Strabismus is a state of impaired ocular alignment with a prevalence of 2-4% in the western world. In children, strabismus can cause severe permanent vision impairment but in adults the main problem is diplopia or social difficulties such as avoiding eye contact, which can be very disabling. There are many causes for strabismus, ranging from refractive errors to severe neurological conditions. The primary aim when treating strabismus is to restore ocular alignment. Various treatment methods are available but extraocular muscle surgery remains the treatment of choice for permanent correction. Anterior segment ischemia is a rare but severe complication to strabismus surgery. The condition occurs due to damage of the anterior ciliary arteries that course along the rectus muscles and therefore gets damaged during surgical manipulation. In its mildest form it results in pupil abnormalities but more severe consequences such as corneal scars have been observed with risk of vision loss. It is generally believed that to reduce the risk of anterior segment ischemia, only two muscles should be operated on during strabismus surgery and a third muscle can only be operated on given that 6 months healing time has passed. However, when the commonly used strabismus surgery procedures were developed a century ago, they were based on empirical observations of clinical outcome. Knowledge of the effect of strabismus surgery on perfusion to the anterior segments of the eye is virtually non-existent. Today modern, non-invasive, techniques are available to monitor perfusion during surgery. Our research attempts to develop perfusion and oxygenation monitoring techniques to assess the effect of strabismus surgery for the first time.

Laser speckle contrast imaging (LSCI) uses a 780 nm laser for illumination and a high-sensitivity image sensor. When the light hits the area of interest some of it will be reflected and some of it will scatter as it hits moving particles such as red blood cells. The interference of the reflected light can be detected as a speckle pattern by the image sensor. The speckles will change as the red blood cells move in the capillaries. This change can be interpreted as blood flow rate, i.e., the perfusion of the area. LSCI is a non-invasive technique that has proven useful to monitor blood perfusion in skin with high spatial and temporal resolution. LSCI has been used in measurements of perfusion in human periocular flaps and reconstructive surgery with convincing results (Tenland et al., 2020, Berggren et al., 2019). However, the potential of LSCI has not yet been used to monitor blood perfusion in strabismus surgery.

Hyperspectral imaging (HSI) uses a broad light spectrum with detection between 600 and 1000 nm, forming a spectral map, from which for example the blood oxygen saturation (sO2) can be calculated. HSI has yet not been clinically established, but a few studies have demonstrated its applicability in monitoring oxygenation in in humans (Klaessens et al., 2013, Calin et al., 2017, Holmer et al., 2016, Goetze et al., 2020, Meier et al., 2012). However, the technique has never been tested for monitoring oxygenation in strabismus surgery.

The purpose of this project is to use LSCI and HSI for perfusion and saturation monitoring during strabismus surgery. Hopefully, adequate monitoring techniques during surgery could lead to the prediction and elimination of anterior segment ischemia after strabismus surgery. Furthermore, patients that are considered to have high risk of developing anterior segment ischemia are often not offered surgery at all or operated on with a suboptimal surgical method when a perfusion examination might show that standard procedures could be performed. Surgical tradition set aside, we imagine that in many cases it would be possible to operate on three muscles at a time, and a fourth after a period of healing. Furthermore, we believe that patients with increased risk could be identified preoperatively with this technique. In conclusion, if successful in monitoring perfusion and saturation during strabismus surgery, LSCI could enable individualized surgical procedures leading to lower risk of complications such as anterior segment ischemia as well as better clinical outcome.