The first ophthalmic ultrasound image was published in 1956 [1]. Since then, ultrasound imaging technologies have evolved to offer crucial diagnostic information in many ophthalmic conditions, such as complications of ocular trauma [2–5]. Ultrasound imaging is especially helpful when visual (optical) inspection is impossible to perform or does not provide a definitive diagnosis. Ultrasound in ophthalmology is employed in several clinical applications such as general-purpose ocular imaging (B-mode), ultrasound biometry that pursues precise distance measurements (A-mode), and ultrasound biomicroscopy (UBM) limited to the anterior segment, which uses high frequencies (e.g., 50 MHz) to provide a very high resolution of 33 μm and less [6–10]. Although very useful, specialized ophthalmic scanners or UBM devices are not available in the vast majority of emergency settings. Modern multipurpose ultrasound systems, on the other hand, are increasingly available for emergency imaging needs, and they have been demonstrated to provide excellent ophthalmic images when fitted with high-frequency probes [11]. Furthermore, high-end systems of this class employ sophisticated focusing, harmonic imaging and other optimization techniques that are not yet feasible in the “ophthalmic” ultrasound systems. In the absence of slit lamp capability or other imaging options, this imaging modality may seek to obtain information that is normally outside the generally recognized scope of ocular ultrasound.
The condition of the iris and its response to stimuli is of interest in a variety of conditions. Reviews on pupillary light reflex (PLR) and its clinical implications have been published [3, 12]. For example, absence of the PLR has been shown to be a risk factor independently associated with death in craniofacial trauma [13]. Prognostic consideration of pupillary diameter and constrictive ability was also recommended by the American Association of Neurological Surgeons [14]. Due to the importance of PLR evaluation, it seems prudent to consider other potential means of PLR assessment for instances when soft tissue damage, corneal opacity or hyphema may obstruct visual access to the pupil. Of the various pupillometry methods that have been described, the majority still require specialized hardware and expertise and are not available in the majority of emergency settings [15–17]. Multipurpose ultrasound imagers, however, are used in emergency settings worldwide, rapidly establishing a new role of sonography as a first-line modality for emergency departments at the patient’s bedside [18–20]. These conventional multipurpose ultrasound scanners fitted with high-frequency probes in the 10–12 MHz range can be used to evaluate the condition of the globe and its components [21]. Taking advantage of the capabilities of these scanners, we developed a practical method to assess pupillary response to light.
The experiments described here have been part of a large ground-based study conducted by NASA to develop ultrasound imaging procedures for long-duration space flight. Since the only ultrasound device currently flown in space is a multi-purpose system (HDI-5000, ATL/Philips, Bothell, WA, USA), procedures and protocols were sought to take utmost advantage of the capability in the majority of foreseeable medical conditions in space, including ophthalmic trauma, blunt head injuries and body injuries, to name a few.
This work is based on the finding that besides antero-posterior planes, coronal or near-coronal sections of the eye are attainable, resolving the pupil in real time, clearly and separately from the strongly reflective lens (Fig. 1, Video 1). The ability to resolve the pupil using ultrasound allows for PLR evaluation. Consensual PLR, which is demonstrated in the procedure described, has been shown in human and animal experiments to be weaker, yet, is comparable to the direct PLR in terms of the degree of constriction [22–24]. Besides evaluation for instances such as neurological trauma, the method, with some modifications, may also be of interest for general ophthalmology and neurosurgery practice, as well as for research in neurophysiology, since it allows recording a variety of additional parameters such as rate of constriction, latency time, and dilatation time.