elective operation in the hypertensive patients.

Anaesthesiology 79, 50: 285–92.

4.Hurst S, Valentin Fuster, R Wayne Alexander, Robert A, O’ Rovrice: The heart (10th ed): 2001.

5.Hypertension control: Report of a WHO expert committee, WHO technical report series. 862, 2000.

6.Kapllan M: Clinical hypertension: Normal (7th ed): 2000.

7.Magnussen J, Thulin T, Wernex O et al: Hemodynamic effects of pretreatment with metoprolol in hypertensive patients undergoing surgery: Br J Anaesth 86, 58: 251–60.

8.Prys–Roberts C, Meloche R, Foex P: Studies of anaesthesia in relation to hypetension: I Cardiovascular responses of treated and untreatede patients: Br J Anaesth 71, 43: 122.

9.Stone JG, Foex P, Sear JW et al: Risk of myocardial ischemia during anaesthesin treated and untreated hypertensive patients. Br J Anaesth 88, 61: 675–79.

Preoperative 8 Evaluation for SICS

Kamaljeet Singh

Sumeet Jain

In all types of surgeries good preoperative evaluation helps in giving great postoperative results and more importantly a grateful patient. The usual pattern of preoperative examination especially for SICS should be

following:

Detailed History

Detailed History of patient should be taken:

1Diabetes These patients are likely to have more incidence of postoperative uveitis, neovascularisation of iris and diabetic retinopathy. Therefore, a

diabetic should be thoroughly examined.

2.Hypertension If history of hypertension is present it should be well-controlled, to prevent any untoward incidence of expulsive hemorrhage.

3.Ocular history History of recurrent redness, pain, discharge and previous treatment must be asked.

4.Refractive error Patient should be asked whether patient is ammetropic or emmetropic at the age of 40 years. It is important from two angles:

i.IOL power calculation

ii.Scleral rigidity is low in myopic. Nucleus delivery becomes difficult in these cases.

Examination

Detailed examination under slit lamp gives many clues.

i.Corneal endothelium can be examined by using 25x or 40x ocular by using specular reflection, or by using Eisner lens. By these two techniques good assessment of corneal endothelium can be made. It helps in excluding patients having low endothelial cell count e.g. in Fuch’s dystrophy, glaucoma, chronic iritis, trauma, old keratitis, multiple injuries and old age. Keratic precipitates should also be looked for.

ii.Iris pupil examination under slit lamp Any evidence of posterior synechiae, pigments on lens, or bound

down pupil can be easily assessed under slit lamp. Pupil should also be examined after dilating it. It will give further details for iris and also help in knowing whether pupil dilates easily or not.

iii.Lens examination Preoperative examination of lens should be done by dilating pupil because it is easier to assess the grade of hardness of cataract.

iv.Fundus examination Fundus should be examined by +78D lens under slit lamp. It gives a very good view of macula even if media is hazy due to lenticular changes. It will avoid surprise postoperative findings of macular degeneration, diabetic maculopathy and optic atrophy.

v.Hypotony Hypotonic eye is not suitable for SICS become making a scleral tunnel in a hypotonic eye is very difficult and there are chances of tunnel getting ragged.

Most important aspect of SICS is expressing the nucleus in the anterior chamber out of capsulotomy or capsulorhexis. In a hypotonic eye prolapsing the nucleus in AC expression of the nucleus becomes very difficult. Hence it is advisable not to apply pinky ball before the operation. Instead gentle massage of the eye can be done after giving peribulbar injection

vi.Age of the patient As the age advances the size and hardness of the nucleus increases. The size of the lens at the age of 65 years is 1/3rd more than at 25 years. Hence very old patients with hard and large nucleus are not suitable cases for SICS. This is the preferred choice of surgery for comparatively young patients. Therefore in older persons if we are doing SICS, the incision should be comparatively bigger.

vii.Small pupil Small contracted pupil makes capsulotomy or capsulorhexis very difficult. Prolapsing the nucleus in anterior chamber becomes almost impossible and hence it is better to do ECCE than SICS.

viii.Eyes with uveitis The patients who had recurrent episodes of uveitis along with synechiae are not suitable because of the following reasons.

a.Proper capsulorhexis or capsulotomy is difficult as the pupil does not dilate fully because of the adhesions between the capsule and iris.

b.Prolapsing the nucleus in AC is difficult.

c.There are more chances of PC rent and vitreous prolapse as the posterior capsule is weak in these cases.

d.Postoperative inflammation is more in patients with uveitis.

e.There are chances of miosis, zonular weakness, raised IOP and CME. SICS should better be avoided in these patients or else can be done under cover of steroids. Prednisolone one mg/ kg daily should be given one week prior to the surgery.

