Slit lamp illumination for vitreoretinal surgery

jeudi 10 avril 2008 par Didier ducournau

Although the great majority of ophthalmologists worldwide systematically use a slit lamp for the clinical diagnosis of ocular diseases, by a rather strange paradox, we neglect this emblematic instrument as soon as we enter the operating room. During the last 20 years, I have been using a surgical slit lamp attached to the operating microscope and positioned at 5° from the axis, as my sole illumination system for more than 16 000 vitrectomies and 10 000 RDs without vitrectomy. The advantages of this illuminating system are linked to its three characteristics : first, it is a slit ; second, it is fixed to the microscope ; and third it is tilted between 5 and 7 degrees from the axis. We will first look at the general advantages provided by this system, before studying the more specific advantages for macular and retinal detachment surgeries. We will finish by proposing some tricks.


A. General advantages

If one considers the general evolution of the different illuminating systems proposed to vitreoretinal surgeons during the last twenty years, one will be surprised to discover that they did not follow the surgeons’ requirements and evolutions in surgery :

1) Complete Control

The period of Kloti’s Vitrectomy Machine is over, when surgeons operated a mono speed cutter and the assistant aspirated with a syringe. Now the surgeon controls the cutting speed, the phaco power, the aspiration flow, the diathermy power, the microscope focus/zoom… like a one-man band ! Forcing him to monopolize his left hand to illuminate the surgical field seems to be archaic ! Why sacrifice the left hand of a one-man band ? What kind of phacoman would accept to sacrifice his left hand in order to illuminate the anterior segment, and ask his assistant to maintain the crystalline nucleous while performing the cracking procedure with his right hand ? video1 Anterior segment surgeons naturally consider the microscope as a viewing system combined with an illuminating system. Why should posterior segment surgeons be punished and forced to sacrifice their left hand ?
Equipped with a surgical slit lamp, the microscope, like consulting biomicroscopes used in the clinic, is a viewing and illuminating tool. When removing a suture in the office, at the slit lamp, when both hands are necessary to do the job (the left hand maintaining the patient’s lids open and the right hand removing the suture), one would naturally take advantage of the biomicroscope illumination. With illumination provided by the microscope, the left hand is free to control the plano-concave lens positioning, stabilize the forceps and therefore control the manipulation precision video2, clean the instrument tip, make the scleral depression, and perform a real bimanual surgery.

2) Bimanual surgery
The period of performing vitreous removal only is over. Now, we also remove the pre and sub retinal proliferation, maintaining the tissue with one hand and cutting it with the other. The need for this bimanual surgery is so obvious that the industry has developed illuminating systems fixed to the sclera or to the surgical instruments—but this does not go without disadvantages. If one wants to cut and eat a steak in a dark room, three solutions are offered :

  • One can fix one or two lights on the edge of the plate ; this is what is provided with all the systems fixed to the sclera (MIS, chandelier…). However, they present some disadvantages, as they damage the sclera and increase the risk of vitreoretinal incarceration and crystalline lens injury.
  • One can fix a light on the knife and/or the folk ; the illuminating instruments present even greater disadvantages that led to their eventual disease, mostly due to the inescapable pitfall of the induced shadow. If an illuminated fiber is stuck on one side of the instrument, a large shadow is projected on the other side. If fibers are placed all around the tip, the instrument size is greatly increased and a centered shadow appears at the exact location where one is working. In addition, the photo toxicity risk is a problem when an illumination is provided so close to the retina.
  • One can switch on the room light placed above the plate. This is what is done with illumination provided by the microscope.

3) Mini-invasive surgery
The period of “the great surgeons perform large incisions” is over. In all meetings today we can hear this “mini-invasive” concept. This is logical in that functional results are related to the surgical trauma intensity and as cosmetic considerations are becoming more important. This is observed in all surgical fields. But how can one praise the concept of mini-invasive surgery while ignoring all the advantages given by illumination placed outside the eyeball ?

