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External/Anterior Segment

External/Anterior Segment

The anterior segment is the front third of the eye that includes the structures in front of the vitreous humour: thecornea, iris, ciliary body, and lens.

Within the anterior segment are two fluid-filled spaces:

  • the anterior chamber between the posterior surface of the cornea (i.e. the corneal endothelium) and the iris.
  • the posterior chamber between the iris and the front face of the vitreous.

Aqueous humor fills these spaces within the anterior segment and provides nutrients to the surrounding structures.

Introduction

Optical coherence tomography (OCT) is a high-resolution imaging technique, which allows a noninvasive tissue observation through sectional cuts of the ocular structure. This concept is based on the measurement of low-coherence delay infrared light reflected onto a tissue to be examined with the use of interferometry. The system with a wavelength of 1310 nm has allowed greater penetration into opaque tissues such as sclera and limbus for the visualization of angular structures. Anterior segment OCT (AS-OCT) technology has improved in the last years with the development of higher resolution systems, considering time domain and Fourier-domain systems.

Anterior segment OCT is an ancillary exam suitable for glaucoma evaluation, allows the measurement of the anterior chamber inner diameter, and provides a pachymetric map, also suitable for evaluation of ectatic disorders, corneoscleral abnormalities, corneal transplant follow up, and anterior segment tumors.

There are limitations in the evaluation through AS-OCT considering the lack of information of the posterior chamber, such as the ciliary body and sulcus. High-frequency ultrasound methods (35 to 50 MHz) usually allow better penetration into opaque and high-density structures , requiring an immersion technique and contact to the globe.

The usage of AS-OCT has been limited to the adult population. The application of this method in the evaluation of pediatric ocular diseases has not been well defined.

In the pediatric population, reports using a posterior segment OCT system (820 nm) demonstrated mean values to the foveal thickness (186 microns) and nervous fiber layer thickness (108.27 microns), additionally allowing an analysis of the optical disc and a detailed morphological description of the macula in premature children. The observation of prevalent intraocular tumors and simulatory lesions in children was also validated with posterior segment OCT method. Handheld posterior segment OCT devices are available to help surgeons at exams under anesthesia in the pediatric population.

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