Ultrasound biomicroscopy: An invaluable asset in glaucoma

Abstract

Ultrasound biomicroscopy (UBM) is a non-invasive and high-resolution approach to studying anatomical characteristics and relationships among various anterior segment components. This imaging approach provides real-time cross-sectional images even in opaque corneas. It is especially valuable to assess lesions and document anatomical abnormalities, i.e., irido-cilliary complex, zonular abnormalities, and mass lesions in the anterior segment. This review article gives an overview of the UBM imaging technique and its applications to various types of Glaucoma.

INTRODUCTION

Ultrasound biomicroscopy (UBM) is a non-invasive imaging technique that produces in-vivo, cross-sectional images in real-time. Since its invention by Pavlin and Foster in 1990, this approach has been a valuable tool for analyzing anterior segment features.^[1]

Even when the anterior media segment is obscured by significant opacities, UBM’s real-time in-vivo imaging allows us to visualize, interpret, and measure anterior segment anomalies. The sole method available to image structures behind extensive corneal scars and to picture the iridociliary complex is UBM. The cross-sectional pictures generated by UBM are demonstrated to be equivalent to histological abnormalities.^[2]

PRINCIPLE

Images from High-frequency ultrasound biomicroscopy are obtained using transducers that oscillate between 35 and 50 MHz. This frequency range provides the ideal balance of resolution and depth of penetration for analyzing the anterior segment. The images have a depth of up to 4 mm, a lateral resolution of 50 microns, and an axial resolution of 25 microns.^[3]

THE TECHNIQUE OF UBM

This imaging procedure can be carried out under topical anesthesia on cooperative patients and under general anesthesia for resistant pediatric patients. The UBM can measure and record the precise location, thickness, and extent of any lesion in the anterior segment.

The marker on the probe enables us to determine the accurate orientation of the image and captured real-time cross-sectional images are seen on the computer monitor display.

For picture acquisition, the UBM probe must be submerged in coupling media. It can be performed with the traditional immersion method/scleral shell or the sterile balloon (ClearScan) method. As a coupling agent, either water or methylcellulose is used [Figures 1 and 2].

Figure 1: Equipment used in UBM imaging (from left to right) methylcellulose, sterile balloon, UBM probe cap, immersion chamber, eyelid speculum, syringe filled with normal saline, and distilled water ampoules (top)

Figure 2: The UBM probe is sealed with cap, after being filled with the coupling agent

1. UBM with Immersion chamber/scleral shell:

• In this conventional method, the eyelids are held open using the scleral shell, after applying a drop of surface anesthetic. The scleral shell is usually composed of plastic or silicon.
• When using a scleral shell as a water bath, the contact with the ocular surface should be tight enough to create a watertight seal. The cup should not push against the cornea, since this might cause discomfort and corneal aberration. The scleral shell is particularly helpful for studying corneal tissue and central ocular anatomy, but not preferable when evaluating the angle or peripheral lesion adjacent to the limbus because the probe needs to be directed perpendicular to the area of interest.
• The size of the palpebral aperture should be taken into consideration while choosing the suitable scleral shell. It can be easily fitted in the palpebral aperture of a buphthalmic eye due to the increase in corneal diameter and globe size. However, in cases with microphthalmos and newborns, the balloon approach should be utilized [Figure 3].

Figure 3: The immersion chamber method is being used for UBM imaging

2. UBM using sterile balloon method:

• In this technique, the palpebral aperture is kept open with help of a speculum. The UBM probe is covered with a distilled water-filled balloon where sterile water is filled up to the flexible sealing collar and is kept directly over the ocular surface to capture images. The sterile balloon method is very comfortable for the patient without any chances of corneal damage and is particularly helpful in presence of corneal irregularities, thin cornea, and ocular surface malignancy [Figures 4 and 5].

Figure 4: The sterile balloon is filled with distilled water and the UBM probe sealed with water filled balloon

Figure 5: UBM imaging is performed with the sterile balloon method

UBM VERSUS GONIOSCOPY

The common practice for visualizing anterior chamber angle structure is gonioscopy. Although gonioscopy is a simple slit lamp-based technique, it requires the presence of transparent ocular media for angle analysis. The benefit of UBM is that it can analyze angle structures even when there is dense hyphaema, thick arcus, or dense corneal opacity. While gonioscopy shows features of superficial structures in the anterior chamber angle, UBM reveals anatomical structures deeper to the trabecular meshwork, the patency of the Schlemm canal, the degree of iris insertion, and also reveals the pushing or pulling mechanism of angle closure. Aside from providing a cross-sectional picture, UBM’s built-in tool can also aid with lesion dimensions and angle parameters.

