Ultrasound Biomicroscopy

From EyeWiki
Original article contributed by: Stephan Chiu, Joseph Christenbury MD, JoAnn A. Giaconi, MD
All contributors: Alpa S. Patel, M.D., JoAnn A. Giaconi, MD and John Davis Akkara
Assigned editor:
Review: Assigned status Not reviewed on August 12, 2016.


Background

Ultrasound Biomicroscopy (UBM) is a technique primarily used for imaging of the anterior segment of the eye. It was first introduced in the early 1990’s by Foster and Pavlin as a way to obtain cross-sections of the eye at microscopic resolution [1]. Compared to regular ultrasound modalities such as A-scan or B scan (10 MHz), UBM uses a much higher frequency transducer (35-100 MHz) [2]. This results in resolutions up to 25um axially and 50um laterally, though depth of tissue penetration is lower. UBM can be used for imaging much of the anatomy of the anterior segment, as well as associated pathologies, including angle closure glaucoma, ciliary body cysts, neoplasms, and angle trauma.

Structures visualized with UBM

  1. Cornea
  2. Iris
  3. Anterior chamber angle
  4. Scleral spur
  5. Ciliary body
  6. Posterior chamber
  7. Anterior chamber
  8. Lens

Clinical applications

UBM is a useful tool for evaluating the presence of narrow anterior chamber angles, angle closure glaucoma, and pathology of the ciliary body. While it can be an alternative to gonioscopy, which can be subjective and difficult to perform with varying interpretations depending on slit lamp light intensity, it is an expensive technology and is more often used as an adjunct test to follow up on abnormal gonioscopic findings. Studies have shown moderate agreement between UBM and gonioscopy for evaluating the anterior angle [3].

UBM is frequently used in research to better understand the angle and can be used for qualitative analysis of the anterior chamber angle (ACA). Analysis and interpretation of images begins with identification of the scleral spur, a protrusion of the sclera into the anterior chamber that attaches anteriorly to the trabecular meshwork. UBM can describe the presence of angle closure glaucoma, which is indicated by irido-corneal contact anterior to the scleral spur. It can also distinguish between different types of angle closure, including pupillary block, plateau iris, and other causes. It may also elucidate causes of open angle glaucoma, such as pigment dispersion syndrome, if reverse pupillary block is seen on scans.

UBM may also be used for quantitative analysis of the ACA [4] [5].

Standard measurements include:

  1. Angle opening distance (AOD), the perpendicular distance between the trabecular meshwork at a point 500um anterior to the scleral spur, and the iris.
  2. Angle recess area (ARA), the triangular area bound between the AOD line and the angle recess.


These measurements may be used to monitor changes in the ACA over time, assess the effect of drug instillation, and evaluate changes in glaucoma patients post-operatively [6].

Additionally, UBM can be used for imaging of the cornea. Pathologies that can be identified include keratoconus, corneal dystrophies, edema, and scars. It can also be used to resolve different layers of the cornea and visualize LASIK flaps post-operatively [7]. UBM has clinical utility in the diagnosis and management of anterior ocular trauma, anterior segment cysts and neoplasms, uveitis/scleritis, and localization of IOL implants in suspected uveitis-glaucoma-hyphema syndrome, among other uses [8].

UBM vs AS-OCT

UBM and AS-OCT are both imaging techniques use to image to anterior chamber. Compared to AS-OCT, the main strength of UBM is its ability to visualize structures behind the iris, including the ciliary body and lens [9]. However, downsides compared to AS-OCT include the need for a water-bath immersion, longer image acquisition times, and the need for a skilled operator. A study comparing the two techniques demonstrated relative agreement in measurements of the anterior chamber [10].

Additional Resources

References

  1. Pavlin CJ, Sherar MD, Foster FS. Subsurface ultrasound microscopic imaging of the intact eye.Ophthalmology. 1990;97:244–50.
  2. Kumar RS, Sudhakaran S, and Aung T. Imaging. From Pearls of Glaucoma Management. 2016.
  3. Narayanaswamy, A. et al. Anterior chamber angle assessment using gonioscopy and ultrasound biomicroscopy. JpnJ. Ophthalmol. 2004; 48(1):44-9.
  4. Pavlin CJ, Harasiewicz K, and Foster FS. Ultrasound biomicroscopy of anterior segment structures in normal and glaucomatous eyes. Am J Ophthalmol. 1992 Apr 15;113(4):381-9.
  5. Ishikawa H, Liebmann JM, Ritch R. Quantitative assessment of the anterior segment using ultrasound biomicroscopy. Curr Opin Ophthalmol. 2000; 11:133-9.
  6. Ishikawa H, Schuman JS. Anterior segment imaging: ultrasound biomicroscopy. Ophthalmol Clin North Am. 2004 Mar; 17(1): 7–20.
  7. Pavlin CJ, Harasiewcz K, and Foster, FS. Ultrasound biomicroscopic assessment of the cornea following excimer laser photokeratectomy. J Cataract Refract Surg. 1994; 20 Suppl:206-11.
  8. Silverman R. High resolution ultrasound of the eye – a review. Clin Experiment Ophthalmol. 2009; 37(1):54-67.
  9. Kumar RS, Sudhakaran S, and Aung T. Imaging. From Pearls of Glaucoma Management. 2016.
  10. Dada T, et al. Comparison of anterior segment optical coherence tomography and ultrasound biomicroscopy for assessment of the anterior segment. J Cataract Refract Surg. 2007 May;33(5):837-40.