Article

Advances in anterior segment imaging

Technologies shine a light on infections, lesions and keratoconus.

As cornea specialists, the ongoing evolution of ophthalmic technology continues to benefit clinicians and patients. Here, I discuss three diagnostic tools that have made a favorable impact on diagnosis and management in our field: confocal microscopy, anterior segment optical coherence tomography (AS-OCT) and corneal tomography.

CONFOCAL MICROSCOPY

Zeroing in on keratitis

Infectious keratitis is one of the most common conditions that we see as cornea specialists. The majority of these patients have mild disease that is bacterial in nature from trauma, improper contact lens wear or poor hygiene. However, severe cases can occur from a variety of pathogens, and the clinical exam does not always reveal which organism is responsible. Confocal microscopy is particularly helpful in diagnosing cases of fungal keratitis and Acanthamoeba keratitis.

Characteristic findings in Acanthamoeba keratitis on confocal microscopy are the presence of hyperreflective cysts 15-28 mm in size.1 They can be arranged in chains and clusters or in a single-file arrangement (Figure 1). Some research has found that the presence of cysts in chains and clusters portends a worse visual prognosis.2 Visualizing the cysts on confocal microscopy has a diagnostic sensitivity of 90% and specificity of 100%.1

Figure 1. Confocal microscopy image of Acanthamoeba ulcer showing hyperreflective cysts.

For fungal keratitis, the hyphae of filamentous fungi such as Fusarium and Aspergillus species appear as numerous high-contrast lines 200-300 mm in length that branch at an angle.3 For Candida species, these appear as many high-contrast elongated particles 10-40 mm in size representing pseudohyphae.3 Moreover, the presence of inflammatory cells is also visible in the epithelial layer in nearly all cases of fungal keratitis.3

Cases in point

One patient that comes to mind is a 39-year-old male referred by a colleague. The patient had a history of improper contact lens wear and over the course of a month had been to several other optometrists and ophthalmologists who struggled to diagnose and properly treat the infection in his left eye. His presenting vision was 20/80. Slit lamp examination showed a dense 1-mm x 1-mm corneal ulcer with adjacent bullae and a ring-like appearance to the infiltrate as well as a 0.5-mm hypopyon. The patient took Besivance (besifloxacin ophthalmic suspension, Bausch + Lomb) every 2 hours and prednisolone acetate 1% twice a day. A corneal culture was taken at this first visit. Confocal microscopy showed the presence of numerous cysts in the stroma, highly suggestive of Acanthamoeba keratitis (Figure 1). This prompted starting the patient on chlorhexidine 0.02% q1h. A corneal biopsy was also performed to send tissue to the local ophthalmic pathologist, which showed signs of degraded Acanthamoeba cysts.

Because the patient’s condition was advanced on presentation, medical therapy was not successful in curing the infection. The risks, benefits, alternatives and indications for corneal transplant were discussed at length with the patient, and he wished to proceed with surgery. A penetrating keratoplasty (PK) and partial lateral tarsorrhaphy were successfully performed. The tarsorrhaphy was incorporated to protect the cornea transplant from degradation as the patient showed evidence of being neurotrophic on clinical exam. Postoperative vision improved to 20/30. The postop photo at month 5 can be seen in Figure 2.

Figure 2. Slit lamp photo of Acanthamoeba ulcer after therapeutic PKP showing clear graft.

Another interesting case was referred in for a non-healing corneal ulcer after possible HSV keratitis infection. Presenting vision was counting fingers at 2 feet. Slit lamp examination showed a 5-mm central/temporal corneal ulcer and small hypopyon (Figure 3). Corneal cultures were taken, and the patient was started on fortified vancomycin 25 mg/ml and tobramycin 15 mg/ml q1h. The culture grew Fusarium species. Hourly natamycin 5% was started as well as oral fluconazole.

Figure 3. Slit lamp photo of large Fusarium corneal ulcer.

The patient was followed closely over the next 2 weeks without significant improvement in the ulcer. Also, confocal microscopy (Figure 4) showed long branching fungal filaments. Several rounds of intrastromal voriconazole 50 mcg/0.1 mL were injected around the lesion to attempt to reduce its size. However, this approach was not successful. Risks, benefits, alternatives and indications were discussed with the patient, and he agreed to undergo a therapeutic PK. The surgery was performed successfully without complications, and intraoperatively an intracameral injection of voriconazole 50 mcg/0.1 mL in a 0.1 mL quantity was administered. The patient has recovered from the surgery, and the transplant is free from recurrence at 6 months postop.

Figure 4. Confocal microscopy image of branching filaments of Fusarium keratitis

In severe cases of infectious keratitis that are not conclusive on clinical exam or corneal culture, confocal microscopy can be a very useful tool for diagnosis and treatment.

AS-OCT

Evaluating conjunctival lesions

Another challenging diagnosis for cornea specialists to make is determining whether a conjunctival lesion is benign, pre-malignant or malignant. Often these lesions must be excised to reach an answer. With the wide adoption of OCT in ophthalmology, clinicians are able to see structures of the eye that previously were not visible.

