INTRODUCTION

Keratoconus (KC) is a noninflammatory eye condition characterized by progressive corneal ectasia, myopia, and irregular astigmatism, generally with bilateral involvement. The etiology and progression of keratoconus have been closely linked to genetic factors, which appear to be multifactorial. However, the majority of cases are not linked to genetic factors^(1,2). It is important to detect progression in KC before making collagen cross-linking (CXL).

The Valsalva maneuver (VM) is an attempt at a powerful exhalation against a closed airway. VM is frequently performed during normal daily activities: examples include lifting heavy items, performing physical exercise, vomiting, and coughing. Various physiological changes happen during these activities, including elevated blood pressure, increased intrathoracic pressure, increased peripheral venous pressure, and stimulation of the peripheral sympathetic system^(3-5). The clinical consequences of VM may have some effects on the anterior chamber, such as a significant narrowing of the iridocorneal angle and a shallowing of the central anterior chamber^(6,7). When patients hold their breath and perform an involuntary VM during ophthalmological examinations, the progression of ectasia could be measured incorrectly.

The aim of the present study was to explore the effects of VM on ectatic corneas during corneal topography scans using a Scheimpflug camera.

METHODS

This prospective observational study involved 100 eyes of 50 patients with bilateral KC. Institutional Review Board approval was obtained, and the study was conducted according to the ethical standards of the Declaration of Helsinki.

Patients with corneal ectatic disease other than KC or other ocular surface and/or intraocular pathology, patients who had undergone any previous eye surgery, and patients who received CXL were excluded from the study. In addition, patients with any kind of systemic disease that could make VM intolerable, and users of anticoagulants, were also excluded. VM was performed during the examination by blowing through a mouthpiece attached to a manometer. Expiratory pressure ranged between 35 and 40 mmHg⁽⁸⁾.

An anterior segment analyzer, Pentacam (Oculus Optikgerate, Wetzlar, Germany), utilizes the Scheimpflug principle in photography with a view to capturing slit images and producing a variety of data in a non-contact fashion. The system includes a rotating Scheimpflug camera and a light source that emits UV-free blue light with a wavelength of 475 nm⁽⁶⁾. This system enables the diagnosis of KC, as well as grading the condition and disease monitoring.

Corneal topography was initially performed with the participant in a resting position, immediately followed by repeating the procedure while the patient performed VM. The same ophthalmologist (Y.K) performed all the measurements. Topography images were taken using the Pentacam, which enabled images to be taken of the anterior and posterior surfaces of the cornea and iris, and of the anterior and posterior surfaces of the lens in a movable virtual eye⁽⁹⁾. After positioning the participant's head in a chin rest and head rest, the participant was asked to fixate on the center of the blue slit light. The joystick was adjusted by the examiner until appropriate alignment was obtained. A 9.00 mm diameter spherical reference, was used in elevation maps. The evaluated parameters are presented in table 1.

The results were analyzed using SPSS 16.0 software for Windows (SPSS Inc., Chicago, IL). Data are presented as mean ± standard deviation (SD). Parameters studied before and during VM were compared using paired sample t-tests, and a general linear model (repeated measures) was used for analyzing the parameters according to the stage of KC. A p-value of <0.05 was considered to indicate statistical significance.

RESULTS

The mean age of the participants with KC in both eyes was found to be 24.14 ± 6.59 years. Of the 50 participants, 27 (54%) were female and 23 (46%) were male. Both of the participants' eyes were included in the study. Of the eyes included in the study, 7% were at stage 1 KC, 47% were at stage 2 KC, 32% were at stage 3 KC, and 14% were at stage 4 KC.

The indices of KC (ISV, IVA, KI, CKI, IHA, IHD, and Rmin) were not significantly affected by performing VM and did not change according to the stage of KC. Similarly, no statistically significant effects were found for the stage of KC, D value, and the average pachymetric progression index over VM. Table 2 presents results showing the effects of VM on anterior segment parameters. When the effects of the KC stage on the parameters were compared, there was a greater decline during VM in CFS Km, Kmax, and D in KC stage 4 cases. These differences were statistically significant with respect to the stage of KC (Table 2). The differences in the other parameters before and during VM did not change with the stage of the KC.

