Abstract

Echographic (A scan) examination performed after the intravitreal injection of aqueous in four rabbit eyes demonstrated echoes of a 10 dB dynamic range. These are similar to those demonstrated in cases of malignant glaucoma.

After paracentesis, the dinamic range of these echoes was markedly reduced. The possible role that intraocular pressure plays in the variation of the chemicophysical status and, therefore, in the acoustic impedance of the vitreous humor. is discussed.

Introduction

So-called "malignant glaucoma" can appear after antiglaucomatous surgery.

It is characterized by an aqueous secretion, coming from the ciliary body, in the vitreous. The anomalous direction of the secretion causes a forward displacement of the iridolcnticular diaphragm, a shallow or flat anterior chamber and a rise in intraocular pressure.

As yet, the only effective therapeutic measure against this condition is posterior sclerotomy with aspiration of the acqueous; several authors (Buschman 1978; D'Ermo et al. 1979; Steindler 1980) have proposed a technique for the echographic localization of pseudocystic acqueous formations in the vitreous humor. Locali­ zation of the sacs is essential in performing an exact diathermic puncture.

Our experience has also confirmed that the surface dividing the aqueous from the vitreous in cases of malignant glaucoma is evidenced with the use uf ultrasound A scan. Diathermic puncture in the echographically localized area has always demonstrated that the method of localization is exact.

In all cases treated in our Institute, the sacs of acqueous were found to cor­ respond to an echo which appeared at maximum sensitivity of the instrument, presenting a dynamic range of about 10 db. We therefore wanted to establish which structure determines the echo and whether this structure is a surface of separation between the aqueous and vitreous with such reflecting potential.

As diathermic puncture often demonstrated the outflow of mixed acqueous and vitreous, it does not seem likely that the aqueous sacs are totally separated from the vitreous. This suggests that the difference in acoustic impedance between aqueous and vitreous, which determines an echo, can appear as a result of a change in the chemico-physical status of the vitreous which is caused by intraocular hypertension. We undertook the following experiment in order to confirm this hypothesis.

Materials and methods

Intraocular pressure of a rabbit eye was measured with a tonometer (Mackey Marg) after intravenous injection of Nembutal (1.5 cc). Then 0.2 cc of rabbit aqueous humor yas injected into the vitreous using a tuberculin syringe with a 25 - gauge needle placed 6 mm from the limbus. Immediately after, the pressure (measured with the Mackey Marg T) rose to 45 mm Hg. An echographic exami­nation, performed with an 8 MHz sound (Kretz Technic Instrument - 7200 MA) demonstrated a 10 db echo similar to those demonstrated in cases of malignant glaucoma (Fig. 1).

Therefore, a paracentesis of the anterior chamber was carried out to render the eyeball hypotonic. Echography performed according to the above-mentioned method, demonstrated a peak with a variable height and with a dynamic range of about 5 db (Fig. 2). This experiment was repeated in four rabbit eyes. The same results were obtained.

Conclusion

It is well known that the intensity of echoes is proportionate to the reflectivity of the surface of separation between the two different humors, and the reflectivity itself also expresses the acoustic impedance between the two humors.

We can therefore assume that a variation in the echo, corresponding lo a rise in intraocular pressure is the expression of a modification in the chemico-physical status of the vitreous humor with consequent changes in its acoustic impedance. If such an hypothesis were confirmed, then the ultrasound localization of aqueous deposits in the vitreous humor would give a correct result, expecially when inlraocular pressure is high.

References

• BUSCHMANN W.: In « Handbook of Clinical Ultrasound», Ed. De Vlieger, Rotterdam, The Netherlands, pag. 842, 1978.

• D'ERMO F., STETNOLER P., PERRONE s.: Preoperative echographic localisation of aqueous humor deposits in the vitreous chamber in post operative hypertonia. Comuncaz. Congr. Alpe Adria 1979, Abbazia (YU).

• STEINDLER P., PERRONE S.: Sulla localizzazione ecografica delle sacche di umore acqueo nei casi di cosiddello glaucoma inaligno. "Boll. Ocul.", 1980, in press.