Cornea is the soft tissue structure located in the outer layer of the eye globe. The transparent cornea has 70% of the refractive power of the eye.1 The human cornea is considered to have both viscous and elastic properties.2,3
Corneal pathologies such as keratoconus are associated with marked corneal stromal thinning and multiple breaks in the Bowman layer. These structural changes may lead to localized biomechanical decompensation resulting in changes of corneal geometry and irregular astigmatism.4–6
In this case report, we presented a case of marked corneal thinning with normal Scheimpflug tomography findings and suspicious corneal biomechanical properties.
A 30-year-old man presented to the outpatient clinic of the Nourelain Center for Lasik and Corneal Surgeries, Tanta, Egypt. The patient complained of bilateral defective vision due to bilateral refractive error.
The uncorrected distance visual acuity (UDVA) in the right eye was 0.06, with corrected distance visual acuity (CDVA) to 1.00 by a refraction of −2.50 × −2.75 × 15, whereas the UDVA in the left eye was 0.05, with CDVA to 0.80 by a refraction of −4.75 × −2.50 × 64.
Slitlamp examination (RM-8900, Topcon Medical Systems, Inc.) of the anterior segment was unremarkable with bilateral normal corneal diameter (mm), normal scleral color, clear corneal stroma and no corneal scars or endothelial abnormalities. No iris abnormalities or lens opacities were detected. The intraocular pressure and fundus examination were performed for the patient, and all of them were free. There was no positive family history of refractive errors, ectatic corneal disorders, or associated skeletal deformities.
The patient sought refractive laser correction in our center. Corneal tomography was performed for both eyes by slit‐scanning Orbscan IIz (Bausch & Lomb, Inc.), and the patient was diagnosed as normal corneal tomography with symmetrical bowtie, but the thinnest pachymetry was extremely low. The thinnest pachymetry was 355 μm in the right eye and 358 μm in the left eye with normal topographical map.
The mean keratometry was within the normal range, in the right eye 42.94 diopter (D) and in the left eye 42.97 D. Both front and back elevations were within the normal range in both eyes. The white-to-white diameter was 12.60 mm in the right eye and 12.75 mm in the left eye (Figure 1, A).
Follow-up of the patient for 1 year by corneal tomography was the decision to exclude any possibility for progression for such an extremely thin cornea, and the results were the same (Figure 1, B).
Despite normal corneal tomography, another corneal tomographic examination was performed with another Scheimpflug-based tomography device (Oculus Optikgeräte GmbH), which confirmed the same results of the previous device (Figure 1, C).
Despite normal corneal tomography, we decided to perform bilateral corneal biomechanical assessment due to this ultrathin corneal thickness. Evaluation of corneal biomechanical properties was performed using corneal visualization Scheimpflug technology (CST).
The patient was asked to focus on the red LED. The CST takes about 4,300 frames of the central 8.0 mm horizontal portion per second with a high-speed Scheimpflug camera; the camera records and displays the entire response of cornea deformation to an air puff from the instrument. The data were analyzed by Corvis_ST_1.2r1126 software (Corvis_ST_1.2r1126).7
The data analysis of the CST device included the following: maximum Ambrosio relational thickness (ART max) was 393.5 in the right eye and 338.5 in the left eye, the corvis biomechanical index (CBI) was 0.85 in the right eye and 1.00 in the left eye, and the tomographic biomechanical index (TBI) was 1.00 in both eyes (Figures 2, A and B).
Slit‐scanning and Scheimpflug tomography have become a cornerstone technique for anterior segment imaging because it provides more information about corneal pachymetry front and back elevations.8–10
Corneal thinning is considered the key pathological feature of corneal ectasia, which is generally greatest at the cone apex.11,12 Multiple factors influence the corneal biomechanical properties such as hydration, elasticity, viscosity, and the thickness of the corneal stroma.13
To our knowledge, this is the first reported case of a markedly thin cornea (the thinnest pachymetry was 355 μm in the right eye and 358 μm in the left eye) with no detected ectatic changes by corneal tomography (mean keratometry readings and no back elevations) with CDVA better than 0.70 D.
Nearly all Orbscan IIz corneal tomographic parameters for our case were within the normal range except the extreme bilateral corneal thinning. As regards the bowtie (in the Orbscan and Oculus Scheimpflug-based tomography device), they were nearly symmetrical, especially in the absence of skewing or angulation.
There was slight decentralization of the thinnest point of the left eye (0.9 mm), and the decentralization in the right eye was (0.4 mm) in both Orbscan follow-up visits, so we considered it together with this extremely low pachymetry as an indication for further investigations.
Tomography maps detect minimal alteration in the shape of the cornea, such as thinning and increased curvature. However, they are not able to evaluate the biomechanical stability, which is thought to occur before detectable changes in corneal morphology take place.14,15
We performed corneal biomechanical evaluation by corneal visualization Scheimpflug technology, which uses a high-speed Scheimpflug camera to evaluate entire response of cornea deformation to an air puff and analyzes the data by software, which included the CBI that is a useful tool in the diagnosis of corneal ectasia. Our results showed high CBI (>0.50), which is the cutoff point with 98.4% specificity and 100% sensitivity as determined previously in a study of by Vinciguerra et al.16
The TBI is a novel parameter developed by Ambrósio et al.17 It combines parameter based on tomography data from the Pentacam and biomechanical assessment from the Corvis ST. Our results showed high TBI (>0.79), which is the cutoff point with 100% sensitivity for detecting clinical ectasia and 100% specificity, as well.
