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Arachnoid cyst


The arachnoid is one of the membranes that surrounds and protects the central nervous system, i.e. the structure of the brain and spinal cord. The membranes are three in number: the dura mater outside, the arachnoid in the middle, and the pia mater inside. Between the arachnoid and the pia mater, is a space called the subarachnoid space which contains a fluid called cerebrospinal fluid (CSF). An arachnoid cyst is a formation developed when the arachnoid fills with cerebrospinal fluid. These cysts may appear as a result of trauma or even spontaneously. They can cause a phenomenon of compression of nerve roots or spinal cord injury, at the head of various neurological symptoms. In most cases, these cysts are formed in the lower part of the spine, often in the lumbar vertebrae.


An arachnoid cyst may cause neither compression nor any symptom, in which case it is called an asymptomatic cyst. It is frequently discovered during spine imaging performed for another reason. In the case of symptomatic cysts, clinical signs depend on the location of the cyst. Symptoms found for example in the case of lumbar arachnoid cysts are:
- lower back pain;
- pain in a portion of the lower limbs, one or both legs according to the compression zone;
- disorders of the sensitivity of the same areas, like paresthesia, tingling sensations, numbness, or sometimes burns and electric shock;
- movement disorders with more or less sever paralysis if the bone is too compressed.
For other locations rarer symptoms may exist.


The diagnosis is made using medical imaging: depending on the symptoms, the height where the problem lies is targeted, and a CT scan or MRI will often highlight the cyst and the phenomenon of compression. If necessary, a sample (biopsy) will determine if the cyst is benign or cancerous.


Excision or surgical removal of a symptomatic arachnoid cyst can be difficult given its location. Cysts that do not give rise to any symptoms are usually left in place and monitored.