ix.Patients who have undergone glaucoma filtering surgery are not ideal for non-phaco SICS because of hypotony.

x.Fuch’s endothelial dystrophy In this, there occurs bilateral non-inflammatory loss of endothelium. Since in SICS the nucleus is prolapsed in the AC

before its expression, manipulations in AC will lead to significant endothelial cell loss.

xi.There are few other conditions in which the technique should not be done.

a.Microphthalmos Here one has to make a very large incision as nucleus is very large. Moreover, there are increased chances of vitreous loss and other congenital anomalies.

b.Extensive congenital anomalies

c.Rubella cataract

d.Rubeosis iridis

e.Subluxated lens

To summarise, the key to successful manual SICS is proper selection of the cases for that the patients have to be thoroughly examined, screened and planned accordingly.

FURTHER READING

1.Natchear G: In Manual small incision cataract surgery. Arvind Publications, India 2000.

2.Rozakis GW: In: Cataract Surgery: Alternative small incision technique. 1st (edn): Thordofare, Inc. 1995.

3.Shah Anil: In small incision cataract surgery (Manual Phaco) Best out of Waste Bhalani Publishing House: India. 2000.

Lens implantation surgery is a one-time surgery. The refractive power of the pseudophacos is final and the patient must live with any mistake

committed or be subjected to a very dangerous operation, namely to the removal and replacement of the intraocular lens. Later correction is only achieved with extraocular aid in the form of glasses or contact lens.

So to ensure that our patients have the optimal correction, the power of the lens to be implanted must be determined individually in every case.

The problem of implant power calculation arose along with the first ever IOL implant, when Ridley in 1949 observed in his patient a postoperative refraction of 24.0 D + 6.0 × 30°. Ever since, various workers have been working on this problem of implant power estimation to obtain the best result.

The methods used to estimate implant power might be classified in two broad heads:

1.Methods based upon primary refraction.

2.Methods based upon measurement, viz axial length, corneal curvature, etc.

Estimation of Implant Power

Based on Primary Refraction

In the early days this method was the most used method. The following assumption is made while adopting this method of IOL power calculation:

1.The refractive power of the natural lens is + 23.7 D.

2.The cardinal plane of the natural lens is 6 mm behind the corneal apex.

3.The radius of curvature of the cornea and the distance between the lens and the retina do not vary between patients.

If the above conditions are true, then the placement of an IOL with a power of + 20.0 D in the posterior chamber would result in a postoperative refraction equal to that existing preoperatively. It explains that an IOL of

+20.0 D would be sufficient to mimic the natural lens of + 23.7 D, because the cardinal plane of the IOL placed

in the posterior chamber would be further away from the retina than the natural lens.

Such lenses in which the pre and postoperative refraction remains the same are called Idem lenses. Depending upon the plane of placement of the optic, the power of the IOL will vary in any given eye. The Table 9.1 gives the power for idem lenses depending upon the plane of placement.

Table 9.1: Rules of thumb for idem lens

Emmetropia Lenses

While idem lenses are sufficient for patients who are preoperatively emmetrope, for patients with known refractive errors, it would be more desirable to implant a lens which would result in emmetropia postoperatively. The following formula gives the implant power required for emmetropia.

IOL power for emmetropia = Idem lens power + (1.25 × Refractive error)

Example:

For a preoperative myopia of –2.00 D

For a preoperative hypermetropia of + 1.00 D

One should remember while trying to fit emmetropising lenses, it is pertinent to note that the preoperative glasses used by the patient need not reflect his/her real refraction. A careful history will overcome this problem.

Limitations

Estimation of IOL power based on refraction suffers from the basic assumption that the power of the natural lens is + 23.7 D. Though this may be true in a majority of cases. Postoperatively, the use of this method more than often leads to very high refractive errors. The clarity of an image on the retinal surface of a person’s eye is determined by the sum total of:

a.The refractive power of the corneal surface

b.Power of the lens

c.The distance between the lens and retina.

Each of these factors is variable from person to person and eye to eye. The refractive power of the corneal surface could vary from +39 D to + 49 D, and that of the crystalline lens from + 17 D to +27 D. The length of eyeball, which in turn determines the distance of the lens from the retina, varies from < 20 mm to about 28 mm. Thus in a real situation a highly refractive corneal surface may be compensated for by a short eye to result in emmetropia or vice versa.

Implant Power Calculation

Based on Measurements

With the advent of keratometry and A scan sonography, which provided accurate measurements of the radius of curvature of the cornea and the length of the eyeball, two important parameters required for a precise estimation of implant power.

