Thyroid-associated orbitopathy
Updates to Article Attributes
Thyroid associated orbitopathy (TAO) is the most common cause of proptosis in adults and is most frequently associated with Graves disease.
On imaging, it is characterised by extra-ocularenlargement of the extraocular muscles' bellies enlargement (frequently: inferior rectus > medial rectus > superior rectus) sparing their tendinous insertions, and is usually bilateral and symmetrical.
Epidemiology
The demographics of thyroid associated orbitopathy reflects that of patients with thyroid disease and is, therefore, more frequently seen in women. Although Graves disease is the most common cause, Hashimoto thyroiditis has also been implicated. It may precede, occur concurrently with, or after the onset of abnormal thyroid function, occur concurrently, or commence following its onset.
Clinical presentation
- lid retraction
- proptosis, with resultant chemosis and corneal dryness and ulceration
- optic nerve compression, potentially leading to blindness
- diplopia 1
Pathology
Thyroid associated orbitopathy is characterised by enlargement of the extra-ocular muscles (EOMs) as well as the increase in the orbital fat volume 1. While the exact mechanism is unknown, antibodies to thyroid stimulating hormone (TSH) appear to cross react with antigens in the orbit resulting in infiltration by activated T lymphocytes 3, with subsequent release of inflammatory mediators.
The muscles are infiltrated with inflammatory cells (lymphocytes, macrophages, plasma cells and eosinophils), and increased mucopolysccaride deposition. In long standing cases, increased collagen deposition leads to fibrosis 1.
The extra-ocularextraocular muscles are involved in a predictable fashion as remembered by the (I'M SLOW mnemonic). Involvement of the rectusextraocular muscles in decreasing order of frequency:
- inferior rectus
- medial rectus
- superior rectus
- lateral rectus
- obliques
Increase in orbital fat volume is a result of venous congestion from the compression of the superior ophthalmic vein by the enlarged muscles and/or intrinsic adipose inflammation.
Radiographic features
CT
CT is the most commonly used modality, due to its widespread availability and rapid image acquisition. Contrast, although ideal, is not necessary, as the natural contrast between orbital fat and muscle allows for adequate delineation of the orbital contents.
CT findings include:
- exophthalmos
- can be measured by drawing a line between the anterior tips of the zygomatic bones and measuring the distance between the line and the posterior part of the eyeball. The normal distance is more than 10±1.7mm 6 and a smaller distance indicates
exopthalmusexopthalmos
- can be measured by drawing a line between the anterior tips of the zygomatic bones and measuring the distance between the line and the posterior part of the eyeball. The normal distance is more than 10±1.7mm 6 and a smaller distance indicates
- extraocular muscle enlargement and fatty attenuation
- the order of muscle involvement can be remembered by the mnemonic I'M SLOW
- bilateral (76-90%) and symmetric (70%) involvement is typical
-
the anterior tendon is typically spared (although it can be involved in acute cases), with the swelling largely confined to the muscle belly
- this appearance if often referred to as 'coke bottle' in nature (coca-cola bottle sign), given its resemblance to the classic Coca-ColaTM bottle
- enlargement of the muscle belly is usually accompanied by reduced attenuation representing fatty infiltration 7
- the size of the muscles correlates with both the severity of disease and the risk of optic nerve compression 7
- increase in retro-ocular orbital fat
WhenThe greater the extraocular muscle involvement is pronouncedbulk (especially medial rectus bulk nearer to the apex) and the longer and narrower the bony orbit, the more crowded the orbital apex will become. Apical crowding could result in optic nerve dysfunction/optic neuropathy may be crowded at the orbital apex, leadingdysfunctioncompression 1. Variation in the shape of the orbit, especially the angle formed by the medial and lateral walls at the apex, changes the likelihood of optic nerve compression2,2.
Other rarer signs include 7:
- enlargement of the lacrimal glands (lymphocytic infiltration)
- chemosis
- anterior displacement of the orbital septum
MRI
MRI may also be used in evaluation due to its multiplanar and the inherent contrast capabilities. Use of MR prevents ionising radiation to orbits, which is associated with radiation-induced cataracts. The imaging findings are similar asto those described above for CT in terms ofregarding location and the ocular muscles involved.
