|ECR 2019 / C-1853|
|Advanced MRI techniques in arachnoid web diagnosis.|
Aims and objectives
Spinal arachnoid web (SAW) is a rare entity characterized by a pathological thickening of intradural arachnoid tissue. It is a relatively recent description and frequently underdiagnosed.
It have an unclear etiology but it´s thought to be caused in many cases by subarachnoid bleeding after spinal trauma or surgery. The evolution of intrathecal spinal hematoma to haemosiderin and its derivatives may be the cause of arachnoiditis and arachnoid web formation. Pathologically, these are one or multiple, intradural, extramedullary bands that extend from the posterior pial surface of the spinal cord to the dura mater.
The blockage of normal laminar cerebrospinal fluid (CSF) dynamics around the spinal cord can cause an abnormal distortion of the spinal cord such as a characteristic deformity of the spinal cord known as the “scalpel sign” (Fig.1). This sign is a useful and simple fact, that guides the suspicion of the presence of dural membranes. The variable positions and shape of the web within the subarachnoid space determine the morphology and velocity of turbulent flow.
Depending on the location of the SAW, symptoms might include sensitivity and motor disorders, numbness, dorsalgia and tingling in the arms or legs, or difficulty walking. These unspecified neurological symptoms may vary in intensity and usually evolve slowly over time causing progressively neurological dysfunction. After a MRI exam there are often previous diagnoses of transdural hernia or arachnoid cyst, and even studies interpreted as normal. On the other hand, Syringomyelia or myelopathy are late associated radiological findings.
Even though they are already known techniques, advanced MRI techniques applied to the spinal cord still have difficulties to be applied in daily practice. The combination of advanced functional techniques such as Phase-contrast MR imaging (PC-MRI) or cine TRUE FISP HR sensitive to CSF motion and anatomical sequences like T2-weighted high-resolution isotropic SPACE or 3D CISS ( constructive interference in steady state ) sequences have had a great impact on the diagnosis of cystic spinal cord lesions. These sequences can now depict cystic lesions, such as arachnoid or leptomeningeal cysts and the related abnormal CSF flow within the subarachnoid space.
Differential diagnosis of Arachnoid Web:
The differential diagnosis of the posterior deformities of the spinal cord without solid mass includes transdural ventral herniation and dorsal spinal arachnoid cyst.
1. Transdural hernia
They are extremely rare. It´s thought to be a dural defect allowing the subarachnoid space to communicate with the extradural space. This dural defect can be congenital or acquired and are classically located between T2 and T8. (Fig. 2)
Transdural hernia is characterized by:
- Focal distortion with cord pulled rather than pushed forward.
- Convex posterior medullar surface in sagittal plane with complete focal obliteration of the anterior band of CSF.
- In some cases detection of the anterior dural defect with protrusion of the medullary tissue into the extradural space.
- CSF flow present in the intradural space posterior to the marrow. The spinal cord does not separate from the dura during the cardiac cycle (systole)
2. The arachnoid cyst:
Spinal arachnoid cysts are meningeal cyst relatively uncommon usually of traumatic acquired origin. They are CSF-filled sacs contained by the arachnoid mater and may be classified as intradural (type III) or extradural (type IA) according their location. Most primary intradural spinal arachnoid cysts are dorsal to the cord and are located in thoracic segment. (Fig.3).
Arachnoid cyst is characterized by:
- Occasional direct visualization of the walls of the cyst.
- Presence or absence of CSF in front of the marrow, but without an image of anterior transdural herniation.
- Rectification and / or flattening of the posterior edge of the spinal cord. It is a smooth, symmetrical posterior impression and usually more extensive than that produced by the arachnoid wed. No scalpel sign.
- Blockage of the flow of dorsal CSF.
3. Arachnoid web
Dorsal thoracic arachnoid web represent to a thickened band of arachnoid over the dorsal aspect of the cord that usually causes a focal thoracic cord distortion.
The characteristics of the arachnoid web are:
- Anterior displacement of the thoracic cord with focal dorsal impression or indentation, abrupt, without showing mass. (The so call “sign of the scalpel”).
- Direct visualization of the membranes in 3D T2WI is only rarely achieved in specific cases.
- Normal, or more intense, active flow of CSF behind the cord. Detection of points of greater turbulence and acceleration of the flow in the CINE Pc studies. Sometimes sagittal flow sequences show decreased CSF flow posteriorly to the cord due to the arachnoid web.
This paper is a retrospective review of 18 patients demonstrating arachnoid web in the upper thoracic spinal cord.
The objective of this poster is, through the experience in our center, to make known this infrequent entity, little described in medical literature, and explain how you can reach a right diagnosis through a specific MRI spinal protocol.