Theoretic Formulas

It was not until 1967 when Fyodorov presented his theoretical formula based on geometric optics utilizing keratometry and A scan ultrasonography that implant power calculation matured into a rational discipline.

In addition to Fyodorov and Colenbrander, Thijssen, van der Heijde and Binkhorst published theoretical formulas. These formulas are all based on geometric optics as applied to schematic eye using theoretical constants. These apparently different formulae are in fact, identical, except for correction factors. They all can algebraically transformed to

Different theoretical formulae were described by different workers from time to time as follows:

N = Refractive index of aqueous and vitreous L = Axial length in mm

K = Keratometry in diopters

C = Postoperative AC depth in mm R = Radius of curvature in mm.

Empiric Formula

A few workers, including Gills, Sanders, Retzlaff and Kraff developed regression equations based on observed clinical data relating to eye measurements and IOL power. From these equations they developed formulae for predicting IOL power. These formulae claim more accuracy in predicting implant power than theoretic formulae. These are also subject to change as more data are incorporated into developing and regression equations.

Among these the SRK formula developed by Sanders, Retzlaff and Kraff has gained wide acceptance because of its ease of use.

As it can be seen from above formula, a change in the axial length of by 1 mm results in a 2.5D change in implant power or a change in corneal refractive power by 1D results in a 0.9D change in implant power.

The theoretical and empiric formulae worked well for eyes of axial length ranging from 22 to 4.5 mm. For eyes of short or long axial length, while the theoretical formulae predicted too high or too low emmetropia value respectively. The SRK formula had the opposite effect.

To overcome this problem the second generation formulae were developed. The SRK II formula was developed where the basic SRK formula remained unchanged but some additional computations were necessary to suit short/long eyes.

Adjusting Original SRK to SRK II

Average length eye No adjustment needed unless high ametropia desired (Figs 9.1a to c).

SRK II Formula:

P = A – 2.5 L – 0.9 K + C

C = SRK II correction for long and short eyes.

Short Eyes (Less than 22 mm)

•If axial length is 21 to 22 mm—add 1 diopter to emmetropia value

•If axial length is 20 to 21 mm—add 2 diopter to emmetropia value

•If axial length is 10 to 20 mm—add 3 diopter to emmetropia value

Example The patient has a 20.73 mm axial length eye and the original SRK formula shows 24.7 D IOL will give emmetropia. Add 2 D to this emmetropia power 24.7 + 2 = 26.7 D for correct emmetropising power.

Long Eyes (More than 24.5 mm)

If axial length is more than 24.5 mm subtract 0.5 D from emmetropia value.

Large Ammetropic Postoperative

Refraction Desired (More than 1.5D)

If the postoperative refraction desired is more than 1.5 D in nonmyopic patients or more than – 0.75 D in myopic patients (Axial length greater than 24.5 mm) use following:

IOL for desired refraction = Emmetropia power – (RF × Desired refraction)

RF = 1.25 if emmetropia power greater than 14 RF = 1.00 if emmetropia power less than 14

SRK/T

It is in theoretical formula developed using the non-linear terms of physiologic optics and impirical regression methodology for optimisation. It utilises the corneal height formula for predicting postoperative ACD and an axial length correction factor (Retinal thickness) which varies with eye length.

Emmetropia should be the Goal

1.When bilateral pseudophakia is planned

2.When there is hypermetropia of 1.5 D to 2.5 D in an useful fellow eye.

3.When there is known or suspected absence of binocular vision

4.When senile macular choroidal degeneration is present in both eyes

5.When a contact lens is used in an aphakic fellow eye.

Ammetropia should be the Goal

The only indication to make the eye ammetropic exists in unilateral pseudophakia.

Factors affecting accuracy of implant power calculation.

Axial Length Measurement

Two methods are currently used for the measurement of the axial length. They are the applanation and immersion technique. In the applanation method the applanating probe is kept on the cornea, whereas in the immersion method the probe does not come into direct contact with the cornea but acts through an intermediary coupling solution. In the former method there is possibility of a slight depression of the eye leading to a lower estimate of axial length. This is of significance in very short eyes.

Keratometry

This is another probable source of error because in manual keratometry, failure by the operator to calibrate for his/her refractive error, could lead to wrong reading. Autokeratometers are not subject to this error. It would be better if the refractive power of the cornea is estimated from the radius of curvature of the cornea rather than measure the refractive power directly.

Orientation of the Implant

a.Plano convex optic The normal position is with convex surface forward, flipping of the lens would

lead to a decrease in the power of the lens by

0.75 D.

b.Meniscus optic Flipping of this lens is mechanically difficult and is not recommended. However such a flipping would lead to the displacement of the principal axis posteriorly, thereby decreasing the effective power.

c.Biconvex optic There is no change in the power if both the surfaces are equally convex. Most IOLs have 3:1 ratio in convexity between anterior and posterior surfaces, thus reversal of the optic would decrease the effective power.