-
T1:
iso-signalisointense to the other facial muscles, or fatty infiltration - T2: increased signal intensity may be seen due the inflammatory process
- T1 C+ (Gd): enhancement may be present
Treatment and prognosis
Although in many instances the disease is self-limiting, spontaneously improving within 2-5 years 3 often, discomfort, cosmetic issues, the risk of corneal ulceration and optic nerve compression requiresoften require treatment. Options include:
- medical: supportive, steroids
- radiotherapy 3
- surgical decompression
Differential diagnosis
General imaging differential considerations include:
-<p><strong>Thyroid associated orbitopathy (TAO)</strong> is the most common cause of <a href="/articles/proptosis-1">proptosis</a> in adults and is most frequently associated with <a href="/articles/graves-disease">Graves disease</a>.</p><p>On imaging, it is characterised by extra-ocular muscles bellies enlargement (frequently: inferior rectus > medial rectus > superior rectus) sparing their tendinous insertions, usually bilateral and symmetrical. </p><h4>Epidemiology</h4><p>The demographics of thyroid associated orbitopathy reflects that of patients with thyroid disease and is, therefore, more frequently seen in women. Although <a href="/articles/graves-disease">Graves disease</a> is the most common cause, <a href="/articles/hashimoto-thyroiditis">Hashimoto thyroiditis</a> has also been implicated. It may precede, occur concurrently with, or after the onset of abnormal thyroid function.</p><h4>Clinical presentation</h4><ul>- +<p><strong>Thyroid associated orbitopathy (TAO)</strong> is the most common cause of <a href="/articles/proptosis-1">proptosis</a> in adults and is most frequently associated with <a href="/articles/graves-disease">Graves disease</a>.</p><p>On imaging, it is characterised by enlargement of the extraocular muscles' bellies (frequently: inferior rectus > medial rectus > superior rectus) sparing their tendinous insertions, and is usually bilateral and symmetrical. </p><h4>Epidemiology</h4><p>The demographics of thyroid associated orbitopathy reflects that of patients with thyroid disease and is therefore more frequently seen in women. Although <a href="/articles/graves-disease">Graves disease</a> is the most common cause, <a href="/articles/hashimoto-thyroiditis">Hashimoto thyroiditis</a> has also been implicated. It may precede the onset of abnormal thyroid function, occur concurrently, or commence following its onset.</p><h4>Clinical presentation</h4><ul>
-<li>optic nerve compression potentially leading to blindness</li>- +<li>optic nerve compression, potentially leading to blindness</li>
-</ul><h4>Pathology</h4><p>Thyroid associated orbitopathy is characterised by enlargement of the <a href="/articles/extra-ocular-muscles">extra-ocular muscles</a> (EOMs) as well as the increase in the orbital fat volume <sup>1</sup>. While the exact mechanism is unknown, antibodies to <a href="/articles/thyroid-stimulating-hormone">thyroid stimulating hormone</a> (TSH) appear to cross react with antigens in the orbit resulting in infiltration by activated T lymphocytes <sup>3</sup>, with subsequent release of inflammatory mediators.</p><p>The muscles are infiltrated with inflammatory cells (lymphocytes, macrophages, plasma cells and eosinophils), and increased mucopolysccaride deposition. In long standing cases, increased collagen deposition leads to fibrosis <sup>1</sup>.</p><p>The <a href="/articles/extra-ocular-muscles">extra-ocular muscles</a> are involved in a predictable fashion as remembered by the <a href="/articles/extraocular-muscle-involvement-in-thyroid-associated-orbitopathy-mnemonic">I'M SLOW</a> mnemonic. Involvement of the rectus muscles in decreasing order of frequency:</p><ul>-<li>inferior</li>-<li>medial</li>-<li>superior</li>-<li>lateral</li>- +</ul><h4>Pathology</h4><p>Thyroid associated orbitopathy is characterised by enlargement of the <a href="/articles/extra-ocular-muscles">extra-ocular muscles</a> (EOMs) as well as the increase in the orbital fat volume <sup>1</sup>. While the exact mechanism is unknown, antibodies to <a href="/articles/thyroid-stimulating-hormone">thyroid stimulating hormone</a> (TSH) appear to cross react with antigens in the orbit resulting in infiltration by activated T lymphocytes <sup>3</sup>, with subsequent release of inflammatory mediators.</p><p>The muscles are infiltrated with inflammatory cells (lymphocytes, macrophages, plasma cells and eosinophils), and increased mucopolysccaride deposition. In long standing cases, increased collagen deposition leads to fibrosis <sup>1</sup>.</p><p>The <a href="/articles/extra-ocular-muscles">extraocular muscles</a> are involved in a predictable fashion (<a href="/articles/extraocular-muscle-involvement-in-thyroid-associated-orbitopathy-mnemonic">I'M SLOW</a> mnemonic). Involvement of the extraocular muscles in decreasing order of frequency:</p><ul>
- +<li>inferior rectus</li>
- +<li>medial rectus</li>
- +<li>superior rectus</li>
- +<li>lateral rectus</li>
