Role of mannitol in cataract surgeryBT Shah, BT Maskati
Department of Ophthalmology, K.E.M. Hospital and Seth G.S. Medical College, Parel, Bombay-400 012., India
Hypertonic mannitol solution has been found to be most effective in preventing vitreous loss in cataract surgery due to profound hypotony and detergence of vitreous following intravenous infusion. Routine use of mannitol is hence recommended prior to cataract surgery, especially in all one-eyed patients or in cases where there is an increased risk of vitreous disturbances.
Osmotic agents like urea, mannitol, glycerol have been effectively used earlier to lower both the cerebro-spinal pressure and intra-ocular pressure. Mannitol is a hexahydroxy alcohol related to mannose. It occurs as a white, crystalline powder and is soluble in water and stable at room temperature. Intravenous administration of mannitol induces diuresis by elevating the osmotic pressure of the glomerular filtrate to such an extent that tubular re-absorption of water and sodium is hindered. Mannitol also promotes excretion of chlorides. It itself is excreted unchanged in the urine. In the present series, a study is made to evaluate the use of mannitol in cataract surgery.
Fifty patients with different types anyl grades of cataracts were studied. They were divided in two groups of 25 patients each. Group I patients were given an intravenous infusion of 20% mannitol, 60 minutes prior to cataract operation, at a rapid rate of 60-90 drops per minute, the total dose of mannitol being 1.5-2.0 gms/ kg. body weight.
Group II served as controls in whom acetazoleamide was given in a dose of 250 mg. thrice on the previous day and 500 mg. in the morning on the day of operation. The types of cataracts in the patients is shown in [Table 1].
Intra-ocular tension was recorded by means of a "Schiotz tonometer" in each case prior to and after treatment with the diuretic agents and after this, facial and retrobulbar blocks were given by injecting 2% zylocaine locally. In all patients peripheral button-hole iridectomy was done and an intra-capsular cataract extraction was attempted with a cryo-probe. A record was made of the type of extraction and the vitreous face was examined and classified into 4 categories depending on its position:
0 = normal vitreous face-concave and behind iris
+= iris bulging forwards
++= vitreous in anterior chamber
+++ = vitreous loss.
The details regarding the type of cataract extraction, pre- and post-mannitol intra-ocular tensions and position of the vitreous face are recorded and presented in [Table 2] and [Table 3].
The comparison between the average falls in the intra-ocular tensions in both the control and mannitol treated groups is shown in [Figure 1]. The position of the vitreous face as found at operation in different individuals is shown in [Table 4].
Complications: - Mannitol-treated group I patients experienced excessive thirst, diuresis, slight headache and nausea, whereas control group patients had paraesthesia, epigastric burning pain, nausea, vomiting and headache. These were treated accordingly.
Reduction of intra-ocular pressure prior to cataract surgery is essential to keep vitreous in its "physiological position" after lens extraction and to minimise post-operative complications. Osmotic agents have been utilized to soften the eye, prior to surgery from as early as 1904 by Contonnet.  He used hypertonic sodium chloride for the same and since then a number of other substances like dextrose, sucrose, magnesium sulphate, sorbitol, glycerol, urea and mannitol have been used as ocular hypotensive substances.
Virno and others  studied oral glycerol efficacy. in
Intravenous hypertonic urea was effectively used by Galin et al , and Davis et al  in separate studies.' Recently Crews and Davidson,  Tartar  and Scotty studied cases of acute congestive glaucoma of varied etiology with an average ocular tension of 50 mm of Hg and found 84% response with intravenous urea administration. This series also included cases with lens induced glaucoma and anterior dislocated lens with glaucoma which had not responded to therapy with acetazoleamide.
Use of hypertonic mannitol as an osmotic agent was reported with success by Barry et al , in cases with functional renal failure and oliguria and has since then been an accepted part of therapy. Mannitol was also found a very effective agent for reducing cerebro-spinal fluid pressure. Wise and Charter  demonstrated, in anaesthetised dogs after ligation of renal arteries, that the cerebrospinal fluid hypotensive effect of mannitol was independent of its diuretic effect without secondary rebound overshoot of pressure. Effect of mannitol in reducing intra-ocular pressure was studied by Smith et al  in normotensive and glaucomatous eyes along with blood osmolarity study. and the mean: reduction in ocular tension was found to be 48% in normotensive eyes (average fall about 8 mm . of Hg) and ,52% in glaucomatous eyes with return of intra-ocular pressure to its initial value in 2½ to 4½ hours from the time of initial reading. In vivo, this was demonstrated by Robbins and Galin,  to be due to temporary reduction in volume of vitreous gel following administration of hyperosmotic agents. Hyperosmotic agents produce hypertonicity of blood and cause temporary removal of fluids from vitreous gel. Brubekar and Riley  calculated the reduction of vitreous volume from changes in its weight in rabbit's eye and found it to be 2.7% with orally administered glycerine and 3.7% with intravenously administered urea or mannitol.
Galin and others  made an osmotic comparison of urea and mannitol and found urea superior as an ocular hypotensive agent at equal dosage levels. This was due to the lower molecular weight and poor ocular penetrance of urea. The hypotensive effect was also more lasting than mannitol, which is rapidly cleared by the kidney (1% per ml of loading dose). Also mannitol requires more rapid infusion time than urea. The maximum reduction in intra-ocular pressure occurred approximately 45 minutes to 4½ hours following administration of either. The rise to control levels was reached in about 5 hours for urea and 3 hours for mannitol.
Weiss and Shafer  preferred mannitol for clinical use because of the following reasons. Urea is unstable in solution and must be freshly prepared before using a tedious and time-consuming procedure. It cannot be sterilized by heat. Patients complain of severe pain in the arm due to proteolytic effect of urea in 30 % solution. There may be severe local reaction on extravasation with sloughing and tissue necrosis. Urea also had slight fibrinolytic activity and increased bleeding tendency. Mannitol is found to be equally effective and it is clinically a safe drug. Molecular weight of mannitol is three times that of urea; however, it is concentrated in extracellular fluid compartments which comprises only 1/3rd of the total body water, and hence the relative disadvantage of greater molecular weight than urea is largely overcome. Weiss  recommended mannitol as it can be easily administered and is stable in solution, inert and non-toxic as compared to other osmotic substances.
Kornblueth et al , used urea prior to cataract surgery in a large series, which included all types of cataract. Uncomplicated and complicated cataracts accompanied by glaucoma or high myopia were included in the study and intracapsular cataract extraction was done with absence of vitreous loss. The results of their study of anterior vitreous face with same gradation after intracapsular cataract extraction with peripheral buttonhole iridectomy is shown in [Table 5].
Urea was found to give best results in the above study. Keeping the same guidelines, the present study is made in patients, where mannitol was used instead of urea for obvious reasons as stated earlier.
In a series of patients, it was observed that vitreous loss was absent with the use of mannitol prior to cataract surgery with reduced vitreous disturbances.
Tolerance to mannitol being generally good, except for excessive thirst and diuresis, no unpleasant side-effects were noted. There was absence of cellulitis or tissue necrosis following accidental subcutaneous extravasation of mannitol.
We take this opportunity to thank Dr. C. K. Deshpande, Dean, K.E.M. Hospital and Seth G. S. Medical College, Bombay-12 for allowing us to use the hospital data.
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]