Ocular ultrasound in the intensive care unit

(CONSULTANT LEVEL EXAMINATION)

ASHOT E. SARGSYAN, MICHAEL BLAIVAS, THOMAS GEERAERTS and DIMITRIOS KARAKITSOS

Overview

Multipurpose ultrasound systems offer a variety of eye and orbit imaging techniques relevant to critical care. Bedside ocular ultrasound can be used for the following purposes: (1) timely identification of threats to ocular health and vision and prevention of vision-related disability in trauma,1–4 (2) assessment of nontraumatic acute change in vision and the need for immediate ophthalmologic consultation,¹ (3) measurement of the diameter of the optic nerve sheath (ONSD) as a surrogate measure for intracranial pressure (ICP),^5–9 (4) assessing the integrity of the bony orbit and periorbital tissues in severe craniofacial trauma,¹⁰ and (5) assessing the pupillary reactivity when visual access to the pupil is impeded.¹¹ However, ocular ultrasound applications are somewhat unique and not standardized at this time, whereas only ONSD measurements have been partially studied in the intensive care unit (ICU) environment. This chapter illustrates specific guidance for ultrasound equipment choice and adjustment, general ocular scanning techniques, and tips to maximize the utility of the ultrasound method for eye and orbit imaging.

Equipment and settings

In principle, any modern ultrasound system is acceptable for eye/orbit imaging if equipped with an appropriate transducer. At the time, multipurpose systems do not have either specialized transducers or factory-programmed “presets” for ocular imaging. To abide by the current safety standards, a machine should be chosen that displays the safety-related meta-data: mechanical index (MI) and thermal index (TI) on the screen. Older-generation machines do not display these values. Ocular scanning with those machines may be done only for vital indications and using the lowest possible power output. Furthermore, some results (e.g., ONSD measurements) may not be accurate because of old technology or wear and tear.

Because of the relatively shallow depths, small volume of the region of interest (ROI) and the very low attenuation in ocular media, the highest available frequencies are used, most commonly, a 12- or 14-MHz linear array transducer. Higher frequencies offer better resolution and smaller, easier-to-handle probes with a small contact surface. Lower frequencies must be used initially in severe edema of the area or a possibility of orbital fractures, globe rupture, or retrobulbar hematoma (e.g., a 5- to 8-MHz microconvex or a 4- to 7-MHz broadband linear transducer). This will ensure confident penetration without applying undue pressure on the transducer, with a commensurate compromise in spatial resolution.

The choice of transducers also depends on the scope of the examination. Some high-frequency transducers offer brilliant images of the soft tissues and anterior segment but do not focus well at depths greater than 2 cm. ONSD measurement with these probes and examination of the orbital apex area is not advisable. Thus frequency is not the only consideration. For these reasons, initial suitability assessment must be conducted by each facility that chooses to implement and use eye/orbit ultrasound in their practice, and for each machine and each associated transducer.

Equipment controls most frequently used in eye/orbit imaging are gain, TGC (time gain compensation), focus, and power output (see Chapter 1). Additional optimization techniques can be invoked as dictated by specific circumstances.

We highly recommend creating a customized eye/orbit preset that could be selected within seconds from the preset menu of the machine. This will save valuable time; ensure standardization; and serve as a safety, training, and quality assurance mechanism. An example is (1) select the factory-made thyroid preset from the preset menu; (2) adjust depth to 4 cm; (3) move the focus marker to 2.5 cm; (4) adjust MI to 0.23 ¹²^,¹³ with the power output control; (5) set TGC rheostats at their midrange; (6) scan a patient with indications for ocular ultrasound (e.g., to measure ONSD), and after identifying the terminal optic nerve, adjust gain appropriately; (7) save current settings under a new custom preset name (e.g., “eye-orbit”); (8) scan several patients to “fine-tune” the preset, and resave it with improved settings every time; (9) when satisfied, copy the preset to all machines in the department (if same model), or reproduce its features manually if machines are different. Make sure that additional setting changes do not increase MI; if they do, reduce power output accordingly. Note that perfect presets do not exist, and machine adjustments (at least, gain and TGC) will still be necessary in most patients.

General scanning technique and primary views

Eye and orbit scanning in the ICU is done with closed eyelids. The scope of the examination will depend on patient’s history, status, and open clinical questions. An ICU patient may need only a limited posterior pole and optic nerve examination. However, in trauma patients, the scope of the examination may be broader (see Chapters 1, 51, and 57), and thus a holistic approach (HOLA) ultrasound ocular protocol would include scanning of the eyelids, globe and orbital contents, as well as investigation of the frontal, temporal, zygomatic and nasal bones and overlying tissues. The eye examination proper, in its turn, may be directed at checking the overall integrity of the globe or at a more thorough scrutiny of its entire volume.

The operator holds the transducer close to the contact surface, touching the patient’s face with the fourth and/or fifth finger for stability. The following views are assuming a forward gaze; a straight gaze forward is highly desirable but may not be possible in unconscious or sedated patients. Directional terms used for ocular scanning are nasotemporal, superoinferior, and anteroposterior. A location within the eyeball (e.g., for a foreign body) is commonly identified in terms of the clock face (e.g., 1 o’clock) and in the anteroposterior (AP) dimension, relative to the following landmarks: macula, optic disk, equator, ora serrata, limbus, and corneal vertex; the distance from the optical axis or the wall is sometimes also used.

The AP views (sagittal and axial) are used for ocular anatomy survey and allow viewing practically all ocular structures to rule out macroscopic pathology (Figure 6-1A-C). The oblique sagittal view of the optic disk and terminal optic nerve is used to assess the shape of the optic disk, the terminal optic nerve with its meninges, and as a backup approach to measure ONSD in a sagittal plane (Figure 6-1E). This view may offer a possibility to visualize a longer span of the optic nerve by using a small rotation of the imaging plane. Note that the transducer is placed laterally to the cornea so that the imaging plane bypasses the anterior structures (cornea, anterior chamber, ciliary body, and the lens). The axial view from lateral approach allows avoiding the edge-shadowing artifact that in AP views obscures the peripheral zones of the vitreous humor and the globe wall near the equator (Figure 6-1D). The oblique axial view with optic disk and optic nerve head is the quickest and the most reproducible way to measure ONSD (Figure 6-2). The transducer position is on the upper eyelid (preferably with a downward gaze) with the marker to the right (for both eyes), and the plane bypasses the anterior structures. The resulting target image is similar to the sagittal view (see Figure 6-1E). The coronal view of the iris is used to assess the circular symmetry of the iris and the pupillary reactivity to light (Figure 6-3).