Spinal Arachnoid Cyst

Arachnoid cysts form when a pocket of the arachnoid membrane fills with cerebrospinal fluid, sometimes through a one-way valve mechanism that allows the cyst slowly to enlarge. They may be present from birth or develop later, sometimes after inflammation, injury or previous surgery. Whether a cyst causes trouble depends largely on its size and location: a small cyst that is not compressing anything can be left alone and watched, whereas a cyst that is pressing on the cord tends to cause symptoms that gradually progress over time.

Symptoms arise when the cyst compresses the spinal cord or nerve roots, and they depend on the level involved. They can include back pain, weakness or heaviness in the legs, altered sensation or numbness, problems with balance and walking, and in some cases bladder or bowel symptoms. Because a cyst often enlarges slowly, symptoms may creep up gradually. Assessment centres on a careful neurological examination and MRI, which shows the cyst, its position and the degree of compression; occasionally specialised imaging is used to understand how the cyst communicates with the surrounding fluid, which helps in planning treatment.

A small, symptomless cyst usually needs no treatment beyond monitoring. When a cyst is clearly causing progressive symptoms by compressing the cord or nerves, surgery is the effective treatment. The aim is to relieve the pressure — either by removing the cyst wall, or by opening it widely (fenestration) so that fluid no longer accumulates and the cord is decompressed. Where the cyst communicates with the fluid space through a small channel, that connection may be dealt with to reduce the chance of it re-forming. The evidence, from surgical series and reviews, shows that decompression relieves symptoms and halts progression in appropriately selected patients, and that outcomes are best when surgery is done before the compression has caused fixed damage.

The outlook depends on how much and how long the cord has been compressed. Where surgery is performed before permanent damage has set in, symptoms often improve and progression is halted, and most people recover well over the following months with rehabilitation. Where compression has been severe or prolonged, some deficits may persist. My priority is to identify the cyst that is genuinely responsible for progressive symptoms, to distinguish it from harmless incidental cysts, and to intervene at the right time.

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