Spinal extradural arachnoid cyst causing paraparesis in a child

INTRODUCTION

A spinal extradural arachnoid cyst (SEAC) is an uncommon condition in which the arachnoid membrane protrudes through a defect in the dura and progressively enlarges to form a cyst in the epidural space, often resulting in compressive myelopathy.¹ The most common locations of these cysts are the middle and lower dorsal spine; however, they have also been reported in lumbar, lumbosacral, and cervical regions.² They are more commonly found in males, with a peak incidence in the second decade of life.^3–5

MRI is useful in making an initial diagnosis of this rare entity. SEAC is seen as an extradural lesion with signal intensity similar to that of cerebrospinal fluid (CSF) on all sequences. CT myelography helps in detecting the actual site of communication between the subarachnoid space and arachnoid cysts, as well as intercommunication between arachnoid cysts, and thus plays a pivotal role in pre-surgical planning for dural defect repair and cyst excision.⁶

We report a 12-year-old boy with progressive lower limb weakness and loss of sensation. MRI revealed SEAC causing cord compression. The dural defect was localized on CT myelography, and surgical repair was done along with excision of the cyst.

THE CASE

A 12-year-old boy presented with complaints of weakness in both lower limbs. The patient was in his usual state of health 4 months back when he complained of weakness in his lower limbs. This progressed and he eventually developed paraplegia with bowel and bladder incontinence over 3 months. The patient had trauma to his back when he fell off his cycle 4 months before the onset of his symptoms. There was no other significant medical or surgical history. The neurodevelopmental history was normal and the family history was non-contributory. On examination, bilateral lower limbs were spastic with grade II power in each limb. Deep tendon reflexes were exaggerated. There was a loss of sensation to pain, touch, temperature, and proprioception below the D9 level.

MRI of the spine revealed a large, loculated extradural cystic lesion along the dorsal aspect of the spinal canal, extending from the D3 to L4 vertebral body levels. The signal intensity of the lesion was similar to the CSF on all sequences (Fig. 1a-c). Extension of the cyst through bilateral neural foramina was seen at multiple levels (Fig. 2a). The spinal cord was displaced anteriorly with obliteration of the posterior subarachnoid space and cord compression. Imaging features were suggestive of a dorsal spinal extradural cystic lesion, and CT myelography was done to investigate the cause and nature of the cyst.

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Fig 1.

(a) Sagittal T1-weighted (T1W), (b) T2-weighted, and (c) post-contrast T1W showing a large non-enhancing cystic lesion (asterisk in a-c) of cerebrospinal fluid signal intensity extending from D3 to L4 level. Note the extradural fat septations (white arrow in a) dividing the cyst into multiple locules

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Fig 2.

MRI Ax FIESTA (a) showing extension of the spinal extradural arachnoid cyst through the neural foramen (black asterisk). Axial CT (b) shows the administration of contrast through a spinal needle (white arrow) placed in the centre of the thecal sac. Hyperdense contrast (black asterisk) is seen in the dependent location of the sac as the patient was placed in the right lateral decubitus position. CT myelogram (c) showing dural defect with leakage of contrast into the extradural space. Cord herniation is seen through the defect (black arrow). The same findings were appreciated on MRI (d), retrospectively. FIESTA fast imaging employing steady-state acquisition

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The patient was placed in the right lateral decubitus position, and under CT guidance and local anaesthesia using a 24-gauge spinal needle, a lumbar puncture was performed at the L5-S1 interspinous space, and active CSF flow was demonstrated (Fig. 2b). A mixture of 5 ml iohexol (240 mg/ml, GE Healthcare, Princeton, New Jersey) and 2 ml meglumine gadoterate (0.5 mmol/ml, Guerbet, Cedex, France) was injected slowly intrathecally and the needle was removed. The patient was repositioned and turned in lateral and prone positions to facilitate the passage of contrast through the spinal canal. Serial images were acquired in the supine and prone positions. Delayed axial and sagittal T1-weighted (T1W) MRIs were also taken. A lower-than-usual site was selected for a lumbar puncture to avoid entering the extradural cyst, which was extending up to the L4 level.

CT and MR myelograms revealed a defect in the right lateral aspect of the dura at the D8-D9 level, with leakage of contrast into the extradural space (Fig. 2c). Additionally, herniation of the cord through the defect was seen (Fig. 2d). Early CT myelogram saggital image (Fig. 3a) did not show contrast opacification of arachnoid cyst. Gradual progressive opacification of the loculated cyst was seen (Fig. 3a-d). One of the locules at the D7-8 level showed delayed opacification, as indicated by delayed T1-weighted MRIs (Fig. 3b). Communication between the cysts was well seen on multiplanar reformatted images.

