Двойное лучепреломление роговицы человека: обзор

Ocular Coherence Tomography (OCT) and Scanning Laser Polarimetry (SLP) are two game-changing terms that have entered the lexicon of eye care over the last two decades. Fine details of intra-ocular structures are made visible and become amenable to analysis, and the assessment of the retinal nerve fiber layer (RNFL) has become a key feature in the monitoring and overall analysis of the retina. The assessment of the RNFL, like those from the OCT- and SLP-acquired images, is affected by the birefringent properties of the cornea and, to a lesser extent, the crystalline lens [¹,²,³,⁴,⁵,⁶,⁷,⁸,⁹,¹⁰,¹¹,¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷,¹⁸,¹⁹,²⁰,²¹,²²,²³]. The process of estimating the thickness of the RNFL is affected by changes in corneal birefringence following corneal refractive surgery, and this apparent change in the RNFL may be misdiagnosed as resulting from an insidious cause when it is actually an artefact resulting from a change in corneal birefringence [¹,³,⁸,⁹,¹⁰,¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷,¹⁹,²²]. Monitoring corneal birefringence has been proposed as a method for the non-invasive assessment of glucose levels in the anterior chamber [²⁴,²⁵,²⁶,²⁷] and intra-ocular pressure [²⁸,²⁹,³⁰,³¹,³²], though clinically viable instruments that fulfil these roles are not available. Eye care specialists may recognize the term corneal birefringence but may not be familiar with it. So, what is birefringence? Birefringence was first reported to occur in crystals by Bartholin in 1669 [³³], and in the ex vivo cornea by Brewster in 1815 [³⁴]; Valentin (in 1861) described seeing colored fringes and a dark cruciform image after placing a human cornea between crossed polarizers [³⁵]. In 1921, Koeppe [³⁶] was the first to use polarized light for the biomicroscopic examination of human corneas in vivo, and Cogan (in 1941) provided more precise descriptions of the dark cross [³⁷]. An example of corneal birefringence consisting of isochromatic zones and dark regions in the shape of a cross is shown in Figure 1.