Definition and location

The arachnoid web is an intradural extramedullary transverse band of arachnoid tissue that extends to the dorsal surface of the spinal cord within the subarachnoid space, causing mass effect and dorsal indentation. This membrane-like structure shows free moving to and fro in synchrony with the cardiac cycle in intraoperative ultrasound. Pathological examination shows fibroconnective tissue, cuboidal, simple epithelium with meningothelial appearance and nonspecific chronic inflammatory infiltrate.

In almost all cases the arachnoid web affects the upper level of the dorsal spine.

Epidemiology

In the retrospective review of the 16 patients in our hospital, the average age at the diagnosis was 59 (the age range was 42-78), with a slight female predominance 56.3 %, similar to other studies.

Clinical presentation

The clinical presentation is variable, including the signs and symptoms of myelopathy, radiculopathy or neuropathic pain, including episodic lower extremity weakness, sensory symptoms, bowel and bladder incontinence, hyperreflexia, spastic paraparesis, clonus, gait instability.

Etiology

Its etiology is still uncertain and is unknown whether they are congenital o acquired, although there are some theories:

• The arachnoid web may be an incomplete or disrupted formation of an arachnoid cyst, which are thought to be originated from diverticulations of the septum posticum (a membrane that divides longitudinally the posterior subarachnoid space).
• Or it may represent a collapsed arachnoid cyst.
• The presence of chronic inflammatory infiltrate suggests the possibility of an inflammatory process.

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Associated findings and its pathophysiology

The arachnoid web blocks the cerebrospinal fluid (CSF) movement in the longitudinal direction, generating upper and lower morphologic changes, like myelopathy and syringomyelia. 

Chang et al. propose the membrane functioning as a 1-way valve. So, first, the CSF passes through a small opening in one end of an oblique septum, and then, in the reverse flow, the opening will be obstructed leading to increased pressure. ( Fig. 1 , Fig. 2 )

There are two theories to explain the pathophysiology of syringomyelia associated with CSF flow obstruction:

• The first, proposed by Heiss et al., also known as the “suction effect” suggests that the enhanced pulse pressure proximal to the syrinx drives CSF into the spinal cord through the perivascular space.
• The second, proposed by Chang et al. and introduced by Greitz called the “Venturi effect” suggests that the blockage of the fast components of CSF flow causes a distal pressure drop, generating a pressure gradient between the inside (probably the central canal) of the spinal cord and the subarachnoid space, causing spinal cord edema and finally leading to syrinx formation. ( Fig. 3 )

So according to Chang et al. theories, if the caudo-rostral flow is blocked, the syrinx is formed rostral to the web, and vice-versa, with the possibility of finding it on both sides if both flows are blocked.

Differential diagnosis

• Dorsal arachnoid cyst.
• Ventral spinal cord herniation: acquired (occult or repetitive trauma or iatrogenic) or idiopathic.