What causes the Y sign on spinal MRI?
Let me know in the comments.
Click to reveal the answer
Answer:
The Y sign is a Y-shaped deformity of the thecal sac on axial MRI or CT, produced when excess extradural fat compresses the sac circumferentially, indenting it anteriorly from both sides and posteriorly in the midline. It is regarded as characteristic of spinal epidural lipomatosis and is a feature of advanced disease. Severity is commonly graded with the Borré system, based on the epidural fat-to-spinal-canal index; grade III, an index of 75% or more, is the stage at which the Y sign typically appears. The dominant cause is exogenous corticosteroid therapy, followed by obesity, endogenous hypercortisolism such as Cushing syndrome, previous spinal surgery, and idiopathic cases. The lumbar spine is affected most often. The differential for epidural material compressing the sac includes epidural abscess, haematoma, metastasis, lymphoma and angiolipoma, all of which are separated from lipomatosis by their signal characteristics: lipomatosis follows fat on every sequence and suppresses completely on fat-saturated images.
Why is it called so?
The compressed thecal sac takes the outline of the letter Y on axial images.
Pathophysiology
Epidural fat is a normal component of the spinal canal, most abundant dorsally in the lumbar region. Under sustained glucocorticoid excess or in obesity, adipocytes in this compartment hypertrophy and proliferate. Because the bony canal cannot expand, the accumulating fat encroaches on the only compressible structure available, the thecal sac. Fat builds up dorsally in the midline and in the two dorsolateral recesses, so the sac is indented from three directions at once; the resulting cross-section has a narrow posterior midline limb and two anterolateral limbs, which is the Y. Progressive compression accounts for the neurogenic claudication and radicular symptoms.
Alternative names: Y-shaped thecal sac sign
Other associated named signs: Polka dot sign of a vertebral haemangioma as an unrelated axial pattern, and the trefoil canal of degenerative stenosis
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
Improve Content And Help Your Colleagues!
Found an error in the post? Please let us know using our contact page and we will update it with due credits!
If you wish to contribute radiological images for this case (or any other article on the website), you can submit them here and you will be duly credited: Submit radiology case