Teaching Case

A 85-year-old lady presented with six months history of numbness of toes and progressive difficulty in walking. Examination revealed spastic para-paresis with brisk reflexes and a sensory level just above naves. She had advanced osteoporosis and vertebral compression fractures. MRI spine was carried out, which showed a rare finding.

MRI Thoracic Spine:
There are numerous long-standing benign vertebral compression fractures. Degenerative changes are seen.

The spinal cord is markedly abnormal. There appears to be an arachnoid cyst compressing the cord at T7. The cord is tethered at this point and there is cord oedema distal to this, however, there appear to be soft tissue nodules associated with the cord more inferiorly at T10 and T11.

Post Gadolinium Contrast MRI Thoracic Spine:
There is evidence of meningeal nodular thickening with diffuse meningeal and nerve root enhancement. This is most likely to be due to meningeal and nerve root metastases.
The arachnoid cyst which is compressing the thoracic cord is presumably due to abnormal CSF flow consequent on the meningeal metastasis
Lumbosacral:
Spondylotic changes and osteoarthritis of the facet joints. At L3-4 and L4-5 mild central canal stenosis. At L3-4, L4-5 there is bilateral lateral recess stenosis and L5-S1 there is left lateral recess stenosis.

Introduction:
Spinal extradural arachnoid cysts are very uncommon, a condition where a cyst is formed by herniation of arachnoid matter through a dural defect [1]. It is a rare finding seen in about 1-3% of space-occupying lesions of all primary spinal lesions [2]. These cysts are divided into three categories or types.
Type I: cysts where no spinal nerve root fibres are seen,
Type 2: cysts accompanying spinal nerve root fibres and
Type 3: known as spinal intradural meningeal cyst [3].
These cysts occupy various locations in the spine; the majority (65%) seen in the middle to lower thoracic spine [4] (Fig 1 and 2). The exact reason for the formation of these cysts is not categorically known, however, it is assumed that there is a congenital origin or it can be acquired in adult patients through trauma, inflammation, surgery and metastatic malignancy, although it is a rare presentation [4]. Dural metastasis was considered to be the cause of arachnoid cyst formation in our case. The probable cause of cyst enlargement is thought to be due to the active secretion of fluid from the cyst wall.
Clinical presentation:
The presenting symptoms vary and depend on the location, size and severity of physical compression on the underlying nerve roots fibres [5]. If the cyst is located in the cervical area, spastic tetraparesis or Horner’s syndrome may result. Thoracic cysts present as spastic paraparesis, same as in our patient, while lumbar cysts can present as radiculopathy and bowel and bladder incontinence.
Symptoms may not be constant and they may fluctuate with remissions in between.
Diagnosis:
MRI is the preferred initial diagnostic modality to detect the lesions [6] and also shows the relation with the other anatomical structures. The communication between the cyst and the sub-arachnoid space is confirmed by CT-myelography. [3] Scalloping and osseous changes in the spine occur due to long-standing hydrostatic pressure by the cyst.
Treatment:
The management depends on the symptomology, asymptomatic patients are managed with conservative treatment and observation [7]. In symptomatic patients with pain or neurologically worsening symptoms, surgical excision of the cyst and repair of the dural defect has proven effective [8].
Summarizing our case, meningeal metastasis was considered to obstruct the flow and had led to formation of the cyst. A search for the primary was unsuccessful, though CT of the chest showed a few suspicious nodules (Fig. 4a, b) but nothing could be declared with confidence.