  • Two sclerotomies only are required : no need for additional sclerotomy or sclerotomies (in case of MIS or chandelier) so no additional risk of vitreoretinal incarceration video3.
  • No additional risk of vitreous traction induced by intraocular fiber manipulations.
  • No additional risk of crystalline lens damage due to intraocular pipe manipulations or scleral depression in the area of scleral fixated fibers. In addition, the posterior capsule visualization is increased by the optical cut produced by the slit lamp and this is even more protective for the lens.
  • No risk of photo toxicity (or almost no risk)

4) Low cost surgery
The period when procedure cost was a minor problem is over. The cost problem becomes even more crucial as the world tries to reach the “zero risk “ :

  • The widespread evolution towards the “all disposable” is slowly becoming an inescapable development either because of governmental health codes or facilitation of defense in the case of legal proceedings
  • The fixed price for each procedure given by public/private health systems will not follow the cost increase required to reach zero risk.
  • The patient’s acceptance to pay for surgery is difficult to obtain. In fact, since the risk is lowered, healing has become a social right like paid vacation, therefore a patients no longer understand why they should pay for surgery.

By eliminating the cost of disposable intraocular fibers for each procedure, the slit lamp illumination system gives additional economical capacities. In my clinic (2000 retinal procedures a year), we calculated that the use of the slit lamp allowed to save, every year, about 100 000 Euros.

B. Specific advantages for macular surgery

1. The slit lamp is fixed to the microscope

  • The left hand is freed

The freed left hand is then available to center the plano-concave lens on the cornea, manipulate the forceps wheel in order to orientate the jaws perpendicularly to the membrane edges, and to clean the extremities of the instruments. This allows a more fluid procedure.
In any case, the main advantage that amaizes my visitors is that my left hand is available to stabilize the forceps, avoiding trembling movements and therefore increasing work precision. One appears a surgeon with fantastic dexterity, never touching the retina incorrectly and the procedure becomes faster.

  • Decreased photo toxicity
    JPG - 67.9 ko
    Fig 1 : slides kindly provided by M.Mauget MD

In epiretinal membrane, macular hole or edema surgery, the macula is on and the photo toxicity risk is therefore higher. The phototoxicity is related to the square of the distance between the fiber and the retina.
In order to compare the illumination provided by a standard intraocular fiber to the one provided by a slit lamp, I conducted a small experiment. I carried out a chorio sclerectomy on a fresh pig’s eye in order to measure, with a photoelectric cell, the illuminations provided by a slit lamp (7,200 Lux) and by an intraocular fiber at 17mm (7,800 Lux), 8mm (30,500 Lux) and 4 mm (127,000 Lux) from the posterior pole video4.
An easy way to appreciate the intensity of delivered light when performing a vitrectomy is to use a camera with a fixed gain, set up to get a good image when the fiber enters the eye (at 17 mm from the macula) video5. When the fiber is too close to the retina, the surgeon’s papilla myosis compensates for the increase of illumination, something that the camera with fixed gain cannot do. In light of this, one should avoid approaching the intraocular fiber closer than 5 mm from the retina video6, if one wants to avoid inducing macular damage as illustrated in Fig 1.
The advantage of the slit lamp is that the delivered illumination is always the same, as the distance between the light source and the observed target remains constant.
With the scleral fixated illuminated fibers, the distance does not change either. However, the illumination provided by the slit crosses through the natural filters constituted by the cornea and the crystalline lens (or the IOL), and therefore will have in any case a lower retinal toxicity.

2. The slit is tilted between 5 & 7 degrees from the axis

This produces a better visibility of diffracted light. For epiretinal membrane surgery or macular hole surgery, if dyes are not used, one needs to see the light reflected by the gliosed internal limiting membrane. Because the intraocular light source illuminates the macula with an angle of 25°, the great majority of reflected light remains inside the ocular globe as it bounces with an angle of 25° from the axis. On the contrary, the slit lamp light illuminating the macula at an angle of 5°, the great majority of reflected light will reach the surgeon’s eye mirroring the reflection of the membranes video7.
This increased membrane reflection observed under slit lamp illumination provided me with two advantages :

  • The idea and practice since 1985 of systematic ILM peeling.
  • The unneeded use of dyes for most of my macular procedures (excluded edemas or macular hole without membrane reflection)

