AS-OCT AND ULTRASOUND

BIOMICROSCOPYDr Shylesh B DabkeGlaucoma Fellow

Aravind Eye hospital

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Overview

• Because anatomy plays a large role in chronic angle closure, evaluating the angle anatomy is a critical first step in treating this disease.

• Using a combination of various AS imaging technique examiner wil get a clear picture of the angle anatomy and help to make appropriate treatment decisions.

• Gonioscopy is the current gold standard, but imaging techniques such as Ultrasound biomicroscopy (UBM), Scheimpflug photography, and anterior segment optical coherence tomography (AS-OCT) provide additional valuable information.

Gonioscopy• Gonioscopy provides the examiner with a

dynamic, broad view of the angle.

• Has ability to analyze the angle with/without compression (PAS vs appositional angle closure.

• Although quick, inexpensive, and valuable, it is also subjective and difficult to perform.

• Reproduciability, quantitative information cannot be obtained.

• Therefore less than ideal for research and in clinical situations where repeated measurements are needed.

SCHEIMPFLUG IMAGING

• This technique uses light scattering to image the anterior chamber.

• Although the resolution of the images is high this method is of limited usefulness for evaluating the angle because of its inability to image the peripheral angle in many eyes.

AS-OCTUBM

ANTERIOR SEGMENT - OCT

• First described using the same wavelength of light as retinal OCT, namely 830nm.

• OCT imaging of the anterior segment with a longer wavelength of 1310nm was developed later on and had the advantages of better penetration through sclera

• Rapid, non contact method that maybe performed by a technician with patient sitting Upright.

• Obtains a high-resolution cross-section of the anterior segment without touching the eye.

• Enables visualisation of the form, site & position of AC components.

• Defines & measures the distances between them.

• Machine automatically eliminates distortion induced by optical transmission factors.

• Reflex Saturation Beam indicates perpendicularity of the eye to the scanning beam and minimal tilting of the image.

• The scan dimensions are 16 x 6 mm and 4 image frames are averaged.

• Resolution is 2-20microns which makes it suitable for measurement of finer eye structures.

• Qualitative assessment- An important landmark to identify in ASOCT images is the scleral spur.

- This is visible as an inward projection of the sclera at the junction between the inner scleral and corneal curvatures.

- Apposition between the iris & inner corneo-scleral wall has been used as a qualitative method of detecting angle closure

• Quantitative Assessment- Quantitative measurement of the AC angle is possible with in-built software & requires identification of the scleral spur.

- The commonly used quantitative parameters * Angle opening distance(AOD500/AOD750) *Angle recess area(ARA500/ARA750) *Trabecular space area(TISA500/TISA750)

Angle opening distance (in mm): Perpendicular distance between a point 500µm (AOD 500) or 750 µm (AOD750) anterior to the scleral spur and the opposing iris.

Angle recess area (in mm2): The triangular area (ARA 500 or 750) bounded by the AOD 500 or 750, the anterior iris surface and the inner corneo-scleral wall

Trabecular Iris space area (in mm2): Trapezoidal area (TISA 500 or 750) bounded by the AOD 500 or 750, the anterior iris surface, the inner corneo-scleral wall and the perpendicular distance between the scleral spur and the opposing iris.

• SL – OCT(Heidelberg Engineering)Visante (Carl Zeiss Meditec, Inc.)

• The Visante OCT (Carl Zeiss Meditec) is a time-domain OCT device that scans the anterior segment, generating images that extend from limbus to limbus.

• Newer Fourier-domain OCT (FD-OCT) intended for posterior segment imaging are increasingly evolving to include anterior segment imaging.

• FD-OCT units have shorter wavelengths (typically 800-840 nm)

Newer Modalities• A new swept source FD-OCT device (Casia SS-1000; Tomey

Corporation) can rapidly obtain high-resolution scans with 3D reconstruction of the anterior segment.

Clinical use in glaucoma• An adjunct to gonioscopy as well as a substitute when

gonioscopy is not feasible.• Useful as a patient education tool, especially when laser

peripheral iridotomy is being recommended.• Phototherapeutic keratectomy to assess the depth of corneal

opacity.• LASIK for CCT & to measure flap thickness.• Keratoconus & INTACS for CCT & positioning of INTACS.• Cysts of anterior segment to diagnose etiology.• Biometry for phakic IOL.

AS-OCT & Glaucoma• Useful tool for evaluating filtering bleb or GDD in post-op

period.

• Clinical description of bleb is subjective & there may be cases in which clinical appearance does not correlate with bleb function.

• Visualising intra-bleb morphology with AS-OCT may enhance our understanding of different surgical outcomes.

AS-OCT over Gonioscopy• Two common flaws in the performance of clinical gonioscopy

include the placement of pressure on the cornea and the use of excessive amounts of light.

• Both typically produce the illusion of an open angle in apatient who otherwise may have narrow or even appositionally closed angles.