Like dynamic gonioscopy, angle anatomy evaluation with UBM performed under normal and pharmacologically constricted pupils may distinguish between anatomical and synechiae angle closure.

UBM VERSUS ASOCT

ASOCT generates high-resolution pictures in clear media but is inefficient in the densely scarred cornea.^[4] Even in the presence of a transparent cornea, infrared rays utilized in ASOCT are unable to penetrate thick iris tissue and pigment epithelium.^[5] The ability of ultrasound to penetrate thick iris tissue and dense iris pigment epithelium makes it an ideal tool to assess deeper anatomical abnormalities, lesions beneath scarred cornea, hyphema, thick iris tissue, and iris pigment epithelium.

The UBM is especially valuable for capturing iridociliary complex, zonular abnormalities, mass lesions under the iris, sulcus fixed IOL location, iris configuration, and ciliary processes anatomy in plateau iris and malignant glaucoma.

Another benefit of UBM over ASOCT is that it can be performed in the supine position. While younger patients are less cooperative for ASOCT in sitting posture, UBM can be conducted with ease while the patient is supine and under general anesthesia.

CLINICAL APPLICATIONS OF UBM

UBM in angle closure glaucoma

The peripheral iris, ciliary body, and even the anterior choroid may all be seen with UBM, which makes it possible to study the causes of angle closure glaucoma in various conditions like primary angle closure glaucoma (PACG), ciliary effusion syndrome, plateau iris, lens subluxation syndrome, ciliary body cysts, and malignancies.

The underlying cause of angle closure is different in all conditions. The relationship between the angle, peripheral iris, and ciliary body, is crucial for determining the precise pathophysiology and the appropriate management, such as iridoplasty, iridotomy, or medical management.

• Angle closure in PACG and Plateau iris

While angle closure in PACG is caused by the anterior bowing of the iris due to the pressure difference between the anterior and posterior chamber [Figure 6], the angle closure in plateau iris results from the anterior placement of the ciliary process pushing the adjacent peripheral iris and resulting in compression of the iridocorneal angle [Figure 7].^[6] On gonioscopy, this mechanism appears as a “double hump pattern,” however the precise mechanism behind this gonioscopic look can be viewed in UBM.

Contrary to glaucoma found with a plateau iris configuration, which has a normal depth anterior chamber and a flat iris surface with a steeper iris root [Figure 7], PACG has a shallow anterior chamber and convex iris configuration [Figure 6].

Figure 6: Clinical image in a case of plateau iris configuration (a), UBM showing flat iris surface, anterior angulation of peripheral iris at its insertion, steep iris root and iridotrabecular contact (b). The depth of anterior chamber is normal

Figure 7: Clinical image in a case of plateau iris configuration (a), UBM showing flat iris surface, anterior angulation of peripheral iris at its insertion, steep iris root and iridotrabecular contact (b). The depth of anterior chamber is normal

• Malignant glaucoma:

Malignant glaucoma or ciliary block glaucoma is a severe form of absolute angle closure when flattening of the posterior chamber is also seen. Although this condition primarily occurs after incisional surgery, the precise pathophysiology is still unknown.

It is thought to be caused by anterior rotation of the ciliary processes, which in phakic individuals press against the lens equator and in aphakia, against the anterior hyaloid.^[7] This created pressure causes the lens iris diaphragm to move anteriorly, the vitreous gel to move forward, and the aqueous outflow to completely stop. UBM imaging allows for the visualization of this whole anterior segment picture [Figures 8 and 9].

The assessment of the anterior segment structures following surgical management is also aided by UBM. Unlike angle closure in PACG, the forward bowing of the iris is not seen here and angles remain closed even after patent iridotomy.