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Fig 3.

CT myelogram (a) shows opacification of the extradural arachnoid cyst. One of the locules (white arrow) did not opacify completely on early images. Delayed MR myelogram (b) showing opacification of the whole of the arachnoid cyst, including the early non-enhancing locule (black asterisk). Follow-up MRI (c and d) after 12 months of operation shows a normal spinal cord with no residual or recurrent arachnoid cyst

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A straight midline incision was made from D6 to L2 and laminectomy was done. Multiple arachnoid cysts with multiple lamellar septations were found, which were thinning the cord and pushing it ventrally. The lamillations were removed and three dural defects were noted at the T8-T9-T10 level near the nerve roots. A total excision of the cyst was done. The cyst had a thin wall with clear fluid. Primary dural repair was done and fat and glue were placed. D8-11 laminoplasty was done and wound closure was done in layers. The perioperative period was uneventful, with no new neurological or CSF leaks in the postoperative period.

An early follow-up examination (at 2 weeks) in the outpatient clinic revealed improvement in power, with return of sensations in both lower limbs. The patient was able to walk on his own with minimal support. A follow-up MRI done 12 months after cyst excision (Fig. 3c and d) showed a normal appearance of the spinal cord, no CSF leak, and no recurrence of the cyst. The power improved with no residual deficit.

DISCUSSION

SEACs are an uncommon cause of myelopathy. They are commonly located on the dorsal aspect of the cord but can also be found on the ventral or lateral aspects.⁷ Causes may include trauma, surgery, arachnoiditis, and neural tube defects.^4,8,9 In many cases, the aetiology is not clear and most non-traumatic SEACs are thought to be congenital.⁴ Our patient had a history of trauma to the back, which appears to have resulted in a dural tear and formation of the cyst. Herniation of the spinal cord along with contrast through the defect in CT myelography, supports our propositions. Dural defects are often found around the nerve root sleeves. In our patient too, the defect was located along the right lateral aspect, around the exiting nerve root sleeve, presumably occurring due to tension across the relatively fixed nerve roots and the mobile dural sac. Several theories have been proposed to explain the formation and progressive expansion of SEAC: Oneway check valve-like phenomenon occurring at the dural defect causing intermittent pressure surges within the cyst leading to expansion and progressive cord compression; hyperosmolar cyst contents causing fluid absorption and cyst expansion; and the cyst wall secreting fluid thereby resulting in enlargement. Many case reports in the literature favour the one-way valve mechanism to explain the progressive expansion of SEACs.^3,9–11

CT myelography is the imaging modality of choice. It depicts the anatomy of the cysts and dural defects in more detail than MRI.^12,13 Delayed imaging in a prone position can show differential emptying of cysts and help in precisely locating the site of the defect, as in our patient. It helped in preoperative planning of ligation of the communication pathway. MRI is a non-invasive modality that is useful for initial investigation in diagnosing SEAC and assessing the extent of the lesion; however, it is less useful when it comes to delineating the site of the defect. MRI also helps in assessing the degree and extent of cord atrophy and myelomalacia.

In our patient, the cyst appeared loculated and extended over multiple segments of the spine. Progressive filling of the cysts with contrast was demonstrated. One of the locules showed delayed filling on the delayed MR myelogram. We recommend administration of a mixture of iodinated and gadolinium contrast into the thecal sac. Delayed serial imaging with MRI (MRI myelography) serves two purposes: one is to reduce the total radiation dose, and the second is to achieve better resolution of MRI images compared to CT. Intrathecal administration of gadolinium is safe and no major complications have been reported even after 12 months of follow-up in one study.¹⁴

Conservative treatment with observation is recommended for asymptomatic cysts.^9,15 In patients with symptomatic neurological deterioration from SEAC, complete cyst excision followed by obliteration of the communicating pedicle and repair of the dural defect to prevent one-way check valve CSF leak is the mainstay of treatment.^16,17 In our patient, total excision of the cyst was done with repair of the dural defect. Precise pre-surgical localization of the defect and anatomy of the cysts, as determined by MRI and myelography, aids in planning tailored laminotomy and helps avoid multi-level laminectomy, thereby reducing the risk of complications.