How is thyroid eye disease diagnosed?

In the majority of patients with Graves’ disease, a suspected diagnosis of thyroid eye disease may be confirmed on the basis of clues and findings that come to light when a doctor takes a history and performs a physical examination. This type of diagnosis, based on evidence derived from a history and physical examination, is called a clinical diagnosis. Specific symptoms that may be related to thyroid eye disease include a distinctive change in the appearance of the eyes, a sensation of pressure building up in the eye sockets, increased redness and watering of the eyes, increased sensitivity to light, irritation that feels like a piece of sand or grit may be stuck on the surface of an eye, blurring of vision, or double vision. If a patient with active Graves’ disease reports any of these symptoms, a diagnosis of thyroid eye disease may be suspected.

Supportive findings that may be noted on physical examination include a characteristic stare that may be marked by increased prominence of the white rim of the sclera around the iris. Close examination of the surface of each eyeball may reveal dilation of blood vessels consistent with conjunctival injection. A finding of boggy thickening of the conjunctiva itself may be consistent with chemosis. Soft tissue changes consistent with lid edema and periorbital edema may also be noted. Close inspection of the surface of each cornea with a special instrument known as a slit lamp may reveal scratches or scarring that may be consistent with exposure keratitis. Examination of the movement of the eyes may reveal findings that may reflect entrapment of extraocular muscles. Some patients may notice the emergence of double vision when they are asked to track the motion of a fingertip as it moves up-and-down and left-to-right across the field of vision. In severe cases, inspection may reveal that one eyeball does not rotate as far as the other as they both move in certain directions during this maneuver. This finding is called disconjugate gaze. A specific maneuver that involves shining a flashlight directly into each eye may reveal sluggish constriction of a pupil that may reflect the effects of optic nerve compression. Suspicion of this complication may be heightened if eye chart testing reveals decreased visual acuity.

While it may not be essential to obtain specific images to confirm a suspected diagnosis of thyroid eye disease, there are certain instruments and radiographic studies that may be used to provide objective measurements of the degree of involvement. These measurements may help to provide a baseline estimate of the activity of thyroid eye disease that may help to determine whether there is any need for specific treatment. If a decision is made to begin treatment, similar measurements can be followed over time to determine whether the treatment has led to any improvement.

Proptosis due to thyroid eye disease can be measured with a special instrument called an exophthalmometer. This instrument measures the distance between the edge of the rim of bone that forms the outer edge of the eye socket and the front of the cornea that forms the transparent layer of the exposed surface of the eye. This measurement provides an estimate of the extent to which each eyeball may be protruding out of its eye socket. The normal distance between the outer edge of the eye socket and the front of the cornea ranges from about 18 millimeters to 22 millimeters. The simplest type of exophthalmometer is shaped like a small transparent ruler with one end that forms a blunted point. This point is held against the outer edge of the eye socket to provide a baseline for obtaining a measurement. A more sophisticated instrument called a Hertel exophthalmometer incorporates a system of sliding rulers and angled mirrors that provide an indirect view of the side of each eyeball. A sliding ruler can be used to measure the distance between the outer edges of the right and left eye sockets. Ruled lines on the surface of each mirror can be used to measure the protrusion of each eyeball.

An orbital CT scan is a radiographic study that can be used to generate images of the eyeballs and eye sockets. These images provide very accurate information about the position of each eyeball within each eye socket, swelling in each extraocular muscle, and the amount of fat surrounding the extraocular muscles and optic nerves. Characteristic findings that may be consistent with active thyroid eye disease include forward bulging of the thick layer of connective tissue that holds each eyeball in place, preferential involvement of specific extraocular muscles, and enlargement of the middle portions of extraocular muscles. These findings may help to establish a diagnosis of thyroid eye disease in patients with euthyroid Graves’ disease. In patients who are undergoing treatment, serial orbital CT scans may be followed over time to track changes in the dimensions of extraocular muscles that may reflect a response to treatment. Patients with active Graves’ disease undergoing orbital CT scans should not receive any form of intravenous contrast dye, as the iodine present in most of these dyes may be taken up by overactive thyroid tissue. This may interfere with any plans for treatment that involve the use of radioactive iodine.

Orbital MRI scans can also provide images of the eyeballs and eye sockets, though they are less commonly used in practice. An orbital ultrasound is a radiographic study that can be used to measure the dimensions of enlarged extraocular muscles. It uses signal provided by reflected sound waves to generate cross-sectional video images of the middle portions of extraocular muscles. In patients undergoing treatment, serial orbital ultrasounds may be followed over time to track changes in the dimensions of extraocular muscles.