Figure 8: Malignant glaucoma in a phakic patient. Severe form of angle closure glaucoma with absent anterior and posterior chamber

Figure 9: Malignant glaucoma in a pseudophakic patient with patent iridotomy. UBM image is showing very shallow anterior chamber, flat posterior chamber, and absence of choroidal effusion

• Ciliary effusion:

Ciliary effusion is caused by leakage of serous fluid from choriocapillaris and the accumulation of fluid in the supraciliary space. The underlying cause could be idiopathic, secondary to the thickened sclera in congenital scleral anomalies, posterior scleritis, nanophthalmic eyes, sulpha-derived drugs, particularly topiramate, episcleral venous congestion as seen in sturge weber syndrome, or after idiopathic compromised venous outflow.

UBM in these eyes is characterized by exudative detachment of the ciliary body from the overlaying sclera, anterior displacement of the ciliary body and lens iris diaphragm, and secondary angle closure [Figure 10]. While a slit lamp examination of the anterior segment is comparable to that of malignant glaucoma, UBM aids in distinguishing these two disorders since malignant glaucoma is not accompanied by fluid in the supraciliary region. Additionally, UBM may be used to quantify scleral thickness. No alleviation after iridotomy is observed because the underlying mechanism of elevated IOP is not pupillary block but the anterior displacement of the ciliary body.

Figure 10: A patient with nanophthalmos and cilliochoroidal detachment. Note the fluid in the supracilliary space (star), closed angle, and the anterior rotation of the cilliary body

Angle closure glaucoma secondary to posterior synechiae formation

Angle-closure from pupillary block may occur in cases of severe inflammation and is caused by the formation of posterior synechiae between the pupillary margins and the anterior lens surface [Figures 11 and 12]. In these situations, UBM is useful to distinguish from angle closure due to mass lesions.

• Pigment dispersion syndrome.

Figure 11: Secondary angle closure glaucoma in a patient with uveitis. The pupillary margin is attached to the lens capsule, causing entrapment of fluid in the posterior chamber and iris bombe formation

Figure 12: Another case of secondary angle closure glaucoma post-congenital cataract surgery. Occlusio pupillae is caused by the developement of posterior synechiae and the lack of surgical iridectomy

The underlying reason for pigment release and trabecular meshwork occlusion in pigmentary glaucoma is assumed to be the concave iris structure, resulting in a reverse pupillary block. The increased pressure in the anterior chamber than the posterior chamber makes this vicious cycle worse.

UBM can be used to measure a range of characteristics, including iridolenticular touch, iridocorneal angle, and iris concavity [Figure 13]. These factors have been discovered to be linked to an increased risk of pigmentary glaucoma.^[8]

Figure 13: Clinical (a) and UBM image(b) of a case of pigment dispersion syndrome with concave iris configuration, open angles and deep anterior chamber

Additionally, UBM exhibits the effects of peripheral iridotomy on iris configuration.

CYCLODIALYSIS CLEFT

A cyclodialysis cleft is caused by the disinsertion of longitudinal muscle from the scleral spur. It might be post-traumatic or a surgical consequence. The direct contact between the anterior chamber and the suprachoroidal area results in an extra aqueous outflow route, leading to hypotony. This condition may be accompanied by choroidal effusion [Figure 14]. UBM is the best imaging modality for diagnosing and monitoring this,^[9] which reveals scleral spur blunting, Schlemm’s canal collapse, and a direct aqueous passage from the anterior chamber to the suprachoroidal region.

Figure 14: UBM image of the cyclodialysis cleft (arrow), blunting of scleral space and mild cilliochoroidal effusion also seen (Courtesy: Dr Sonam)

ANGLE RECESSION

Angle recession and cyclodialysis cleft can be clearly distinguished from one another using UBM. Angle recession causes the ciliary body face to tear at the iris insertion, resulting in a broadening of the ciliary body face without disrupting the sclera-ciliary body interface [Figure 15a and b].

Figure 15: Gonioscopy (a) and UBM imaging (b) in angle recession showing broad band of ciliary body (Courtesy: Dr Faisal)

UBM in Childhood Glaucoma

• Congenitalcorneal opacities (CCO)

Congenital corneal opacity (CCO) is the major cause of visual deprivation in infants and early treatment is crucial for the prevention of severe amblyopia. CCO can be caused by many conditions, such as anterior segment dysgenesis, congenital glaucoma, hereditary endothelial dystrophy, metabolic disorder, infection, or post-trauma. The underlying reason may be pathology within the corneal layers to the involvement of other anterior segment tissues. UBM has evolved into an ideal way to examine and analyze the entire anterior segment since it offers detailed two-dimensional grayscale images of anterior ocular components, even when the anterior ocular media is obscured with significant opacities.^[3]

The most common causes of CCO are Congenital glaucoma, Peters anomaly, Reiger anomaly, and sclerocornea.^[10,11]

• Primary congenital glaucoma:

The majority of PCG patients have corneal edema and haze, making it difficult to see anterior chamber features. In these instances, UBM is a feasible approach that may be carried out while the patient is lying supine under general anesthesia.

