Corneal endothelium is – in combination with the excrescences from Descemet’s membrane – the key histological layer for diagnosis of FECD. In the morphological picture, two main entities may be estimated:

Typically modern (semi)automated specular microscopes display (1) number of analyzed cells; (2) density of cells per 1 mm²; (3) average size of analyzed cells; (4) standard deviation of analyzed cell size; (5) coefficient of variance of the analyzed cells, depending on the relation of the mean cell size and the standard deviation; (6) size of the largest analyzed cell; and (7) size of the smallest analyzed cell (Fig. 3.6).

Interestingly, cornea guttata can be seen on the slit lamp before it can be demonstrated in the specular microscopy [20]. The extent of cornea guttata seems to follow a clinically identifiable pattern course. The endothelial mosaic has firstly a strawberry pattern [21] which is the first sign for cornea guttata. This strawberry pattern can present small hyperreflective dots suggestive of protrusions of Descemet’s membrane in the center of hyporeflective areas [22]. As cornea guttata progresses, these hyporeflective areas get larger resulting in a craterlike appearance [23]. According to Zoega, the specular image can be divided in five grades. In the fifth grade, guttae occupy more than 50 % of the image [20].

Healthy corneal endothelial cells appear as a regular array of mainly hexagonal cells which exhibit bright cell bodies and dark cell borders [24]. The appearance of guttae in specular microscopy and confocal microscopy is similar [22]. Endothelial cell morphology is one key for the diagnosis of FECD: the main changes include polymegalism (variation in cell size) and pleomorphism (variation in cell shape) [25]. Patients with cornea guttata have reduced proportion of hexagonal cells, but increased proportion of tetragonal, pentagonal, heptagonal, and octagonal cells [23]. This is contradicted by Zoega et al. who did not detect a difference in hexagonality between corneas with and without guttae during a 7-year follow-up [20]. This is in accordance with a study by Kitagawa et al. who also did not detect changes in hexagonality, pachymetry, or visual function in a group of Japanese patients with primary cornea guttata [10]. Therefore, it is insecure whether polymegalism and pleomorphism of the endothelium in the presence of cornea guttata are already diagnostically decisive in order to talk about FECD. Nevertheless, it is a common sense today that in FECD, polymegalism and pleomorphism do increase [26]. The authors attribute this to oxidative stress in general or from hypoxia.

Several attempts have been made to standardize specular microscopy. They focus on the fact that endothelial cell density per square millimeter is notoriously overestimated being very much depending on the individual investigator. It is difficult to find the same area once measured again for a sequential measurement, leading to a considerable variation of images taken from the center of the cornea with guttae [23]. Neither manual nor automated endothelial cell count will give valid results in case of unevenly distributed guttae (Fig. 3.7). There is a possibility to create an effective cell density, correlating the fraction of clear endothelium and the fraction of the image area free of guttae. But this method is based on several assumptions. One assumption is that the scanned area is representative of viable cell density [27]. This depends on the severity of FECD. It could be demonstrated that most of the changes in FECD occur in the central 0.6 mm of the cornea [28]. Corneal edema can lead to light scattering, thus reducing image quality and making central analysis more difficult. In the corneal periphery at 3.7 mm, there is more damage to cells inferotemporally, but no positive correlation with pachymetry could be established so far [28].

The pachymetry may range up to 1100 μm in the late stage of FECD compared to normal thickness of 530 μm [21, 29]. However, the change of central corneal thickness during clinical follow-up visits is more important than the overall value [30]. It is of importance that the thickness measured also depends on the system used. Given the difficulty in defining the exact center of the cornea, separate readings should be obtained if the central corneal thickness is manually measured using ultrasound. Today, tomography (e.g., using the Pentacam) allows better comparison of central corneal thickness values during individual follow-up. A central corneal thickness greater than 640 μm usually indicates corneal edema [31] (Fig. 3.6). Patients with central corneal pachymetry higher than 650 μm have greater than 85 % probability of epithelial edema occurrence [32]. Four millimeters from the center, the cornea is still statistically thicker than normal, but this seems to be clinically much less important. It has been suggested to use a central-to-peripheral thickness ratio (CPTR), i.e., measuring the center of the cornea and at 4 mm to detect individual progression of the disease [31].

