Anatomical and MR correlative study of the proximal sciatic nerve vasculature

Abstract

Objective: The aim of our study is to demonstrate that increased T₂ signal on MRI could be due to intraneural vessels in asymptomatic individuals, and may therefore be a normal finding.

Methods: An initial anatomic cadaveric study was undertaken to gain a better understanding of the vascular supply of the proximal sciatic nerve. Secondly, a retrospective study of MR imaging of patients without sciatic symptoms was performed to assess the prevalence of intraneural vessels, defined as hyperintensity on at least three consecutive slices on both T₂ and gadolinium enhanced T₁ weighted imaging, visible on routine MSK pelvic imaging.

Results: The anatomical study demonstrated a relatively abundant blood supply in the peri-ischiatic region. In the MR study, 20/76 (26%) patients showed visible intraneural vessels. More than one intraneural vessel was depicted in two of the sciatic nerves. Direct branching between the extrinsic and intrinsic systems was seen in only five cases.

Conclusion: Normal intraneural vessels can frequently be seen within the sciatic nerve on routine musculoskeletal pelvic imaging. Advances in knowledge: T₂ hyperintensity in the proximal sciatic nerve can be due to intraneural vessels and should not necessarily be reported as abnormal.

Figure 1.

(a) Posterior view of the sciatic nerve. Black asterisks demonstrate the sciatic nerve. Black circles from superior to inferior show the most superior intraneural vessel which arose from the inferior gluteal artery, the middle intraneural vessel which arose from a branch from the inferior gluteal artery and the most inferior of the three a branch of the the medial circumflex.(b) A diagram from TeachMeAnatomy (http://teachmeanatomy.info) which highlights the local relations of vessels and piriformis to the sciatic nerve.

Figure 2.

Magnified view of superior intraneural artery post microdissection. (a) The full black arrows highlight the edge of the nerve. The dotted black arrows shows the vessel.(b) Magnified view of two middle intraneural vessels post microdissection. (c) Magnified view of distal intraneural artery post microdissection.

Figure 3.

(a) Axial section with the left sciatic nerve highlighted by the dotted black oval. (b) Magnified axial section: white arrows show larger intraneural arterioles within the epineurium and black arrows demonstrate smaller arterioles within the perineurium.

Figure 4.

(a) Axial section through the proximal nerve. The two white dashed line ovals highlight intraneural vessels deep to the epineurium. The black dashed line highlights nerve fascicles.(b) Magnified microscopic view of the intraneural anatomy of the proximal nerve. Grey arrow demonstrates an artery, the black arrow demonstrates a thrombus within a vein and the blue arrow shows an adjacent nerve fascicle. (c) Coronal section of the proximal specimen with dotted black arrows highlighting the vasculature running longitudinal with adjacent nerve fascicles (black solid arrows).

Figure 5.

Three axial images taken from the same level: superior image T₂ Dixon water phase, middle image T₂ Dixon fat phase, bottom image T₁fat suppressed gadolinium enhanced image.The pink rings outline the sciatic nerve and the blue arrows the adjacent extrinsic vessels.

Figure 6.

T₁ Fat suppressed, gadolinium enhanced axial images of a left sciatic nerve.The left image is cranial and the right most image caudal.Red dashed line outlines the sciatic nerve and blue arrows show the extrinsic vessels passing posterior to the nerve. The black arrow shows an arising intraneural vessel.

Figure 7.

Axial T₂FS images of a left sciatic nerve (left superior, right inferior). The red dashed line outlines the sciatic nerve and the blue arrow demonstrates an intraneural vessel. The yellow arrow demonstrates a further intraneural vessel.

Figure 8.

Coronal T₁ MPR demonstrating an example of a left-sided sciatic variant. The red arrows show the branching of the sciatic nerve with the superior branch passing through the piriformis muscle and the inferior branch passing below. This is in keeping with the most common variant Type 2. The white arrows demonstrate the left piriformis muscle.

Figure 9.

Beaton and Anson’s schematic of the variant proximal sciatic anatomy. (a) Normal passage of the whole sciatic nerve below and behind the piriformis, (b) most common variant a bifurcation of the sciatic branch passes between the pirformis, (c) variant in which nerve bifurcates and passes above and below piriformis, (d) the whole nerve passes throught the piriformis, (e) and (f) further hypothetical variant routes.