Ultrasound’s non-invasive and high-resolution approach makes it ideal for studying anatomical characteristics and relationships among various anterior segment components including the iris, anterior chamber angle, scleral spur, ciliary body, Schlemm’s canal, and lens, regardless of optical medium quality [Figure 16]. There is evidence that the numerous UBM-measured metrics can be used to diagnose patients, plan surgeries, and predict outcomes.^[12] In contrast to normal eyes, PCG eyes are demonstrated to have longer zonules, a wider trabecular iris angle, a thinner iris, and a short or nonexistent Schlemm’s canal. Aberrant iris and ciliary process insertion, as well as atypical tissue membranes covering the trabecular meshwork (visible as a hyperreflective membrane at the anterior chamber angle), are furthermore frequent^[13–15] UBM is also useful for planning management of these. If there is a visible Schlemm canal (a lucent black region exactly adjacent to the scleral spur), the patient will be a suitable candidate for catheter-assisted trabeculectomy.^[14] The UBM is particularly beneficial in acute hydrops in congenital glaucoma when a high IOP-induced rupture of Descemet’s membrane results in total clouding of the cornea [Figure 17].

Figure 16: Primary congenital glaucoma with hazy cornea (a), UBM image of the right eye showing high iris insertion, a patent Schlemm’s canal, thinner iris and long ciliary processes (b)

Figure 17: Acute hydrops in congenital glaucoma (Clinical image), UBM showing rupture of the Descemet’s membrane

• Peters anomaly

Peter’s abnormality is one significant reason among numerous underlying factors that contribute to corneal opacity in infants.^[16]

Sine qua none of Peter’s anomaly— Descemet’s membrane discontinuation with a defect in the adjacent stroma, can be seen even amid acute corneal edema or scarring.

UBM findings in Type 1 Peters anomaly include central Descemet’s membrane defect along with the defect in the adjacent corneal stroma. This gap is sometimes bridged by membranous anomalies [Figure 18], occasionally with cystic intrastromal fluid clefts [Figure 19] or a fluid cleft with irregular membranous structures [Figure 20]. During a clinical examination, the central portion is revealed to be a clear dot-like space surrounded by oedematous and opaque tissue.

In the Type 2 Peters anomaly, Descemet’s membrane and endothelium defect is seen as a heterogeneous layer, combined with adherence of a few iris strands or a complete iridocorneal adhesion to the margin of the defect, with shallowing of the anterior chamber.

Type 3 also contains keratolenticular adhesions, a cataractous change, and an atypical lenticular form in addition to the aforementioned features.

Additionally, UBM can be used to analyze the remodeling of corneal layers in Peter’s abnormality. Interestingly, Descemet membrane rupture and corneal edema may also be caused by acute hydrops in congenital glaucoma, keratoconus, or post-trauma, but none of these conditions produces fluid clefts in the corneal stroma, as seen in Peters abnormality.

• UBM in Congenital Primary aphakia (CPA):

CPA is severe anterior segment dysgenesis where nondevelopment of the lens is accompanied by dysgenesis of other adjacent anterior segment structures. The “featureless anterior segment where no structure is apparent except for a thin cornea with increased corneal curvature and a rudimentary iris” is the most distinctive feature of UBM [Figure 21]. It may present as microphthalmos or buphthalmos depending upon the disproportionate dysgenesis of ciliary processes and trabecular meshwork. The clinical examination is the mainstay of the examination for this rare anomaly where one can see a typical silvery shiny haze on the cornea. In these cases, the limbus is not defined, and a sclerocornea-like structure is visible in the circumferential portion of the assumed limbal area. It is crucial to avoid operating on these eyes since clinical studies consistently demonstrate the presence of phthisis bulbi in these situations.