A theoretical advantage of anterior segment OCT is the possibility to measure corneal thickness layer by layer and not as overall thickness, therefore being able to better differentiate between stromal and epithelial edema. This method holds a second advantage in FECD patients: Descemet’s membrane is visualized as a thickened band of two opaque lines. The anterior line is smooth and represents the stromal face of Descemet’s membrane, while the posterior line is irregular and wavy with local thickenings strongly corresponding to the histological image [33]. This might allow to determine the progression of the disease. Unfortunately, this clear separation is restricted to ultrahigh-resolution anterior segment OCTs. In contrast, commercially available anterior segment OCTs have limited ability even to depict Descemet’s membrane at all.

IVCM is able to demonstrate microstructural changes that correlate well with histological studies. IVCM showed marked diminishment of total nerve number and number of nerve branches in the subbasal corneal nerve plexus with increasing stages of FECD compared to a control group [11]. Corneal sensation is mildly reduced in FECD patients at stage IV when tested with a Cochet-Bonnet esthesiometer [11]. It has to be kept in mind, however, that there is also a decrease of nerve density in the subbasal plexus with age [6]. But nerve alterations in the subbasal plexus can already be demonstrated in early stages of FECD [34]. They seem at first not to have an impact on corneal sensitivity. But in a study on 42 eyes after Descemet’s stripping endothelial keratoplasty, corneal sensitivity remained lower than in controls for 36 months after surgery [35]. Pleomorphism and polymegalism can be demonstrated in IVCM and noncontact specular microscopy [36–38]. Guttae can be demonstrated in IVCM as hyporeflective round structures with ill-defined borders. Sometimes, there is a sharp hyperreflective spot in the center of a gutta [36, 37]. In addition, IVCM can help to assess endothelial cell count and morphology in analogy to specular microscopy

In posterior polymorphous dystrophy, the endothelial cells undergo a transformation to epithelial-like cells [45] (Fig. 3.10a, b). Scattered endothelial vesicles can be shown with retroillumination [46]. Of note is that the vesicles in retroillumination can be visualized in IVCM as large oval structures that contain abnormal endothelial cells [37] which allow a clear distinction from FECD corneas. A railroad-shaped pattern of the corneal endothelium can be seen at the slit lamp and is almost pathognomonic [37] (Fig. 3.10c). Eyes with posterior polymorphous corneal dystrophy (PPCD) often need no therapy for decades. However, the intraocular pressure has to be monitored on a regular basis.

Congenital corneal clouding in the form of a milky ground glass appearance is the landmark of CHED, usually already present at birth [47] (Fig. 3.11). Specular microscopy in patients only mildly affected has not yet been published. But specular microscopy in parents of CHED patients may show cornea guttata [47].

According to the heredity, only males are affected [48]. The so-called endothelial moon craters are the hallmark of this disease (Fig. 3.12a) that can also be demonstrated in female carriers. These alterations compromise the whole corneal endothelium. In later life, a band keratopathy (Fig. 3.12b) may develop. Congenital corneal clouding can occur. Until now, no data on noncontact specular microscopy or corneal thickness are available in the literature because of the rarity of this disease.

Differential diagnosis of other alterations of the corneal endothelium include:

Whereas fibroblastic metaplasia and the production of abnormal and excess extracellular material by corneal endothelium are believed to be a nonspecific common response to stress, injury, or disease, the production of PEX fibers is a specific and pathognomonic feature of the PEX syndrome. Because, however, this pathognomonic feature is not evident throughout all five stages [49] of the disease, the diagnosis of PEX keratopathy may rely on other clinical and histopathologic criteria, such as diffuse edema, irregular thickening of Descemet’s membrane, and pronounced melanin phagocytosis in the presence of an ocular PEX syndrome [49–52].

Circumscribed excrescences of the posterior Descemet’s membrane (“guttae”) are the clinical and histopathologic hallmark of FECD. The endothelium is attenuated, but mostly intact and shows moderate fibroblastic transformation and moderate amounts of phagocytosed melanin granules. FECD shows obvious similarities and differences with PEX endotheliopathy: the diffuse decompensation pattern on PEX eyes can be easily clinically distinguished from the corneal edema in classical FECD, which usually starts centrally and spreads peripherally, and also from bullous keratopathy (e.g., after anterior chamber lens implantation), which usually starts in the limbal region and spreads centrally (Fig. 3.13). Further clinical differences between Fuchs and PEX endotheliopathies include the presence of typical guttata in FECD which is usually lacking in PEX eyes (Figs. 3.14 and 3.15) and the extent of endothelial loss, which is often more pronounced in PEX eyes (Table 3.1). Besides more severe polymegalism and pleomorphism, PEX eyes may show hyperreflective retrocorneal whitish deposits at slit-lamp examination (Fig. 3.16).