3. In conclusion, slit lamp use allows a less traumatic and faster procedure

All the above specific and general advantages allow :

  • A more mini-invasive procedure :
    • Two sclerotomies at 11 o’clock & 12 o’clock only are necessary.
    • Without vitreous traction induced by an intraocular fiber
    • Core vitrectomy only is necessary (no need to remove vitreous colored by tripan blue)
    • Protection of the crystalline lens.
  • A faster procedure
    • Only two sclerotomies and core vitrectomy only
    • No need for coloring agents in most cases
    • Faster peeling due to the improved precision given by forceps stabilization
    • As numerous visitors could observe it along the year, I routinely perform around 14 macular surgeries in one single operating room from 9 am to 1 pm, with 8 minutes operating room cleaning after each procedure.
C. Specific advantages of retinal detachment surgery

1. The slit lamp is a slit lamp

This means that it allows an optical cut of the transparent tissues. This is why the great majority of ophthalmologists worldwide systematically use a slit lamp in the clinical diagnosis of anterior segment diseases. This is why it is our emblematic instrument.

  • It facilitates vitreous analysis
    It is really astonishing that posterior segment surgeons neglect this device as if it was not useful to study the vitreous (a transparent tissue as well) in vitreoretinal diseases. Retinal detachment is the result of a vitreoretinal conflict and analyzing only the retinal part of this conflict does not provide a thorough understanding of the pathogenesis. The optical cut provided by the slit allows identification of the posterior hyaloïd location and analysis of vitreous traction characteristics video8. With the help of slit illumination, we can then distinguish a static vitreous traction video9 from a dynamic one video10 ; we can discern if vitreous strands are relaxed enough by indentation and analyze the tears’ relationship with the posterior hyaloïd. All these details are important if you want to adapt the surgical strategy to the clinical findings and the pathogenesis of the detachment.
  • It facilitates the anterior hyaloïd removal in phakic eyes.
    As the optical cut enables the identification of the posterior capsule of the lens, it becomes easier to remove the anterior hyaloïd. One can move the cutter close to the capsule, avoiding any contact with it video11.
  • It allows better control of the cryopexy
    The optical cut allows us to observe the progression of the ice ball. In addition, this is even easier if one is working with microscope magnification ; excess of cryopexy is then easier to avoid than with indirect ophthalmoscopy.