With respect to the anterior segment, OCT can be used to delineate the anatomical characteristics of a variety of ocular surface squamous neoplasms (OSSN). Recent studies indicate that AS-OCT can help delineate between intraepithelial and invasive ocular surface tumors.4

Characteristics of intraepithelial OSSN include a clear plane of separation between thick hyperreflective epithelium and hyporeflective areas and a hyperreflective basal membrane.4 Moreover, hyporeflective zones within the tumor surrounded by thickened hyperreflective epithelium were found to be characteristics of invasive OSSN.1

For melanocytic tumors, researchers have demonstrated that a thick basal epithelial hyperreflective band with normal overlying epithelium without cysts is a characteristic of primary acquired melanosis (PAM).5 Conjunctival melanomas typically have epithelium of normal thickness with a variable hyperreflectivity of the basal layer with the lesion itself being subepithelial.6

AS-OCT in three patients

This case of a pigmented conjunctival lesion in a younger patient in Figure 5 could be anything from a nevus to PAM to a conjunctival melanoma. It is darkly pigmented and has irregular margins and possibly feeder vessels. However, AS-OCT shows the lesion is hyperreflective and confined to the epithelium within the substantia propria also with several cysts. With this information, the most likely diagnosis is a conjunctival nevus. Given the benign nature, a biopsy was deferred, and the patient was followed closely.

Figure 5. Slit lamp photo of conjunctival lesion and anterior segment OCT of conjunctival nevus.

In Figure 6, we see a darkly pigmented lesion at the limbus. The AS-OCT shows a hyperreflective line at the level of the basement membrane without detectable invasion beneath. The patient underwent biopsy, and the pathology showed PAM with clustering of melanocytes along the basement membrane. The AS-OCT showed the melanocytes clustering along the basement membrane but was unable to discern the degree of atypia of PAM.

Figure 6. Slit lamp photo and anterior segment OCT of primary acquired melanosis.

Figure 7 shows another pigmented conjunctival lesion. However, this one appears elevated with fleshy color, poorly defined margins and larger feeder vessels. AS-OCT shows a hyper-reflective inhomogeneous lesion with a cleavage component in the epithelium and intact tissue architecture between the epithelium and the stroma. This patient underwent a biopsy that revealed the lesion to have a large collection of atypical melanocytes within the stroma and cells that stained positive for Melan-A; the diagnosis was amelanotic melanoma.

Figure 7. Slit lamp photo and anterior segment OCT of conjunctival amelanotic melanoma

AS-OCT allowed the determination of whether to biopsy each of these suspicious conjunctival tumors and played a role in the management of the care of these patients.

CORNEAL TOMOGRAPHY

A tool to identify keratoconus

Advances in corneal imaging have also helped in the diagnosis of keratoconus and other corneal ectatic disorders. With the advent of corneal collagen cross-linking (CXL), the progression of keratoconus or other etiologies of corneal ectasia can now be halted. Corneal topography is beneficial to image the anterior cornea and can easily diagnose moderate and advanced keratoconus from Placido disk imaging of the axial (central 3 mm) and tangential (9 mm) maps.

However, in early onset keratoconus or forme fruste keratoconus, the anterior corneal surface can look normal. These patients, particularly if they are seeking refractive surgery, may be at risk for progression of disease.

Corneal tomography is an advanced diagnostic tool that uses Scheimpflug imaging to take photographs of the anterior and posterior cornea. As a result, the posterior cornea, which can be abnormal in early onset keratoconus, can be visualized.

While there are many corneal tomographers on the market, two of the most common available in the United States are the Pentacam (Oculus Optikgeraete GmbH) and the Galilei G4 or G6 (Ziemer Ophthalmic Systems AG). Our institution uses the Galilei G4. It has the ability to simultaneously capture Placido disk images of the anterior cornea along with two Scheimpflug images. It also has numerous metrics to screen for keratoconus, including keratoconus probability index (KPI), inferior-superior index (I-S), irregular astigmatism index (IAI) and differential sector index (DSI).7

Cut-off thresholds have been studied for each of these metrics for diagnosis of clinical keratoconus as well as pre-keratoconus.8-13 Values of the KPI of 10% or less, I-S ratio of 1.6 or less, IAI of 0.45 or less and DSI of 1.72 or less are generally accepted as cutoff values for pre-keratoconus.8-13 The higher each value and the more values that are elevated simultaneously, the higher the physician’s index of suspicion that keratoconus or corneal ectasia is present. Therefore, clinicians can say with a large degree of certainty that a patient has pre-keratoconus or clinical keratoconus.

Tomography at work

The following examples compare the corneal tomography maps of several patients from our clinic. The first patient (Figure 8) has thick corneas, regular astigmatism and no obvious change in the anterior or posterior cornea. There is no abnormality in the anterior or posterior cornea and the KPI, I-S ratio and IAI are at or below pre-keratoconus cutoffs. The DSI is above the threshold of 1.72, but the other data points are normal. This patient may be a good candidate for refractive surgery.

Figure 8

The second patient (Figure 9) has a normal-appearing anterior corneal surface in the right eye but some inferior steepening on the anterior cornea in the left eye. The posterior corneas in both eyes appear to have elevated areas in the inferior portion correlating to where the cornea is steep. Moreover, the corneas are thinner inferiorly, more notably in the left eye. This patient likely has early stage keratoconus in both eyes. The KPI, I-S, IAI and DSI are all above cutoff values. This patient is likely a good candidate for CXL to halt progression.

Figure 9

In the last patient (Figure 10), we see a significant amount of inferior corneal steepening on the anterior and posterior corneal maps, and all of our index scores are significantly above their cutoff values. This patient has moderate to severe keratoconus in the right eye and should undergo either CXL very soon or possibly a corneal transplant either by PKP or DALK.

Figure 10

In conclusion, corneal tomography is a very useful tool for screening refractive surgery patients as well as patients who are referred for keratoconus thanks to its Scheimpflug imaging of the anterior and posterior corneal surfaces. Moreover, modern tomographers provide sophisticated metrics that can provide additional certainty to diagnosing keratoconus and corneal ectatic disorders.

CONCLUSION

As technology in the field of corneal and refractive surgery and the anterior segment continues to evolve, ophthalmologists will be able to more expediently and precisely diagnose and treat a variety of conditions. OM

REFERENCES

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