What is Thyroid-associated orbitopathy (TAO)?
Thyroid-associated orbitopathy (TAO), also known as thyroid eye disease or Graves’ eye disease, is the most common specific inflammatory condition affecting the orbit (eye socket) and periorbital tissues. The management of TAO involves both surgical and medical components.
TAO is associated with Graves’ thyroid disease, and can present at any time in the course of the disease, whether the patient is in a euthyroid (normal thyroid), hypothyroid (underactive), or hyperthyroid (overactive) state.
The cause of TAO is unknown. In theory, the immune system promotes inflammation directed at the structures around the eye. Extraocular muscles (muscles that move the eye) are the primary site of inflammation. Orbital fat and eyelid muscles are also commonly involved.
Demographically, TAO is most prevalent among middle-aged Caucasian women, though it occurs in all races. It is particularly rare among Asians. Though less often affected, men tend to have a more severe course than women.
There are two phases of thyroid-associated orbitopathy. The active, inflammatory phase may last from 6 months to 5 years. Signs and symptoms change or progress over weeks to months. The non-active, post-inflammatory phase begins once the signs and symptoms have remained stable for at least 6 months.
Signs and symptoms of active thyroid-associated orbitopathy include: eyelid retraction (particularly lateral flare) causing “thyroid stare”, dry eye syndrome, periorbital edema (swelling), conjunctival swelling (boggy, wet eyes), restrictive strabismus with double vision, proptosis (bulging eyes), and vision loss due to compressive optic neuropathy (damage to the optic nerve – the optic nerve connects the eye to the brain).
The active phase of thyroid associated managed by corneal lubrication (artifical tears), oral corticosteriods (prednisone), corticosteriod injections to the orbit, orbital radiation, and orbital decompression in the rare case of optic nerve compression.
Generally, surgical management is reserved for the post-inflammatory or non-active phase of the disease, except when vision-threatening disorders (e.g., optic neuropathy or severe corneal exposure) are present.
The signs and symptoms of the non-active phase include eyelid retraction, exposure keratopathy, restrictive strabismus (tightness and pulling sensations when moving the eyes causing double vision), proptosis and compressive optic neuropathy with vision loss.
If mild to moderate, management may only require artificial tears. If more severe, then surgery is usually required. Surgical management of TAO must follow a staged sequence of procedures:
- Orbital Decompression
- Strabismus Correction
- Correction of Eyelid Retraction
Because of the variable nature of TAO, surgery to correct disease-related functional abnormalities is carefully timed and individualized. Though not all stages are necessary for every patient, orbital decompression is performed first, followed by strabismus correction, and, finally, correction of lid retraction.
1. Orbital Decompression
Orbital decompression, if required, is performed first in the surgical staging of TAO. There are several indications for orbital decompression in patients with TAO: compressive optic neuropathy, exposure keratopathy due to proptosis, orbital pain, elevated intraocular pressure, and cosmetic deformity.
Orbital decompression in TAO is achieved by removal of orbital bony wall and/or orbital fat. Removing portions of one or more of the bony walls of the orbit expands the volume available to the orbital fat and extraocular muscles.
Typically, a balanced orbital decompression is performed, which involves removal of the lateral orbital wall (outside wall) and medial orbital wall (inside wall), along with orbital fat.
2. Strabismus Correction
The goal of surgical therapy is not to eliminate double vision entirely, but rather to move the region of single binocular vision into a more functional area (straight ahead and down). Because of the unpredictable nature of restricted extraocular muscles, surgery is usually performed with adjustable sutures. Adjustable sutures allow the alignment of the eyes to be fine-tuned in the postoperative period when the patient is awake and alert, thus improving the final surgical outcome.
3. Correction of Eyelid Retraction
Upper lid retraction can cause dry-eye symptoms and corneal exposure, and may even induce a corneal ulcer due to inadequate lid closure. It also contributes significantly to cosmetic disfigurement. Due to the tendency for spontaneous improvement, surgery for isolated upper lid retraction is usually done after at least 1 year of observation.
Eyelid retraction surgery is performed after decompressive and strabismus surgeries have been completed and the lid position has been stable for 6 months or more.
Upper lid retraction is corrected with a levator recession operation. The levator muscle (muscle that lifts the eyelid) is lengthened, thus allowing the upper lid to cover more of the eye. One can think of this operation as the opposite of a ptosis (drooping eyelid) repair.
Lower eyelid retraction is a common problem in TAO patients. Patients with lower eyelid retraction complain of tearing, dryness, and foreign-body sensation. They frequently have evidence of exposure keratopathy. The most commonly used method of elevating the lower lid involves placing a tissue graft, thus effectively elongating the lower lid providing better coverage of the eye.