2. The slit lamp is fixed to the microscope

  • For external procedures as compared to indirect ophthalmoscopy
    • Improved control thanks to microscope magnification
      With the slip lamp and a contact lens (three mirrors lens or others), you can observe the vitreoretinal disease with the microscope. You can therefore benefit from the microscope magnification, ie from an increased separating power : a small peripheral 50 microns hole that cannot be seen with indirect ophthalmoscopy (because it is optically impossible) will be detected at the highest microscope magnification. Such improved visibility of details has two major advantages :
      • Surgical strategy
        If one can be certain, after a pre and/or per operative examination (with or without the help of additional scleral depression), conducted in good conditions (good mydriasis, posterior capsule and vitreous conditions…), that the amount of retinal breaks you have detected are the only breaks present at the time of your examination, one can buckle only where the breaks are located. Such a small detail, which acts on your gut feeling, has enormous repercussions on your surgical strategy. Due to its optical limits, indirect ophthalmoscopy does not allow to be certain of having identified all the breaks and of not having missed one or two additional 50 microns holes : in order to avoid the risk of failure related to an undetected break, one takes the option of performing a 360° indentation despite the post op CME risk by strangulation. Indeed this option does reduce the recurrence risk, but the post operative functional results as well. Using the slit lamp allows me to never perform a 360° circular indentation in PVR stages lower than C2. Indeed, I have 7 to 8% recurrence in stage A (a limited indentation involves less relaxation of the vitreous base than a circular one) but a higher functional result (no significant CME rate) video12.
      • Intra operative control
        All intra operative maneuvers that involve the skill of retinal surgeons (accurate break localization, intensity of cryotherapy, control of the indentation relaxation effect, control of perfect break reapplication on the pigment epithelium) are achieved thanks to microscope magnification. Additionally, as well it is easier to control proper IOL positioning in the bag with the microscope than without, in the same way these controls conducted under the microscope are easier to achieve and therefore more accessible for a beginner. No experience is necessary to recognize the proper spreading of the tear edges over the indentation ; you only have to place the Goldman lens and increase the magnification : the optical cut will allow you to identify the different plans with respect to the others and to control the absence of residual subretinal fluid.
    • Less time and energy
      Intraocular verifications made with indirect ophthalmoscopy require a lot of energy and time : the surgeon must stand up, the nurse switches on the light, removes the operating table, gives the ophthalmoscope to the surgeon who fixes it on his head, takes the lens, moves around the patient, conducts the verification, changes the gloves…all the maneuvers being made in reverse order when the surgeon goes back to his/her seat. I will not even consider the problem of flaws regarding asepsis rules ! All this incredible waste of time and energy compared with one single maneuver under microscope !
      An intraocular verification made with the slit lamp simply requires placing a contact lens on the cornea and switching on the lamp with the foot pedal. This is completed in ten seconds and the surgeon remains seated. Three consequences can be observed :
      • Less time : a retinal detachment procedure including cryo, buckling, drainage and gas tamponade requires, with the slit lamp and the microscope an average time, of 12 to 15 minutes. You can then accept additional emergency cases and eliminate waiting lists, which should be avoided in this pathology
      • Less energy : after ten retinal detachments (each of them requiring between four to eight intraocular observations) the surgeon remains alert and aware
      • Less time and energy : the surgeon does not hesitate to conduct additional security checks until he is sure of having accomplished the right buckling procedure. The overall quality of the surgery is increased video13.
  • For vitrectomy as compared to intraocular pipe
    The main advantage provided by the microscope with slit lamp fixation is that the surgeon’s left hand remains free
    • For simultaneous self performing scleral depression
      In retinal detachment surgery with vitrectomy, as much vitreous as possible must be removed. A scleral depression is therefore required. Scleral depression decreases the risk of touching the crystalline lens with the cutter, presses the retina on the induced indentation and therefore facilitates the catching of adherent vitreous fibers. It is particularly necessary during peeling of the posterior hyaloïd from the retina - from the posterior pole to the equator-, having incarcerated the posterior hyaloïd in the cutter port : meanwhile, one needs to size up the amount of scleral depression according to the specific area one is dissecting ; the location and/or the depth of the depression needs to be constantly adjusted (more centripetal, more posterior, more anterior…) video14. Such a delicate balance between scleral depression and vitreo retinal dissection can be only best and safely conducted by the surgeon himself, performing a naturally coordinated bimanual maneuver, even if he/her has been working with the same assistant for a long time video15.
      Some surgeons do use endoillumination pipes but are aware of the advantages of doing the scleral depression themselves. They subsequently perform the retro equatorial vitrectomy with the endocular pipe and then remove it to perform the anterior vitrectomy under the microscope coaxial light and self scleral depression. However, they actually lose the continuous visualization of the vitreous cavity from the posterior pole to the ciliary body and may miss some remaining adherent vitreous fibers. The continuity of retinal observation is another advantage provided by slit lamp illumination.
    • For decreasing vitreoretinal incarceration risk
      Globally, combining all these advantages, working under slit lamp illumination is the only system that I have found for performing a complete vitrectomy without any exit of the handpiece, and this decreases vitreoretinal incarceration risk. The 300 inferior degrees peripheral vitrectomy is performed with the help of a manual indentation. When approaching the infusion placed at 12 o’clock, the plano concave lens is removed and replaced by a Goldman lens. I can therefore remove the peripheral vitreous in front of the infusion and around the sclerotomy of the handpiece without any scleral depression. The plano concave lens is then replaced to finish the peripheral vitrectomy all around the globe. All this procedure is done at once and it is only when it is completed acheived that I can remove my handpiece video16.
    • For performing bimanual intraocular maneuver
      Under slit lamp illumination, when a PVR is too difficult to dissect or/and when a manipulation requires the action of the second hand, it is very simple to perform a third sclerotomy and use your freed second hand video17.
  • For vitrectomy as compared to scleral fixated light
    The latest large field illumination systems, fixed to the sclera, provide sufficient illumination for the retro equatorial work (if we accept the risks mentioned above). However, they do not allow the illumination of the entire vitreous base. In addition, the scleral depression made behind the fixed fibers naturally pushes them into the crystalline lens video18. Two options are offered :
    • Performing the depression, taking the risk of inducing a cataract
    • Not removing the vitreous in this area, taking the risk of a post operative anterior PVR
      A third option could be to systematically remove the crystalline lens, but this cannot be considered as mini invasive surgery.