Apart from these, the other factors which affect the accuracy of the implant power are differences in the ultrasound equipment used, surgical technique, postoperative chamber depth, postoperative change in corneal curvature and manufacturing variation in implant power labeling.

Surgeon’s Personal A Constant

Quite often differences between the expected and the observed postoperative refraction were noted by Sander’s et al despite the use of the same style and make of the IOL. They were able to trace this anomaly to be due to differences between surgeons. Based on their studies they are able to develop a method for calculating personal A constant for each surgeon. This calculation may be done retrospectively from records or on continual basis. It is important to calculate separate A constant for each style/make of the lens. The following data are required for the same.

A1 = I + (Ra × Rf) + 2.5L – 0.9K – C

FURTHER READING

1.Hoffer KJ: The Hoffer Q formula: A comparison of theoretic and regression formulas. J Cataract Refract Surg 20: 677, 1994.

2.Olsen T, Oleson H, Thim K et al: Prediction of postoperative intraocular lens chamber depth. J Cataract Refract Surg 16: 587-90, 1990.

3.Retzlaff J: A new intraocular lens calculation formula. Am Intra-ocular Implant Soc J 6: 148, 1980.

4.Retzlaff, Sanders DR, Kraff MC: Development of the SRK/T intraocular lens implant power calculation formula. J Cataract Refract Surg 16: 333-40, 1990.

5.Sanders DR, Kraff MC: Improvement of intraocular lens power calculation: Regression formula. Am Intra-ocular Implant Soc J 6: 263, 1980.

6.Sanders DR, Retzlaff J, Kraff MC: Comparison of the SRK II formula and the other second generation formulas. J Cataract Refract Surg 14: 136-41, 1988.

7.Sanders DR, Retzlaff J, Kraff MC et al: Comparison of the accuracy of the Binkhorst, colenbrander and SRK implant power prediction formulas. Am Intra-ocular Implant Soc J

7: 337-40, 1988.

Most of the ocular surgeries can be performed under local anaesthesia. However, local anaesthesia is the best for SICS. We will describe here various types of local anaesthesia used in

cataract surgery. Following blocks are used:

•Retrobulbar

•Peribulbar

•Sub-Tenon’s

•Topical

•Intracameral

Retrobulbar Anaesthesia

This was the preferred anaesthesia till eighties for cataract surgery. Its advantage is that it gives very good anaesthesia and akinesia in very small volume. 1.5 to 2.0 ml is enough to give a very good effect. The technique is simple. A special 4 cm long 26 gauge retrobulbar needle is used for this purpose (Fig. 10.1). One ml xylocaine mixed with equal amount of bupivacaine with freshly prepared hyaluronidase (15 units per ml) is taken in a 2 cc syringe. Lower orbital margin is palpated and we go behind the eyeball close to the orbital margin at the junction of medial two-third and lateral one-third. Previously it was the practice to ask the patient to look up and in. In this position there are chances of damaging the optic nerve sheath and the macula also comes closer to the needle. Therefore, now the patient is just asked to look straight or even down to prevent the injury to optic nerve. In retrobulbar block we get two resistance. One, while piercing the skin. Other, when we pierce the muscle cone since our aim is to block the ciliary ganglion, which lies within the muscle cone about 7 mm anterior to orbital apex. Following effects are achieved with retrobulbar block.

•It causes anaesthesia

•It causes akinesia

Kamaljeet Singh

VK Srivastava

Fig. 10.1: Retrobulbar anaesthesia

Courtesy: Alcon (India)

•It causes a little proptosis, which is helpful in performing the surgery.

•It reduces the intraocular pressure.

•It dilates the pupil

Complications

Brainstem anaesthesia This leads to respiratory arrest. It occurs when the needle pierces the optic nerve sheath. Through the subarachnoid space the local anaesthetic may reach in the brain. The symptoms start within two minutes of injection and start with confusion, cranial nerve palsy, convulsions, hemiplegia, quadriplegia, cardiovascular instability and even respiratory depression and arrest. The whole episode may take 20 minutes to set in. Although the injection into the optic nerve is supposed to be the mechanism behind this, yet the exact mechanism of this complication is not very clear since some other authorities believe that hyaluronidase could also be responsible for this.

Retrobulbar haemorrhage The orbital apex is highly vascular structure. There are all likelihood of rupturing the vessels, which lead to retrobulbar haemorrhage. When it occurs there is sudden proptosis and lids become tight. The operation in such situation is postponed after doing a patch.