The complaint of double vision can cause confusion

Thyroid eye disease, myositis, and myasthenia gravis are three entities associated with diplopia that may be difficult or confusing to distinguish, said Dr. Cockerham, associate professor of ophthalmology, University of California, San Francisco. She detailed the key symptoms and signs of these diseases, the typical onset, and workup for each disorder.

Thyroid eye disease most commonly occurs in women during the third to fifth decades of life. In addition to diplopia, these patients can present with proptosis, eyelid retraction, and a vague ache on eye movement. They may experience tearing and irritation as well as photophobia.

Myositis, on the other hand, can occur in either men or women, typically in the second to fifth decade, and usually has an acute onset. Patients typically complain of excruciating pain and can remember the day and time that the pain, not the double vision, first occurred. Often, only one muscle on one side is involved, so if diplopia occurs with right gaze, it will usually be only in right gaze. The patients will have unilateral proptosis, ptosis, and pain, she said.

Myasthenia gravis, an abnormality of the nerve-muscle junction, is usually seen in more women than men up to age 40. After age 40, the distribution is evenly split between men and women. It, however, can present at birth in children with myasthenic mothers, or occur as early as the teen-age years and as late as the seventh decade.

Patients who have myasthenia gravis typically present with painless ptosis and variable onset of diplopia. "They usually don't remember the day of the week or even the month that it happened," Dr. Cockerham said. "It can be variable, fluctuating hour to hour and day to day, and definitely worse at the end of the day."

Thyroid eye disease usually affects both eyes, although it can be sequential or asymmetric, Dr. Cockerham noted. Other findings include eyelid retraction/lagophthalmos, conjunctival injection, intraocular elevation on upgaze, proptosis, and periorbital edema. Some patients exhibit the "W sign," with a puffy lower eyelid and prolapse of the lacrimal gland in the lateral upper eyelid, she continued.

Myositis is typically unilateral in presentation, although it can be sequential and bilateral. Patients can exhibit conjunctival injection, proptosis, and periorbital edema.

Myasthenia gravis can be unilateral or bilateral and is associated with ptosis and the inability to sustain upgaze, she said.

The workup for diplopia involves external photos and baseline visual field examination. For individuals suspected of having thyroid eye disease, thyroid function tests should be performed such as T₄ and TSH as well as thyroid stimulating immunoglobulins (TSI).

In thyroid eye disease, CT or MRI will reveal enlarged multiple extraocular muscles, the tendons spared, a normal lacrimal gland, and expanded intraconal fat. With myositis, MRI typically shows single muscle involvement, tendons affected, an inflamed lacrimal gland, and streaking intraconal fat.

"With myositis, additional workup is usually not warranted," Dr. Cockerham said. A biopsy, however, should be performed if the patient is not within the age range for the disease or if it is recurrent, she noted. Laboratory evaluation is indicated in bilateral cases to look for specific causes of inflammation such as Wegener's granulomatosis.

Those patients suspected of having myasthenia gravis should be asked about systemic symptoms, such as difficulty swallowing or breathing, nasalization of speech, or weakness on lifting arms or climbing stairs.