-</ul><p>Increase in orbital fat volume is a result of venous congestion from the compression of the <a href="/articles/superior-ophthalmic-vein">superior ophthalmic vein</a> by enlarged muscles and/or intrinsic adipose inflammation.</p><h4>Radiographic features</h4><h5>CT</h5><p>CT is the most commonly used modality, due to its widespread availability and rapid image acquisition. Contrast, although ideal, is not necessary as the natural contrast between orbital fat and muscle allows for adequate delineation of the orbital contents.</p><p>CT findings include:</p><ul>-<li>exophthalmos<ul><li>can be measured by drawing a line between the anterior tips of the zygomatic bones and measuring the distance between the line and the posterior part of the eyeball. The normal distance is more than 10±1.7mm 6 and a smaller distance indicates exopthalmus</li></ul>- +</ul><p>Increase in orbital fat volume is a result of venous congestion from the compression of the <a href="/articles/superior-ophthalmic-vein">superior ophthalmic vein</a> by the enlarged muscles and/or intrinsic adipose inflammation.</p><h4>Radiographic features</h4><h5>CT</h5><p>CT is the most commonly used modality, due to its widespread availability and rapid image acquisition. Contrast, although ideal, is not necessary, as the natural contrast between orbital fat and muscle allows for adequate delineation of the orbital contents.</p><p>CT findings include:</p><ul>
- +<li>exophthalmos<ul><li>can be measured by drawing a line between the anterior tips of the zygomatic bones and measuring the distance between the line and the posterior part of the eyeball. The normal distance is more than 10±1.7mm <sup>6</sup> and a smaller distance indicates exopthalmos</li></ul>
-<li>muscle involvement can be remembered by the mnemonic <a href="/articles/extraocular-muscle-involvement-in-thyroid-associated-orbitopathy-mnemonic">I'M SLOW</a>- +<li>the order of muscle involvement can be remembered by the mnemonic <a href="/articles/extraocular-muscle-involvement-in-thyroid-associated-orbitopathy-mnemonic">I'M SLOW</a>
-<li>anterior tendon is typically spared (although it can be involved in acute cases) with the swelling largely confined to the muscle belly<ul><li>this appearance if often referred to as 'coke bottle' in nature (<a href="/articles/coca-cola-bottle-sign">coca-cola bottle sign</a>), given its resemblance to the classic Coca-Cola<sup>TM</sup> bottle</li></ul>- +<li>the anterior tendon is typically spared (although it can be involved in acute cases), with the swelling largely confined to the muscle belly<ul><li>this appearance if often referred to as 'coke bottle' in nature (<a href="/articles/coca-cola-bottle-sign">coca-cola bottle sign</a>), given its resemblance to the classic Coca-Cola<sup>TM</sup> bottle</li></ul>
-<li>size of the muscles correlates with both the severity of disease and the risk of optic nerve compression <sup>7</sup>- +<li>the size of the muscles correlates with both the severity of disease and the risk of optic nerve compression <sup>7</sup>
-</ul><p>When muscle involvement is pronounced the <a href="/articles/optic-nerve">optic nerve</a> may be crowded at the <a href="/articles/orbital-apex">orbital apex</a>, leading to optic nerve dysfunction <sup>1</sup>. Variation in the shape of the orbit, especially the angle formed by the medial and lateral walls at the apex, changes the likelihood of <a href="/articles/optic-nerve-compression">optic nerve compression</a> <sup>2</sup>.</p><p>Other rarer signs include <sup>7</sup>:</p><ul>- +</ul><p>The greater the extraocular muscle bulk (especially medial rectus bulk nearer to the apex) and the longer and narrower the bony orbit, the more crowded the orbital apex will become. Apical crowding could result in optic nerve dysfunction/<a title="Optic neuropathy" href="/articles/optic-neuropathy">optic neuropathy</a> due to optic nerve compression <sup>1,2</sup>.</p><p>Other rarer signs include <sup>7</sup>:</p><ul>
-</ul><h5>MRI</h5><p>MRI may also be used in evaluation due to its multiplanar and the inherent contrast capabilities. Use of MR prevents ionising radiation to orbits, which is associated with radiation-induced cataracts. The imaging findings are similar as described above for CT in terms of location and ocular muscles involved.</p><ul>- +</ul><h5>MRI</h5><p>MRI may also be used in evaluation due to its multiplanar and inherent contrast capabilities. Use of MR prevents ionising radiation to orbits, which is associated with radiation-induced cataracts. The imaging findings are similar to those described above for CT regarding location and the ocular muscles involved.</p><ul>
-<strong>T1:</strong> iso-signal to the other facial muscles, or fatty infiltration</li>- +<strong>T1:</strong> isointense to the other facial muscles, or fatty infiltration</li>
-</ul><h4>Treatment and prognosis</h4><p>Although in many instances the disease is self-limiting, spontaneously improving within 2-5 years <sup>3</sup> often discomfort, cosmetic issues, the risk of corneal ulceration and optic nerve compression requires treatment. Options include:</p><ul>- +</ul><h4>Treatment and prognosis</h4><p>Although in many instances the disease is self-limiting, spontaneously improving within 2-5 years <sup>3</sup>, discomfort, cosmetic issues, the risk of corneal ulceration and optic nerve compression often require treatment. Options include:</p><ul>
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