Semilunar sign on cornea a telltale sign of scleritis
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Scleritis is inflammation of the sclera and is predominantly autoimmune. Corneal thinning, astigmatism and peripheral ulcerative keratitis are the common corneal changes following scleritis.
Scleritis can be anterior or posterior. Anterior scleritis is classified as diffuse or nodular, depending upon the clinical presentation. In 1976, Watson and colleagues described about 29% of anterior scleritis causing corneal changes predominantly involving the anterior and stromal layers of cornea. In 1974, Holt Wilson and Watson reported four cases of non-syphilitic deep nummular keratitis with vascularization involving the corneal endothelium in scleritis. We report a novel semilunar sign observed in the cornea, which can be a telltale sign of previous noninfectious scleral inflammation.
Semilunar sign
Semilunar sign is seen on the posterior cornea (Figure 1) and is often associated with prior or concurrent scleritis. The clinical features include presence of posterior corneal opacity, concave semilunar pattern of opacity on diffuse direct illumination, absence of blood vessels and normal anterior cornea.
Source: Dhivya Ashok Kumar, MD, FRCS, FICO, FAICO, and Amar Agarwal, MS, FRCS, FRCOphth
Evaluation of semilunar sign
Serial documentation by digital slit lamp photography (10 times magnification), spectral domain OCT (Figure 2), specular count analysis (Figure 3) and confocal microscopy are the modalities that can be used to assess the progression of semilunar sign. High-resolution anterior segment OCT using the spectral domain platform (1 µm to 5 µm axial resolution) can be used to measure the extent of the posterior corneal opacity, Descemet’s status and corneal thickness.
Differentiate from interstitial keratitis
One has to differentiate semilunar sign from the interstitial keratitis seen in conditions such as syphilis, post-viral keratitis or tuberculosis. Semilunar sign is often adjacent to the location of scleritis, unlike interstitial keratitis, which can be in any location on the cornea. The absence of blood vessels or ghost vessels also differentiates it from conventional interstitial keratitis. The typical concave or semilunar pattern is predominantly followed in scleritis-induced semilunar sign whereas no specific pattern is seen in interstitial keratitis.
Clinical pattern and demographics
A good history and clinical examination form the initial clinical assessment in patients with scleritis. Identification of systemic signs such as joint pathologies, skin problems and integumentary disorders to detect autoimmune conditions is crucial. A recent study by us showed that in 76 eyes of 72 patients with anterior scleritis evaluated in the outpatient clinic, 11 patients (15.3%) presented with semilunar sign. There were nine women (81.8%) and two men (18.1%) with a mean age of 40.5 ± 11.8 years (22 to 56 years). Anterior scleritis was noted in all 15 eyes in either active state (n = 3) (Figure 4) or inactive state (n = 12). Four out of 11 patients (36.3%) had bilateral presentation. The referring symptoms were pain (n = 3, 27.2%) and blurred vision (n = 4, 36.3%). No corneal edema or Descemet’s fold was noted. The pupil was not covered totally in any of the eyes. There was a 0.2- to 0.5-mm clear zone noted between the opacity and the limbus in 11 eyes. No clear zone was seen in four eyes. Full-thickness corneal involvement was not seen in any of the eyes.
The study showed a mean surface area of the endothelial opacity and the standard deviation as 7.7 mm2 and 5.2 mm2, respectively. The overall surface area involved ranged from 2 mm2 to 17 mm2. The percentage of the involved corneal endothelial surface contributed 5.8% of the overall endothelium. The central endothelial cell density ranged from 1,652 cells/mm2 to 2,924 cells/mm2 (mean 2,540.8 ± 351.7 cells/mm2). There was a statistical difference in endothelial cell density between the uninvolved central cornea and the involved peripheral cornea (P = .05). The mean OCT thickness of the endothelial opacity was 212.5 ± 129.3 µm. The corneal epithelium remained uninvolved. There was no statistically significant correlation between the posterior corneal opacity thickness and best corrected visual acuity. However, in three eyes in which the opacity was partially crossing the pupil, the BCVA was reduced.
Investigations
All patients with semilunar sign will need proper laboratory evaluation. Blood investigations include complete hemogram total count, differential count, hemoglobin and erythrocyte sedimentation rate, C-reactive protein, rheumatoid arthritis factor, venereal disease research laboratory, antinuclear antibody, antineutrophilic cytoplasmic antibody P and C, and Mantoux test. Conjunctival swab should be obtained in eyes with active scleral disease for Gram and Ziehl-Neelsen staining to rule out infective etiology. In our previous study, although Mantoux was positive in 54.5% of eyes, Mantoux sensitivity showed no significant correlation with the posterior corneal opacity.
Autoimmunity and semilunar sign
A probable reason for formation of semilunar sign could be the embryological association (the corneal endothelium and the predominant part of the sclera are derived from neural crest cells). Apart from the embryological element, there have been a few common factors, such as close anatomical proximity, closely related collagen structures and immunological response. The frequent corneal pathologies following anterior scleritis were noted as sequelae of a common immune response in both the corneal and scleral tissue in noninfectious anterior scleritis. Thus, noninfectious scleritis is commonly associated with semilunar sign. Unlike common autoimmune conditions, corneal endothelial opacity does not decrease with steroid treatment.
Conclusion
Semilunar sign in scleritis must create awareness of a dormant condition that can affect endothelial function. Hence, necessary precautions should be taken by anterior segment surgeons while operating on eyes with semilunar sign (Figure 5). Steps such as proper planning of surgical incision, use of limited ultrasound power and viscoelastic-guided endothelial protection can prevent endothelial loss. Treating scleritis can reduce the extent of corneal involvement. Follow-up of the patients and timely intervention are necessary in eyes with semilunar sign.
- References:
- Holt Wilson AD, et al. Trans Ophthalmol Soc UK. 1974;94:52-57.
- Kumar DA, et al. Indian J Ophthalmol. 2022;doi:10.4103/ijo.IJO_2073_21.
- Kuroda Y, et al. Br J Ophthalmol. 2017;doi:10.1136/bjophthalmol-2016-308561.
- Watson PG, et al. Br J Ophthalmol. 1976;doi:10.1136/bjo.60.3.163.
- Watson PG, et al. Exp Eye Res. 2004;doi:10.1016/s0014-4835(03)00212-4.
- For more information:
- Amar Agarwal, MS, FRCS, FRCOphth, director of Dr. Agarwal’s Eye Hospital and Eye Research Centre, is the author of several books published by SLACK Books, sister company of Healio publisher Ocular Surgery News, including Phaco Nightmares: Conquering Cataract Catastrophes, Bimanual Phaco: Mastering the Phakonit/MICS Technique, Dry Eye: A Practical Guide to Ocular Surface Disorders and Stem Cell Surgery and Presbyopia: A Surgical Textbook. He can be reached at 19 Cathedral Road, Chennai 600 086, India; email: aehl19c@gmail.com; website: www.dragarwal.com.
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