Introduction

Classical malignant glaucoma, one of the most serious complications of surgery for angle closure glaucoma, is characterized by shallowing or flattening of the anterior chamber and elevation of intraocular pressure (IOP). Medical therapy, consisting of the administration of Mydriatic-Cycloplegic drops, systemic carbonic anhydrase inhibitors or osmotic agents, gives successful results in approximately 50% of cases. 2.3 For the remaining 50% of cases, however, surgery is required.

Among the most effective of the operations recommended in cases of malignant glaucoma are Shaffer's lens extraction and deep incision into the vitreous, and Chandler's pars plana vitreous puncture with aspiration. It is believed that the mechanism upon which the success of the two surgical procedures depends is the piercing of the vitreous through the anterior, and presumably detached, posterior hyaloid with the consequent outflow of fluid trapped in or behind the vitreous. By means of slit-lamp examination of some phakic and aphakic eyes with malignant glaucoma, Simmons and co coworkers were able to observe optically clear areas within the vitreous cavity (interpreted as pockets of fluid) and, in a few cases, a smooth vitreous face anterior to the middle of the vitreous cavity (interpreted as the posterior hyaloid). It is, however, evident that A-scan echography is a more reliable method for the investigation and localization of the interfaces between the aqueous and vitreous because ultrasounds are not affected by the transparency of the dioptic media.

In this study A-scan echography was used to localize aqueous humor in the vitreous chamber in patients with malignant glaucoma before performing diathermic puncture. To verify experimentally whether aqueous sacs in the vitreous chamber can, in fact, be accurately localized by ultrasonography, rabbit aqueous humor was injected into the vitreous cavity of rabbit eyes and observed by A-scan echography and indirect binocular ophthalmoscopy.

The clinical and experimental findings are reported.

MATERIALS AND METHODS

Clinical Study

In four phakic eyes of patients aged between 52 and 67 years (average 60 yrs) with unilateral malignant glaucoma who had previously undergone iridencleisis (2 eyes) and trabeculectomy (2 eyes), A-scan echography (Kretz technik 7200 MA equipment, 8MHz probe, maximum sensitivity) showed one low-to-medium reflectivity narrow based unstable echo spike within the vitreous cavity (Fig 1).

Eight meridians (starting from the 6 o'clock position) were examined. Echoes within the vitreous cavity were found only in one quadrant, the lower temporal in three patients and the lower nasal in one. Deep diathermic puncture of the scleral wall by way of the pars plana in the echographically localized area always resulted in free outflow of clear fluid. The vitreous was never aspirated and air was injected into the anterior chamber at the end of the operation. In two to three years of follow-up, the IOP has remained within normal values. Postoperative echography showed echo-free vitreous chambers in all cases. All patients were given topical atropine 1% drops in the operated eye twice daily and pilocarpine 1% three times daily in the fellow eye.

The echoes found in cases of malignant glaucoma are due to the slight difference between the acoustic impedance of the condensed vitreous surface and aqueous humor; these echoes are similar to those observed in cases of posterior hyaloid detachment.

Experimental Study

Into 30 rabbit eyes, 0.2 cc of rabbit aqueous humor was injected, through the pars plana, into the vitreous using a 25-gauge needle attached to a tuberculin syringe. The pupil was previously dilated and preplaced 5-0 mersilene U suture knotted after removing the needle. Immediately after the injection, the IOP (measured with a Perkin's applanation tonometer) rose from about 10 mm Hg to 50 mm Hg. With the Kretz unit at maximum sensitivity and the probe aimed at the site of the injection, it was possible to observe in the vitreous cavity one medium to high reflectivity echo with a dynamic range of about 10 dB (Fig 2).

After about 20 minutes, the vitreal echo disappeared, the IOP dropped to about 10 mm Hg, and the optically clear roundish area that could be seen in the vitreous cavity by indirect binocular ophthalmoscopy completely disappeared. Paracentesis of the anterior chamber was performed in ten rabbit eyes five minutes after the intravitreal injection of aqueous. As expected, the IOP values dropped abruptly, but the rate of disappearance of echographic and ophthalmoscopic vitreal evidence remained unchanged. No decrease in the echo amplitude was observed immediately after paracentesis.

RESULTS AND DISCUSSION

The aim of this study was not to reproduce an experimental animal model of malignant glaucoma, but only to investigate the echographic, tensional, and ophthalmologic changes in rabbit eyes in which aqueous had been injected into the vitreous.

The experimental findings allow us to conclude that

1. The interface between the vitreous and aqueous can be reliably localized by means of A-scan echography

2. In all cases we found only one vitreal echo; the aqueous sacs touched the retina

3. The rate of disappearance of aqueous was not influenced by IOP

According to Kinsey and coworkers one-half the water in rabbit vitreous is replaced every ten to fifteen minutes. It is not surprising therefore that in our experiment about the same period of time was required for a complete outflow of the injected aqueous from the vitreous chamber.

Our clinical findings indicate that

1. A-scan echography is a useful and accurate device for the localization of aqueous sacs in the vitreous

2. Only one vitreal echo was found in our cases, and in none did we find two vitreal echoes with an echo-free space between them as reported by Buschmann in cases of malignant glaucoma (he used a 6 MHz probe)¹

3. The pars plana vitreous diathermic puncture without aspiration employed by us is based upon the same working hypothesis as Chandler's pars plana vitreous puncture with aspiration; this probably accounts for our successful results²

   We were not able to discover whether the posterior hyaloid was detached in the cases of malignant glaucoma we studied because the vitreal echo in cases of posterior detachment and that arising from the interface between vitreous and aqueous in cases of malignant glaucoma are similar to those observed in clinical cases.

We think that this last point deserves further investigation.

REFERENCES

• Buschmann W. Special techniques. In: De Vlieger M, et al., eds. Handbook of Clinical Ultrasound. New York: John Wiley & Sons, 1978; 842.

• Chandler PA, Simmons RJ, Grant WM. Malignant glaucoma: medical and surgical treatment. Am J Ophthalmol 1968; 66: 495 - 502.

• Frezzotti R, Bardelli AM, Nuti A, Casini P. Follow-up a distanza della nostra casistica di glaucoma atalamico. Anni 1961-1980. Boll Oculist 1981; 60: 829 - 40.

• Kinsey VE, Reddy DVN. Chemistry and dynamics of aqueous humor. In: Prince JH, ed. The Rabbit in Eye Research. Springfield: Charles C Thomas, 1964; 218 - 319.

• Shaffer RN. The lens and glaucoma. Symposium on Glaucoma; Transactions of the New Orleans Academy of Ophthalmology. St Louis: CV Mosby, 1975; 249 - 56.

• Simmons RJ. Malignant glaucoma. Br J Ophthalmol 1972; 56: 263 - 72.