Using Imaging to Understand the Causes and Sequelae of Hypotony

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Speaker 1 (00:46): Welcome to GT: The Podcast. In this episode, Dr. Poojitha Balakrishnan recaps her article “Using Imaging to Understand the Causes and Sequelae of Hypotony,” written with Dr. Jithin Yohannan and featured in the March/April issue of Glaucoma Today. In this episode, Dr. Balakrishnan discusses hypotony as a significant complication of glaucoma surgery and the challenge identifying its causes in sequelae with only slit-limp examinations and indirect ophthalmoscopy. She suggests that ocular imaging modalities such as OCT, A and B scan ultrasound, and ultrasound biomicroscopy may be valuable complimentary tools for diagnosis and management. For more on this, listen now to Dr. Balakrishnan.

Poojitha Balakrishnan (01:36): Hypotony, which is typically defined as IOPs less than five to six mmHg, may be a significant complication of glaucoma surgery and is most often frequently observed during the postoperative period, especially in the early postoperative period following filtering surgery. Although the condition is often visually insignificant or transient, persistent and visually significant hypotony can occur and leads to several adverse outcomes. Clinically significant hypotony may induce neurosensory retinal and structural alterations, potentially resulting in irreversible visual impairment. Therefore, identifying the causes and sequelae of hypotony is important, but can also be challenging with the usual slit-lamp examinations and indirect ophthalmoscopy alone. Ocular imaging therefore is important in identifying hypotony and looking at the underlying causes. Modalities such as OCT and ultrasound are some of the more valuable tools used for prompt diagnosis and management. Hypotony maculopathy from persistently low IOPs can create retinal folds and choroidal folds in the macula as well as the peripheral retina.

(02:51): This can reduce or distort patient's vision and peaks and valleys in these folds from severe maculopathy are sometimes visible on fundus examination, but subtle cases can be difficult to detect. In such subtle cases of hypotony maculopathy, OCT of the macula can be important in highlighting chorioretinal folds involving both the retinal pigment epithelium and the Bruch membrane. And in more advanced cases, one can see extensive folds in the inner retinal layers. The surgical reduction of IOP can cause choroidal expansion, which shifts the eye's focal point more posteriorly relative to the retina, and this can actually cause a hyperopic shift. Conversely, when there's decreased IOP, there can be a relative pressure gradient in anterior movement of the lens iris diaphragm. This can temporarily induce a myopic shift typically seen in the early postoperative period. Although pre and postoperative manifest refraction can assist with this clinical diagnosis, A-scan ultrasonography and optical biometry can provide a more reliable and accurate measure of the axial length, the anterior chamber depth, and the effective lens position.

(04:12): In cases of hypotony, choroidal detachments are often accompanied by a very shallow anterior chamber. Patients may experience a decrease in vision if the choroidal detachment extends over or involves the macula. It can also be visually significant if there is shallowing of the anterior chamber. This can then cause a refractive shift. There are two types of choroidal detachments, serous and hemorrhagic. The serous choroidal detachment is more common. Mechanistically, both are seen with a decrease in IOP, which results in a pressure gradient between the choroidal space and the choroidal capillaries. In serous choroidal detachments, the differences in hydrostatic pressure allow movement of transudative fluid from the choroidal capillaries into the suprachoroidal space, causing a dome-shaped separation that is typically seen of the choroid separating from the sclera. Common risk factors for serous choroidals include increased age, extreme axial lengths, both high myopia or nanophthalmos, as well as chronic uveitis.

(05:23): Symptoms typically include decreased vision and mild dull pain that is caused by scleral stretching and rotation of the ciliary body itself. On the other hand, hemorrhagic choroidal detachments result from an abrupt substantial decrease in the IOP, which ruptures the short posterior choroidal arteries or choroidal veins. The hemorrhage collects in the suprachoroidal space, separating the choroid from the sclera. For this reason, hemorrhagic choroidals tend to present more suddenly and with more significant pain compared with serous detachments. Risk factors for hemorrhagic choroidal detachments include high preoperative IOP, hypertension, high myopia, post-vitrectomy status, and systemic anticoagulation. Visualization on indirect ophthalmoscopy is possible when choroidal detachments are large, encroaching on the macula or are kissing choroidals. However, when the detachment is more anterior, the diagnosis and classification can be achieved more routinely and accurately through B-scan ultrasound imaging. A well-circumscribed smooth and dome-shaped membrane will be visible rising in a convex shape into the vitreous cavity on ultrasound.

(06:41): Unlike retinal detachments, choroidal detachments are seen as more immobile and can extend past the ora serrata on B-scan. For serous choroidal detachments, the space beneath the membrane appears hypoechoic, and in hemorrhagic choroidal detachments, that space appears hyperechoic. This distinction can be very crucial for management. Chronic and severe hypotony can cause the globe to shrink, and this can often be accompanied by a complete or significant loss of vision and eye function. Diagnosis typically can be reliant on clinical history and examination. Common signs include pseudoenophthalmos and dystrophic changes of the cornea and lens. B-scan ultrasound can help confirm this reduction in axial length and also highlight internal disorganization of anatomic structures and an absence of retinal or choroidal detachments. B-scan can also further identify dystrophic tissues, which can demonstrate posterior shadowing and hyperechogenicity due to calcification. Finally, hypotony may also result from a cyclodialysis cleft. This is often seen in the setting of blunt ocular trauma or iatrogenic issues, including after MIGS surgery.

(08:05): The cleft often occurs due to a physical disinsertion of the longitudinal ciliary muscles from the scleral spur. Gonioscopy is of course the gold standard for diagnosis, and usually the extent of the anatomical separation between the ciliary body and scleral spur is indicated by a white sheen reflecting the bare scleral wall and the abnormal posterior displacement of the iris root. However, gonioscopy can be quite difficult in settings, especially after trauma, when hypotony, corneal edema or hyphema is often present. All of these are especially common in a setting where there is a cyclodialysis cleft. Ultrasound biomicroscopy is therefore an important complimentary method for gonioscopy and can help diagnose and evaluate the circumferential extent of a cyclodialysis cleft. On imaging, the cleft appears as a distinct hypoechoic space between the anterior chamber and the suprachoroidal space. UBM can be crucial for preoperative planning as well as postoperative management of large clefts.

(9:18): UBM can also help monitor the resolution of smaller clefts.

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