(Very) High-Frequency Ultrasound of the Anterior Segment

After having shown the applications of (very) high frequency ultrasound (VHFU/HFU) to refractive surgery, in this chapter, we mainly consider the study of the anterior chamber angle/iridocorneal angle (ACA) in case of glaucoma, the corneal opacities and the traumas of the anterior segment. For the study of the ACA, we first define the scleral spur, a structure that must be recognized because in relation to this structure, we can evaluate and measure the opening of the ACA. The practical realization is then detailed: bilateral and symmetrical, the examination following ocular biometry and biometry of the anterior segment. A quantitative approach is preferred for the analysis of the angle and study of the risk of closure. This angle should be systematically measured in the light (photopic condition) and the dark (scotopic light level). The measurement in degrees is to be banished because of the too-variable morphology of the angle and root of the iris. The most frequently used measurement is the angle opening distance at 500 µm from the scleral spur (AOD500) or sometimes at 750 µm (AOD750) or even sometimes at the distance where the angle is the narrowest; however, we can also measure surfaces: angle recess area (ARA) and trabecular iris surface area (TISA), the most frequently used angular surfaces being ARA500 and TISA750. We can also measure the thickness of the iris (mainly at 750 µm from the scleral spur), as well as its morphology, central convexity or peripheral angulation. Finally, the situation of the ciliary body should be studied and the lens vault should be measured. We then describe the different mechanisms of chronic narrow angle glaucoma: pupillary block, plateau iris, basal insertion of the iris and creeping angle glaucoma, then pigmentary glaucoma. This VHFU is also useful after treatment: laser peripheral iridotomy, iridoplasty, filtering surgery or drainage implants. We then review the contribution of VHFU to the study of corneal opacities, mainly neonatal (Peters syndrome 1 and 2, glaucoma, sclerocornea, Rieger anomaly, Axenfeld, or Axenfeld-Rieger syndrome, and dermoids) but also in adults. VHFU is also useful for anterior segment trauma. Then we cover corneal edema, hyphema, angle recession, iridodialysis and iris tears; cyclodialysis and lens anomalies: intumescence; cataract; capsular rupture and (sub) luxation with or without vitreous herniation in the anterior chamber. Finally, the VHFU allows for localizing the foreign bodies with great precision.