ART Max is one of the most valuable combined indices as a tomographic relational thickness. It had statistically better diagnostic value than single-point values, which depends mainly on the symmetrical change in corneal pachymetry in different points to identify early ectatic changes.18
The ART Max measured by the Corvis CST in our case was normal (393.5 in the right eye and 338.5 in the left eye), which matches the previous normal clinical and tomographic data. Thus, the relatively high biomechanical indices (CBI and TBI) can be explained by the low corneal thickness, as both parameters have been developed based on logistic regression analysis, which combined deformation response data with corneal thickness profile.16 These contradictory results of corneal tomography and biomechanical indices necessitate long-term follow-up of this rare case of extremely thin cornea together.
Ultrathin cornea with normal clinical and tomographic features is a rare condition. Evaluation of biomechanical properties in this case showed suspicious results, which were mainly explained by this extremely low corneal pachymetry.
1. Viswanathan D, Kumar NL, Males JJ, Graham SL. Relationship of structural characteristics to biomechanical profile in normal, keratoconic, and crosslinked eyes. Cornea 2015;34:791–796
2. Lombardo M, Lombardo G, Carbone G, De Santo MP, Barberi R, Serrao S. Biomechanics of the anterior human corneal tissue investigated with atomic force microscopy. Invest Ophthalmol Vis Sci 2012;53:1050–1057
3. Elsheikh A, Wang D, Pye D. Determination of the modulus of elasticity of the human cornea. J Refract Surg 2007;23:808–818
4. Rabinowitz YS. Keratoconus. Surv Ophthalmol 1998;42:297Y319
5. Jhanji V, Sharma N, Vajpayee RB. Management of keratoconus: current scenario. Br J Ophthalmol 2011;95:1044Y50
6. Roberts CJ. Biomechanics of INTACS in keratoconus. In: Ertan A, Colin J, eds. Intracorneal Ring Segments and Alternative Treatments for Corneal Ectatic Diseases. Ankara, Turkey: Kudret Eye Hospital; 2007:157Y66
7. He M, Ding H, He H, Zhang C, Liu L, Zhong X. Corneal biomechanical properties in healthy children measured by corneal visualization Scheimpflug technology. BMC Ophthalmol 2017;17:70
8. Ambrósio R, Alonso RS, Luz A, Coca Velarde LG. Corneal-thickness spatial profile and corneal-volume distribution: tomographic indices to detect keratoconus. J Cataract Refract Surg 2006;32:1851–1859
9. Smadja D, Santhiago MR, Mello GR, Krueger RR, Colin J, Touboul D. Influence of the reference surface shape for discriminating between normal corneas, subclinical keratoconus, and keratoconus. J Refract Surg. 2013;29:274–281
10. Oliveira CM, Ribeiro C, Franco S. Corneal imaging with slit-scanning and Scheimpflug imaging techniques. Clin Exp Optom 2011;94:33–42
11. Montalbán R, Alio JL, Javaloy J, Piñero DP. Comparative analysis of the relationship between anterior and posterior corneal shape analyzed by Scheimpflug photography in normal and keratoconus eyes. Graefes Arch Clin Exp Ophthalmol 2013;251:1547–1555
12. Ambrosio R, Caiado ALC, Guerra FP, Louzada R, Sinha RA, Luz A, Dupps WJ, Belin MW. Novel pachymetric parameters based on corneal tomography for diagnosing keratoconus. J Refract Surg 2011;27:753–758
13. Hatami-Marbini H. Hydration dependent viscoelastic tensile behavior of cornea. Ann Biomed Eng 2014;42:1740–1748
14. Roberts CJ. Biomechanics in keratoconus. In: Barbara A, ed. Textbook of Keratoconus: New Insights, 1st ed. New Delhi, India: Jaypee Brothers Medical Publishers; 2012:29–32
15. Scarcelli G, Besner S, Pineda R, Yun SH. Biomechanical characterization of keratoconus corneas ex vivo with Brillouin microscopy. Invest Ophthalmol Vis Sci 2014;55:4490–4495
16. Vinciguerra R, Ambrosio R, Elsheikh A, Roberts CJ, Lopes B, Morenghi E, Azzolini C, Vinciguerra P. Detection of keratoconus with a new biomechanical index. J Refract Surg 2016;32:803–810
17. Ambrósio R Jr, Lopes BT, Faria-Correia F, Salomão MQ, Bühren J, Roberts CJ, Elsheikh A, Vinciguerra R. Integration of scheimpflug-based corneal tomography and biomechanical assessments for enhancing ectasia detection. J Refract Surg 2017;33:434–443
18. Faria Correia F, Ramos I, Lopes B. Topometric and tomographic indices for the diagnosis of keratoconus. Int J Keratoconus Ectatic Corneal Dis 2012;1:92–99