• AS-OCT provides little light artifact. It can actually dynamically show how an angle imaged in bright light may appear open but look narrow when assessed in the dark.

Newer Uses for AS-OCT• Measuring the tear film

Although this work is still mostly showing value in research situations

• Measuring contact lens fit

Advantages• Non contact.• More physiological.• Shorter imaging time.• Less learning curve for operator.• Target may be used to induce accommodation in eye

imaged.• Less inter operator variability.

Limitations• No penetration of pigmented iris epithelium.

Failed flat bleb.Trabeculectomy site seen but no subconjunctival bleb

High elevated tense encapsulated bleb

Same patient with scanned section of bleb only

Penetrating keratoplasty

Intacs

Descement’s Detachment

Granular Dystrophy

Ultrasound Biomicroscopy(UBM)

• High frequency ultrasound technology.

• Detailed 2D gray scale image of ant segment.

• Probe Frequency – 20, 35 and 50MHz.

Principle• It acts on a principle similar to that of the B-scan (10Mhz)

• Frequency 50Mhz

• More frequency, less penetration(5mm) and more resolution

• Limited depth of penetration is also associated with a smaller angular field (4x4 mm)

Patient in supine position

Local anesthetic

Eye cup (plastic or silicone) which is used to create a small water bath

The crystal of the transducer is placed in saline approximately 2 mm from the eye surface.

Methyl cellulose or normal saline can be used as coupling solution

Qualitative and Quantitative Examination

• Structures- Conjunctive, cornea, Ant Sclera- Ant. Chamber angle structures- Ciliary Body- Ant. layers of lens, zonules & IOL- Pars plana

• UBM pro 200 software in the machine automatically measures all angle parameters by marking the scleral spur.

Quantitative parameters of UBM

Qualitative Ultrasound Biomicroscopy

• Glaucoma• Cornea• Tumors• Trauma• sclera• Intraocular lenses• Ocular adnexa

GLAUCOMADetermine the mechanism of elevated intraocular

pressure (angle-closure versus open-angle) by showing the relationship between the peripheral iris and

trabecular meshwork.

Open-angle Glaucoma

• Measure the anterior chamber angle in degrees

• Assess the configuration of the peripheral iris

• Evaluate the iris insertion in relation to the trabecular meshwork

• See if there is an anterior insertion of the iris or an anteriorly displaced ciliary body

Narrow angles

• UBM shows the extent of angle closure

• Depth of the anterior and posterior chambers

• Identifies pathologic processes pushing the lens and iris forward.

PUPIL BLOCK GLAUCOMA

(A) The angle shows appositional closure owing to anterior bowing of the iris. (B) The angle is open with a flattened iris after laser peripheral iridotomy.

To Determine Occludability of the Angle• Perform dark room provocative testing with the UBM, to

study the spontaneous occlusion of the angle under conditions of decreased illumination.

• Better than dark room gonioscopy because the latter is time consuming and standardization of slit-lamp illumination is difficult

PIGMENT DISPERSION SYNDROMEClassical picture• Widely opened angle • Typical Posterior bowing of the peripheral iris

Peripheral Anterior Synechiae• Reveal the extent of irido corneal adhesions, even if the

cornea is hazy or opaque

Malignant Glaucoma• Extremely shallow anterior chamber.• Occluded angle.• Forward rotation of the ciliary body.

Functional Status of Filtering Surgery• Whether the sclerostomy aperture is patent or blocked

internally.• whether the peripheral iridectomy is patent.• whether the filtering bleb is flat, shallow or deep.

Cornea

Graft host junction

Epithelial bullae in oedema

Corneal Dystrophy

Lamellar keratoplasty Stromal scarring

Granular Dystrophy

TUMOURS• Iris tumors, ciliary body tumors and anterior choroidal tumors

can be imaged.

• Borders of the tumor are usually detectable by the change in reflectivity from surrounding structures.

The most common clinical presentation of an irido-ciliary cyst is a peripheral iris elevation - the typical UBM finding

of a thin walled structure with no internal reflectivity is diagnostic.

TRAUMA

CYCLODIALYSIS

Angle recession is imaged as a tear into the face of the ciliary body. Ciliary body tissue is still imaged attached to

the scleral spur.

Zonular rupture

Hemorrhagic cyst within the iris

EPISCLERITIS

INTRAOCULAR LENSES• Ana lysing IOL position

• AC depth after surgery can be measured with a high degree of accuracy.

• Margins of the optic can be easily imaged & decentration analysed.

• Haptic loca tion in relationship to surrounding structures can be determined.

Conjunctival and Adnexal Disease

AS-OCT VS UBM• Optical VS Ultrasound• Resolution : 15m VS 50m• Scan Dimension : 16*6mm VS 5*5mm• Image posterior iris : No vs Yes

Limitations• Contact investigations - patient discomfort

• Supine position

• Cannot be used in- Immediate Post op period- Suspected open globe injury- Younger children

• Time consuming

• Operator dependent