Early Detection of Subclinical Corneal Abnormalities: Biophotonic Imaging Reveals Hyporeflective Bleb-Like Structures in Asymptomatic Eyes

Introduction

Although often asymptomatic, there are several important clinical implications of epithelial basement membrane dystrophy (EBMD) to consider. EBMD is estimated to be the second most common cause of recurrent corneal erosion syndrome and is an important differential diagnosis of dry eye disease [1]. In addition, as reported recently, EBMD plays an important role in the context of refractive surgery (eg, EBMD-associated infectious keratitis after femtosecond LASIK) [2]. Despite its prevalence, EBMD´s subtle clinical manifestations often remain undetected or are assumed to be dry eye. Advanced diagnostic methods like epithelial thickness mapping, confocal laser scanning microscopy (CLSM), or optical coherence tomography (OCT), beyond the traditional slit-lamp biomicroscopy, have introduced specific features of EBMD and should be performed prior to refractive surgery and in dry eye patients to rule out corneal dystrophies. This case report substantiates the critical role of these methods.

Case Report

A 57-year-old man volunteered as a control participant in a clinical study, during which he received a comprehensive ophthalmological evaluation. This evaluation included advanced diagnostic procedures such as slit-lamp biomicroscopy, cor-neal topography, densitometry, optical coherence tomography (OCT), and large-area confocal laser scanning microscopy (CLSM). The participant had a clear cornea (Figure 1), a deep and optically empty anterior chamber, and a clear crystalline lens in both eyes. The best-corrected distance visual acuity was −0.1 logMAR. He reported no ocular pain, tearing, or photophobia. There was no history of ocular trauma, infectious or genetic diseases, or previous ocular surgery. Examination of the anterior segment using slit-lamp biomicroscopy revealed no abnormalities in the corneal epithelium, stroma, or endothelium. Retroillumination did not show any pathological changes. Central corneal thickness measured by Scheimpflug imaging (Oculus Pentacam HR, Oculus Optikgeräte GmbH, Germany) was 564 µm, and corneal endothelial cell density measured by TOMEY Specular Microscopy EM-4000 (TOMEY Corporation, Japan) was 2536 and 2572 cells/mm2 in the right and left eye, respectively. Corneal topography obtained by Scheimpflug imaging showed a regular cornea without astigmatism, abnormal thinning, or thickening. Corneal densitometry yielded regular results without a noticeable light scattering in the anterior, central, or posterior corneal layers. Overall, the patient’s history and the results of the ophthalmological assessment suggested normal ocular health. Further, corneal non-invasive in vivo biophotonic imaging was performed by prototypic high-resolution optical coherence tomography (OCT) and large-area confocal laser scanning microscopy (CLSM) (see Figure 2A for OD and Figure 2B for OS). The OCT imaging was performed using a modified SPECTRALIS imaging system (SPECTRALIS, Heidelberg Engineering GmbH, Germany) with a prototypic objective lens system. The large-area CLSM images were captured with the Heidelberg Retina Tomograph 3 (HRT3, Heidelberg Engineering GmbH, Germany) equipped with an in-house developed Rostock Cornea Module 2.0 (RCM 2.0) [3] and the EyeGuidance system [4].

Atypical hyporeflective bleb-like structures were observed in the corneal epithelium and sub-basal nerve plexus but not in the stroma (Figure 2). Alterations were more intensive in the left eye (Figure 2B) than in the right eye (Figure 2A), but were clearly visible in both eyes. OCT and CLSM revealed these bleb-like structures, which were especially noticeable in the right eye. The first structures appeared approximately 1.8 mm below the central corneal region. High-resolution OCT was able to capture sagittal images and helped to locate these regions, and large-area CLSM allowed detailed en face imaging. With standard CLSM imaging these structures would likely remain undetected, especially in the right eye, or take a great effort to detect. As a comparison, the cornea of a healthy volunteer (a 24-year-old man who volunteered as a control participant) is depicted in Figure 3. No similar structures were found here. However, the epithelial wing cells appeared slightly darker than the remaining epithelium and may not be confused with the case reported here.

Discussion

The authors hypothesize that the morphologically altered basement membranes found here indicate an early form of EBMD. However, it is important to emphasize that these observations do not amount to a definitive diagnosis of EBMD. In slit-lamp images, if visible at all, EBMD is characterized as maps, dots, or fingerprint lines [5], leading to the name mapdot-fingerprint dystrophy. EBMD is also known as Cogan microcystic dystrophy. It is a commonly bilateral dystrophy of the anterior human cornea affecting about 2% of the human population; other studies have reported basement membrane changes in 25% of the general population. EBMD can be visualized in CLSM [6] and is also reported to be visible in SDOCT as either bright or dark spots/regions in the epithelium directly located to the basement membrane [7]. In EBMD, the “blebs” or vesicles should be mentioned. These transparent, round, cyst-like formations, either solitary or clustered together, can only be observed in regredient light and with precise focusing. Histologically, they are not microcysts but rather are discrete protrusions of the basement membrane with consequent deformation of the basal epithelial cells.

The patient reported no ocular pain, tearing, or photophobia. There was no history of ocular trauma, infectious or genetic diseases, or previous ocular surgery. Therefore, and due to the bilateral occurrence, we suspected a systemic cause. EBMD is usually not considered to be inherited, although several patterns of inheritance have been described in the literature [8,9]. Recently, point mutations in the transforming growth factor-beta-induced gene (TGFBI; initially called BIGH3) have been reported to cause EBMD. Nevertheless, only approximately 10% of EBMD patients seem to have a mutation in this gene. Mutations of the gene cause several forms of corneal dystrophies [10] and only among others also EBMD. However, there is a high degree of genetic heterogeneity. We did not consider a genetic test to be indicated at this early stage, especially since the detection rate is variable and the exact form of dystrophy cannot be determined with certainty without a more pronounced clinical picture.

To the best of our knowledge, this is the first case where multi-modal imaging using high-resolution OCT and large-area CLSM were performed, showing potential precursory morphological alterations indicative of EBMD. It is important to note, however, that these observations do not constitute a diagnosis of EBMD. This report supports the hypothesis that epithelial morphological changes occur well before clinical signs. Eker et al recently reported a similar case, but with recurrent attacks of severe ocular pain, tearing, and photophobia, typically upon awakening, in 1 eye [11].

Conclusions

Correlative imaging using OCT and large-area CLSM can contribute to the understanding of corneal diseases. Their combined application could uncover similar cases, distinguishing corneal conditions and contributing to a more accurate and earlier diagnosis. A multimodal correlative approach of combined OCT and CLSM, as prototypically implemented and published recently [12], could play a valuable role in diagnosing and evaluating corneal diseases such as EBMD. Very recent studies enabled monitoring the exact same corneal regions in longitudinal large-area CLSM image series [13], which is almost impossible in standard CLSM image series. This allows for a longitudinal morphological characterization of the blebs and their structure to assess disease progression or potential therapy results. By non-invasively visualizing corneal structures in-depth and in real-time, these techniques enhance our understanding of the disease and aid in developing targeted and effective management approaches.