Pupil diameter increased to a statistically significant degree during VM (p=0.017). Conversely, there was a statistically significant decrease in the angle value during VM (p=0.001). Although the x- and y-coordinates of Kmax changed sides (p=0.007, p=0.006), the change in Kmax value was not statistically significant (p=0.107). Changes in other anterior segment parameters were also not statistically significant.

DISCUSSION

KC is a degenerative disease of the cornea characterized by stromal thinning and conical ectasia⁽¹⁾. KC is usually caused to progress by increased keratometry, irregular astigmatism, and decreased corneal thickness^(10-12). The progression of ectasia was diagnosed based on the corneal tomography and topographic changes. These changes included progressive elevation of the posterior and/or anterior surface, progressive corneal thinning at the thinnest point, and an increase in corneal anterior keratometry steepening with an irregular astigmatism in the cone area.

Choi et al.⁽¹³⁾ reported a study in which 25% of eyes were found to have progression in central keratometry of at least 1.5 D, according to measurements taken by computerized videokeratography (Orbscan II) over a mean of 3.5 years, with a median time to progression of 12 years. Progression in KC disease is determined by a comparison of topographic measurements taken at different times. The present study evaluated whether inaccurate positive or negative results occurred due to VM performed even involuntarily during the measurements. A decline in the value of Kmax was observed during VM in stage 4 KC, which could be interpreted mistakenly as regression of the disease. Thankfully, CXL for progression is not recommended in these advanced KC patients.

Although several studies have tried to investigate the factors that help determine the prognosis, it remains a challenge to predict progression at the time of diagnosis in a specific patient. However, some predictors of progression include a higher cylinder and keratometry (as measured by topography), thinnest pachymetry, and shorter disease duration^(13-15). As these topography parameters are compared between two measurements, those measurements should be performed under ideal conditions without bias from environmental factors. An inaccurate corneal topography could result in mistakes in the diagnosis of progression. Some patients may perform VM involuntarily during corneal topography. However, in this study performing VM did not result in any statistically significant changes in the KC progression indices.

Pekel et al., in their study involving measurements with a Pentacam system, found that AC depth and central corneal thickness in healthy individuals without KC decreased significantly with VM⁽⁸⁾. They also found that VM could affect corneal morphology and that it caused certain reversible changes in the keratometry. Our study also investigated whether there would be changes in KC patients; we found that PD, angle value, and the x- and y-coordinates of Kmax changed. A similar study which evaluated normal corneas demonstrated an increase in PD and decrease in ACD⁽¹⁶⁾. The only organ related to the autonomic nervous system in the study was the pupil⁽¹⁷⁾. There was decreased vagal and increased peripheral sympathetic stimulation during VM, and the noted dilation could be explained by this. Narrowing of the anterior chamber and iridocorneal angle could be caused even by this mild dilation⁽¹⁸⁾.

With subclinical disease, anterior curvature alone may not provide enough information to detect an early corneal abnormality. The goal of the Belin/Ambrosio Enhanced Ectasia Display was to combine elevation and pachymetric-based corneal evaluation in an all-inclusive display⁽¹⁹⁾. The present study found no statistically significant changes in D values, demonstrating early posterior ectatic changes during VM. However, D values in advanced KC have been found to exhibit more changes than in the other stages of KC, between, before, and during VM. These changes are not important in clinical practice because the D value is more important at the early stage of KC.

Previous studies have demonstrated the effect of VM on healthy corneas^(8,16). However, this effect has not been studied in KC patient groups. Before the study was conducted, more anterior segment parameter changes were predicted in KC patients who had thinner and less rigid corneas than in VM studies mentioned. However, the present study found no anterior segment parameter changes that would affect stage changes and treatment plans for KC patients.

In conclusion, no changes were found in the KC indices and stage of the disease while participants were performing VM. Furthermore, it was found that there was an increase in PD and a decrease in angle, and these changes were similar to those from studies on normal corneas. The Kmax value in stage 4 KC may decrease more during VM than in other stages.