Figure 18: UBM in peters anomaly showing defect in the Descemet membrane level with a defect in the posterior stroma, some membranous echoes are also seen in posterior stromal defect

Figure 19: UBM showing multiple fluid clefts in the corneal stroma representing severe corneal stromal edema in peters anomaly

Figure 20: A fluid cleft is seen as a hypo reflective area in the corneal stromal layer with irregular membranous structures in peters anomaly

Figure 21: Clinical and UBM image in a case of congenital primary aphakia, a severe anterior segment dysgenesis where nothing is visible in UBM except for a distorted thin cornea

• Axenfeld Reiger anomaly: Aberrant peripheral iris strands can appear attached to the protruding Schwalbe line, with anterior iris displacement and shallowing of the anterior chamber [Figure 22].

Figure 22: A case of Axenfeld-Rieger anomaly where aberrant peripheral iris strands are attached to the protruding Schwalbe line, with anterior iris displacement and shallowing of the anterior chamber

• Aniridia

Aniridia is a congenital ocular condition characterized by partial or complete iris hypoplasia. There is also evidence of ciliary body hypoplasia and ciliary processes anterior inclination [Figure 23].^[17] In these instances, UBM is an effective tool for determining the degree of hypoplasia, the status of the trabecular meshwork, subluxation, and cataractous changes in the lens, as well as a guide for planning IOL in sulcus fixation.

Figure 23: UBM showing rudimentary iris with high iris insertion blocking the trabecular meshwork in a case of aniridia

• Congenital Acorea

Another form of anterior segment dysgenesis when the pupil is not developed is acorea. The failure of the iris mesoderm to retract during embryonic development is the underlying etiology. The lens’s cataractous alterations and underlying iridocorneal dysgenesis can both be seen with UBM [Figure 24]. To avoid stimulation deprivation amblyopia, prompt care is crucial. UBM aids surgical planning and underlying cause analysis.

Figure 24: Clinical image of a patient with acoria (Left eye). UBM examination demonstrating central irido-lenticular adhesions and extensive peripheral iridocorneal adhesions completely obstructing the trabecular meshwork

Congenital hereditary endothelial dystrophy (CHED)

In the absence of limbal stretching and buphthalmos, CHED manifests as a ground glass cornea. In these eyes, UBM demonstrates a normal anterior chamber angle and thickening at the Descemet’s layer [Figure 25].

Figure 25: Clinical and UBM image in CHED. Note the ground glass appearance of the cornea without limbal stretching. UBM showing a showing normal anterior chamber angle and hyper-reflectivity at the Descemet’s layer

Corneal keloid: A corneal keloid is an elevated, gray-white epicorneal lesion that develops as a result of the aberrant development of fibrous tissue. In these cases, UBM is beneficial for obtaining details about the anterior segment structures underneath [Figure 26].

Figure 26: Congenital keloid (left eye) in Rubinstein-taybi syndrome. UBM image showing full corneal thickness involvement of the lesion along with iris adherence to its base and edge, and a cataractous lens

• Congenital cataract

The characteristics of a cataract, the status of the zonules, the condition of the posterior capsule, the measurement of the sulcus, and assistance with IOL planning can all be determined by ultrasonic biomicroscopy [Figure 27].

Figure 27: A case of congenital cataract and glaucoma in congenital rubella syndrome

UBM IN MASS LESION

In the case of any mass lesion present in the anterior segment, the characteristics of that mass lesion, whether solid or cystic, its relationship with surrounding tissues, extent, and dimensions of the lesion may be assessed using UBM [Figure 28].

Figure 28: A cyst in irido-ciliary sulcus causing localized narrowing of the anterior chamber angle (Gonioscopy and UBM image)

UBM IN SPECIAL CIRCUMSTANCES

• Additionally, UBM can be used to quantify corneal thickness in extensive scars, where a standard pachymetry can result in a falsely high corneal thickness.
• UBM is particularly helpful for assessing corneal thickness in presence of intrastromal fluid clefts, where a standard pachymetry may give an unreliably low reading after measuring just the cyst’s superior wall.
• Following complicated cataract surgery, UBM is useful to find nuclear fragments beneath the iris and to plan scleral fixated IOL.
• To differentiate scleritis from episcleritis

LIMITATIONS OF UBM

The sole constraint of UBM is its requirement for direct tissue contact and supine posture.

In the instance of a corneal ulcer or ocular surface malignancy, a single-use clear scan must be performed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.