Saturday 18 February 2012

Thyroid Ophthalmopathy MRI

A 45 y o male with bilateral orbital proptosis clinically more on left side.  Found to be restless. No visual field defect.
MRI study shows:
Bilateral proptosis, marked on left side.
On axial sections diffuse enlargement of recti, confined to muscle belly. Tendinous portions spared.
Predominantly inferior rectus, medial rectus and superior rectus involved, more on left side compared to right.

Imaging wise diagnosis: Thyroid Ophthalmopathy.
Advised T3, T4 and TSH in further evaluation.

Syn : Thyroid associated Orbitopathy.
The commonest cause of proptosis in adults, most frequently associated with Grave’s disease  with female preponderance.
Clinical presentation includes predominantly proptosis and diplopia.
Patholophysiology is enlargement of the extraocular muscles and increase retro orbital fat. 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 with subsequent release of inflammatory mediators. The muscles are infiltrated with inflammatory cells and mucopolysccaride deposition.
MRI is investigation of choice due to its excellent soft tissue resolution.
Diffuse enlargement is usually confined to muscle belly. Along with diffuse enlargement signal abnormality may be obvious, T1 and T2 hyper intensity can attributed to fatty infiltration and inflammation. Tendinous portions of recti typically spared.
Bilateral (80%) and symmetric (70%) involvement is typical.
Severity of involvement of  extra-ocular muscles is as Inferior > medial > superior > lateral. (Mnemonic: I'M SLOW)

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