Computed tomography
Computed tomography myelogram that relies on the principle of obstruction to CSF flow, can be used for the diagnosis, but it can miss webs as these do not usually cause complete obstruction. In our center, this study is not used.
MRI- conventional sequences
MRI is the gold standard investigation method of the arachnoid web and these are the main features found in the 16 patients included in our retrospective descriptive study between 2013 and 2019 using conventional spinal MRI sequences (T1W, T2W, STIR, DRIVE ("driven equilibrium radiofrequency reset pulse") in a 1.5-T MRI scanner (Intera Achieva, Philips): Transversely oriented band extending from the posterior dura to the pial margin of the cord, which constitutes a direct sign, although almost never found owing to its relatively thin size.
- The “scalpel sign”: focal deformity or indentation at the dorsal surface of the thoracic spinal cord, similar in appearance to a surgical scalpel with its blade pointing posteriorly, which by some authors is considered pathognomonic of the arachnoid web. (Fig. 4, Fig. 5)
- Enlarged dorsal subarachnoid space with the spinal cord positioned abnormally ventrally within the thecal sac.
- Hypointense flow-related artifacts on the dorsal subarachnoid space
- Associated findings: increased T2WI signal above the level of the “scalpel sign” was present in half of the cases (Fig. 6), and only one of them presented syringomyelia. (Fig. 7)
MRI- special sequences, technical considerations
Some authors use special sequences to identify the webs, like MRI with constructive interference in steady-state (CISS) or cardiac-gated phase-contrast cine-mode MRI (CINE) in multiple axial planes.
In our case, we performed a 2D MR myelography in 14 cases, with "single-shot turbo spin-echo" and “Spectral Presaturation with Inversion Recovery” (SPIR), with a “SENSE-spine” coil (Philips) and cartesian acquisition. Images are acquired in the coronal oblique plane along the longitudinal axis of the spinal cord. Parameters for this technique are as follows: repetition time (msec)= 8000; echo time (msec) = 1000; section thickness (mm) = 40; matrix 512 x 142; and acquisition time 144 seconds.
Two-dimensional images showed a focal hypointense band at the same level as the scalpel sign, probably showing the location of the web. (Fig. 8)
Surgery and follow-up
Only four patients underwent surgery confirming the diagnosis.
After opening the dura mater in the midline, the surgeon found an arachnoid membrane, that besides blocking the CSF, also caused compression of the vasculature with a change of caliber at this level. (Fig. 9)
After they dissected the membrane and restored the CSF flow on the dorsal side of the spinal cord, the vessels regained their normal caliber. (Fig. 10)
All the patients showed clinical improvement and the postoperative MR image showed recovery of the normal morphology. (Fig. 11)