3. The slit is tilted between 5 & 7 degrees from the axis

This is perhaps the most difficult advantage to explain, as it calls for a nuanced, “feeling-based” point of view. Indeed an intraocular pipe does provide a fantastic enlarged illumination of operative field, but the image will be a bit flattened. Under a slit illumination, you will benefit from two different aspects. First, the retro illumination will highlight the small breaks while targeting on the adjacent areas video19. Secondly, the illumination axes will be very close to the viewing axis, thereby highlighting the fine details. A concrete explanation can be illustrated by this example : a young mother drops her sewing box—which contains a spool of cotton, a thimble and needles—in her baby’s room. She must find the items. If she uses a halogen lamp, stands up on a chair and illuminates the ground, she will have a large illumination but will only see the spool of cotton because of the lack of magnification : this is what wide-angle systems provide. She can take a flashlight and move around in the room, and will find the thimble ; this is what an intraocular pipe with plano concave lens will provide because of the magnification. But this mother will be relieved only when she will have found the needles. What will she do ? She will crawl on the floor, with her eyes very close to the carpet, illuminating the whole floor with her flashlight using a very tangential illumination. Only in this condition will she see all the needles and leave the baby’s room. This is exactly the same sensation that I have when using the slit lamp. Combining the advantage of the manual depression and this tangential examination, which allows detection of the last vitreous remainders attached to the retina, I have the sensation of having removed all the fibers, a feeling that I never had with intraocular illumination video20.

D. Disadvantages

Slit lamp use presents two disadvantages video21 :

  1. Decreased illuminating field. To avoid reflection, we must use a slit lamp of 2 to 3 mm wide, thus decreasing comfort when working at low microscope magnification. But at higher magnification, this becomes a minor problem. Personally, I usually perform all my dissecting work at the highest microscope magnification, therefore illuminating the entire field.
  2. This kind of set-up is currently cumbersome and prevents us from using certain wide angle viewing systems attached to the microscope. I think that the forthcoming incorporation of the slit lamp within the operating microscope body will further facilitate its universal use.


Each new technique has a learning curve ; only individual experience allows one to solve specific problems of reflection or overcome uncomfortable feelings while working with a three mirrors lens. Here below are some tricks that could help you during this learning curve.

A. Adjustment of the slit

1.On which side of the microscope ?
This depends on your personal habits ; you will act as you do with your office slit lamp. Personally, in my office, I always put the slit at my right side, regardless of which eye I examine. In the operating room I do the same, putting the slit on the right side. Some surgeons like to change, using a right slit for a right eye and a left slit for a left eye.

2.Which width must we select ?
The width choice is a compromise between the desire to have a wider illuminated field (then increasing the slit width) and having the best optical cut without too much reflection causing interferences (then reducing the slit width).

  • My usual slit width is about 3mm. This is convenient for 99% of the cases
  • When the crystalline lens is very clear, the mydriasis very good, and you want to make a movie (therefore requiring a wide illuminated field), you can select a 4mm slit (for the systems which allow to select the slit width)
  • When there is a cortical cataract (increasing the reflected light) or when you want to make a capsulorhexis in a white cataract, it is better to select a 2 mm width

3. Which is the right slit orientation ?
The slit orientation must be frontal, or, more exactly, perpendicular to the axis joining the microscope oculars. How many of us, in the office, put the slit in a horizontal position ? We naturally use the vertical position because it increases the stereoscopic viewing (the image projected in the right examiner’s eye is different from the left one). The same thing occurs in the operating room ; the systems offering a slit orientation parallel to the axis joining the oculars will not provide the same stereoscopic optical cut.

