The tick sign - a new and simple test to diagnose keratoconus at the slit lamp.

Abstract

We present a simple test doable on any slit lamp by which, relying on the positions of the Purkinje reflexes to one another (Tscherning 1898; Gullstrand 1909), keratoconus can be diagnosed. The beam of a small light source is directed onto an eye at about 30 cm distance from an 40°–20° oblique horizontal position temporally and nasally (see Fig. 1, Gellrich 2018). On video still images taken with a slit lamp biomicroscope (Gellrich & Kandzia 2016), we measured the deviation of the second Purkinje image (P2) from the horizontal connecting line between the first (P1) and fourth Purkinje image (P4) in a group of 14 patients affected by keratoconus and 14 normal persons (‘P2-deviation’). P2 deviations exceeding 0.2 mm were associated with a characteristic ‘tick shape’ for the connecting line P1 – P2 – P4 (see Fig. 1). One keratoconus patient's eye had to be excluded due to keratoplasty surgery leaving a total of 24 eyes with keratoconus for our investigation. Classic biomicroscopic signs of keratoconus (Munson's sign, Vogt’ s striae and iron lines) were also documented (Gellrich 2014). Mean P2 deviation was much larger in the keratoconus group than in the normal group (0.29 versus 0.04 mm – p < 0.001, t-test) (Table 1). Among the keratoconus patients, I observed a marked lateral difference in the severity of the disease in nine individuals. The side more seriously affected always exhibited a higher mean P2 displacement in each individual. Those more severely affected eyes had a mean P2 displacement of 0.34 ± 0.10 mm, whereas the less affected eyes’ mean was 0.15 ± 0.08 mm. P2 deviation was larger in eyes with Amsler stage 2 than in those with Amsler stage 1 (0.35 versus 0.23 mm – p < 0.05, t-test). Contralateral eyes clinically not affected by keratoconus had larger mean P2 deviation (0.11 mm) than a normal control group (0.04 mm). These data indicate that the tick test can not only serve as a quick test for keratoconus, but that this method has the potential for documenting severity and possibly even the course of the disease. Furthermore, we were interested in a comparison between the tick sign and the classic biomicroscopic changes associated with keratoconus. All 14 keratoconus patients exhibited at least one positive tick test. From 54 Purkinje patterns in the keratoconus group, 32 revealed a positive tick sign while it was always negative in the control group. On the other hand, only 7 of the 14 keratoconus patients presented biomicroscopic indications of keratoconus in 11 eyes: 10 eyes with Vogt's striae and/or iron lines, one eye with haze. Munson's sign was never present. P2 deviation in our setting for the tick test can be explained by backwards tilt of the posterior in relation to the anterior corneal surface which should be accompanied by higher pachymetric values above than below the corneal equator. For this correlation between corneal dimensions and the tick sign, we analysed topographic data available with the Galilei system from 19 eyes with keratoconus in 11 patients (Steinberg et al. 2015). In the affected eyes, the tick sign was present for 26/38 Purkinje patterns. For topographic correlation with our Purkinje patterns, we determined 1 mm temporally and nasally from the pupil centre the difference in corneal thickness between corresponding locations 0.5 mm above and below the horizontal meridian. We found that if this difference exceeded 20 μm (corresponding to 1.15° angle of tilt between anterior and posterior surface), the tick sign was always positive (20 patterns) and where the tick sign was missing, the pachymetric difference was always <20 μm.