Revealing Henle's Fiber Layer Using Spectral Domain Optical Coherence Tomography

Henle's fiber layer (HFL) contains bundles of unmyelinated cone and rod photoreceptor axons terminating in the pedicles and spherules that synapse in the retinal outer plexiform layer (OPL).1 These fibers are intermingled with Muller cell processes and are obliquely oriented as a result of foveal pit development where photoreceptors migrate inward and ganglion cells migrate outward.2,3 Like axons elsewhere in the central nervous system, the axons of HFL contain microtubules and are long, cylindrical structures.1–3 Their average length is 558 μm,4 and the first synapses occur with dendrites of bipolar and horizontal cells approximately 350 μm from the foveal center.4 Given the large number of central foveal photoreceptor nuclei and this marked displacement, HFL constitutes a significant fraction of the thickness of retinal layers within the macula, as is evident histologically (Fig. 1). HFL is oriented radially about the fovea and is indirectly visible ophthalmoscopically in patients with macular star formation in neuroretinitis.5 HFL also demonstrates the optical property of form birefringence, a property that can be exploited to infer the location of the foveal center as a direct consequence of its consistent effects on polarized light.6 Figure 1. Mammalian foveal histology, courtesy of Roger C. Wagner, Professor Emeritus of Biological Sciences, University of Delaware, http://dspace.udel.edu:8080/dspace/handle/19716/1884. Photoreceptor components are indicated by the rectangle, showing the substantial ... Optical coherence tomography (OCT) uses infrared light to interferometrically derive optical reflectivity information varying by depth in living tissues.7 The use of broadband light sources, a spectrometer, and the application of signal processing techniques has culminated in commercial spectral domain (SD) OCT systems that are capable of imaging with a 5-μm axial resolution in retinal tissue.8,9 These improvements, along with acquisition speeds fast enough to permit frame averaging without significant motion artifact, contribute to improved image quality in SD-OCT devices capable of identifying structures that could not previously be resolved.10 Despite advances in SD-OCT hardware and software, HFL visualization has remained elusive. A seminal paper published in 2004 comparing in vitro OCT images of monkey fovea to histology recognized HFL as a major layer of the retina that could be visualized in vitro and that should be accounted for in vivo.11 However, since that publication, HFL has not been included in diagrams of retinal layers visualized by OCT, likely because of the inability to distinguish a change in reflectivity at the interface between the HFL and the outer nuclear layer (ONL).12–14 Furthermore, segmentation algorithms, normative thickness data, and ONL measurements overlying drusen have recently been published that do not recognize the contribution to macular thickness provided by HFL to the ONL.15–19 Although the need to account for the presence of HFL in OCT images has recently been recognized, the proposed means to measure the contribution of the HFL was an inferential normative model based on high-quality pathologic specimens rather than by a direct means of visualization (Curcio CA, et al. IOVS. 2010;51:ARVO E-Abstract 2286). The only explicit mention of the optical conditions in which HFL may be visualized with OCT can be found in a letter to the editor and was not experimentally validated.20 In our study we describe a systematic method that can be applied to commercial SD-OCT systems to directly visualize and quantify HFL. Additionally, we provide an optical explanation of this phenomenon and demonstrate its clinical relevance.