Thyroid associated orbitopathy

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 enlargement of the extraocular muscles' bellies (frequently: inferior rectus > medial rectus  > superior rectus) sparing their tendinous insertions, and is usually bilateral and symmetrical. 

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 the onset of abnormal thyroid function, occur concurrently, or commence following its onset.

  • lid retraction
  • proptosis, with resultant chemosis and corneal dryness and ulceration
  • optic nerve compression, potentially leading to blindness
  • diplopia 1

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 extraocular muscles are involved in a predictable fashion (I'M SLOW mnemonic). Involvement of the extraocular 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.

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 exopthalmos
  • 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

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/optic neuropathy due to optic nerve compression 1,2.

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 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.

  • T1: isointense 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

Although in many instances the disease is self-limiting, spontaneously improving within 2-5 years 3, discomfort, cosmetic issues, the risk of corneal ulceration and optic nerve compression often require treatment. Options include:

  • medical: supportive, steroids
  • radiotherapy 3
  • surgical decompression

General imaging differential considerations include:

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Article Information

rID: 2180
Section: Pathology
Synonyms or Alternate Spellings:
  • Thyroid associated orbitopathy (TAO)
  • Thyroid associated ophthalmopathy
  • Thyroid ophthalmopathy
  • Thyroid orbitopathy
  • Graves ophthalmopathy
  • Thyroid eye disease
  • Dysthyroid eye disease
  • Dysthyroid ophthalmopathy

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Cases and Figures

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    Thyroid associate...
    Case 1: CT sagittal
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    Thyroid associate...
    Case 1: CT axial
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    Case 2
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    Case 3
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    Case 4: with apex crowding
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    Fig. 1

Enlargeme...
    Case 5
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    Coronal.
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    Case 7
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    Case 8: on MRI
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    Case 9
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    Coronal
    Case 10
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