4.What is the right positioning as compared to the microscope coaxial light ?
Again, this is a question of compromise. The more you decrease the tilt angle, the lower the optical cut. This is why a slit in a coaxial position does not give any optical cut (in our office, we never put the slit in the middle of our oculars to examine the cornea or the lens). However, if the tilt angle is increased too much, the slit light can be stopped by the iris before reaching the retina. A good compromise is to select 5 to 7 degrees from the microscope axis. To obtain this angle I can give you the following trick : let us suppose that you want to put the slit on the right side. Select the lower microscope magnification and move the slit lamp system closer until it begins to encroach upon your right ocular field (you see the shadow of the slit protection in the right side of the image of your right ocular). At that moment, the slit system will be at the appropriate position. As soon as you increase the microscope magnification, the protection shadow disappears.

B. Other adjustments

1.Patient’s head
During a macular surgery, working with an intraocular pipe, one benefits from two anchor points to orientate and move the patient’s eyeball in order to present the macular area to the surgeon’s observation. The hand maintaining the fiber can be dedicated to eye orientation so the right hand can be used for precise peeling work. Working with a slit lamp, one has only one anchor point. One should therefore pay more attention to the patient’s head positioning, avoiding lordosis or kiphosis of the neck in order to have the patient’s eye looking exactly in a vertical direction. One will then be able to use the right hand for peeling work, the other hand maintaining the forceps and therefore increasing precision.

2.For peripheral retina observation
The best visualization of the peripheral retina will be obtained when observing the retina placed in the slit axis ; in other words at 6 or 12 o’clock. When searching the tears or performing the cryopexy, one must ask the assistant to turn the patient’s eyeball in order to present the required peripheral area at 6 or 12 o’clock. To do this, he will pull the sutures placed on the muscles surrounding the dedicated area video22.

C. Selection of useful instrumentation

Some instruments can simplify the manipulation when working with a slit lamp

1. Shorter instruments
The slit lamp system currently proposed by microscope manufacturers is cumbersome and reduces the free space between the microscope and the eye by 5 to 6 cm. One should avoid using instruments that are too long and could therefore touch the slit. For example, I am used to selecting shorter strabismus hooks.

2. Plano concave lens
With intraocular illumination, the light travels only one time through the plano concave lens. With extra ocular illumination it crosses it twice. A very transparent lens, without scratches, is therefore highly recommended. I use a disposable plano concave lens, produced by FCI, for many reasons :

  • It is always transparent
  • It is cheaper than a reusable lens
  • The superior surface has a raised edge that allows the assistant to place it easily on the cornea and to center it as requested video23

3. Three mirrors lenses
The normal Goldman lens is too big for a retinal detachment surgery, especially when performing the cryopexy (the cryo probe touches the lens edges). The lens for children is not appropriate for operating on adults. I recommend you use the disposable three mirrors made by FCI, which are reduced in height, therefore allowing the probe to move around without touching them. In addition, as it is disposable, a chemical sterilization is no longer necessary. The cost is very low (between 25 and 30 Euros in France)

D. How to begin with the slit lamp

To begin the learning curve with the slit lamp, we should select cases where slit lamp use will not greatly change your normal habits or cases where slit lamp use will immediately provide great advantages.

  • In the first category, we can find all macular surgeries. You can start in performing two scelerotomies (the infusion at 12 o’clock and your hand piece at 10:30 or 1:30). If you do not feel comfortable, you can always stop and make a third scelerotomy for the intraocular pipe. Slowly, you will realize that “optional’ illuminating system allows you to save time (and money) and increases precision.
  • In the second category, you can find all the procedures where bimanual action is required, like IOL dislocation, crystalline lens dislocation, or difficult ERM peeling. Slowly, you will realize the advantage of bimanual action, and extend the use of the slit to peripheral vitrectomy and retinal detachment. Be prepared to encounter a lot of minor difficulties in the application of these aforementioned procedures.


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