Case report:
A 42 year old man was referred to our institution by his general practitioner with 3 weeks of acute on chronic back pain. Concerningly, he had also developed new and worsening ataxia and experienced multiple falls over this time.
Neurological examination revealed normal upper and lower limb power with preserved reflexes, however there was reduced sensation throughout the mid to lower body below the level of T5-6.
An MRI of the spine performed externally had demonstrated an abnormality at T6 that was reported as a 'thoracic spinal lesion', prompting referral to the neurosurgical unit and a hospital admission.
A repeat MRI performed at our institution confirmed an abnormality at T6, where there was a focal deformity of the cord with prominence of the dorsal CSF space (Figure 1). A brief background discussion on the three major differentials is presented here.
1. Spinal Arachnoid Web:
Cause: The aetiology underlying arachnoid webs is not fully understood, with various theories suggesting a post-traumatic or post-infectious origin, though some authors speculate that they may form secondary to a collapsed or fenestrated arachnoid cyst4,5. For unclear reasons, the majority of webs are found on the dorsal aspect of the upper to mid thoracic cord1,3. Arachnoid webs are often associated with development of syringomyelia, and though the precise mechanism is not entirely understood, it is likely related to changes in intramedullary pulse pressure1,6.
Signs and Symptoms: Symptoms are variable and non-specific. Some patients are asymptomatic while others present with combination of back pain, radiculopathy, sensory and motor changes3,8. Occasionally, as in our case example, patients can present with gait instability3.
Management: Management depends on the patient’s presentation. Asymptomatic patients are often closely observed with regular imaging while definitive management involves surgical resection of the arachnoid tissue3.
2. Ventral Spinal Cord Herniation:
Cause: Herniation of the spinal cord most commonly occurs through a ventral dural defect with many cases thought to be either post-traumatic or iatrogenic in origin8. If no cause is identified, the condition is termed 'idiopathic spinal cord herniation'9. A smaller number of cases are associated with duplication of the dura, with the cord then herniating through a defect in the inner dural layer10. For reasons that are unclear, the idiopathic form of this condition has a clear female predilection9.
Signs and Symptoms: Though variable, the classic presentation is with progressive Brown-Sequard syndrome. Paraplegia, back pain and leg pain have also been described9,10.
Management: Surgical management involves reducing the herniated segment of the cord, followed by exposure and repair of the dural defect. This requires mobilisation of the cord by first dividing the dentate ligaments at the level of the herniation most commonly via a dorsal approach10. This is a more complex and involved procedure than the procedures required to treat the other two conditions, and, as such, correct pre-operative diagnosis is of great importance to aid in surgical planning.
3. Spinal Arachnoid Cyst:
Cause: The underlying aetiology is not fully understood however, like webs, spinal arachnoid cysts may be post infectious, post traumatic or congenital in nature11. Arachnoid cysts may be either intra-dural or extra-dural and the majority occur in the dorsal aspect of the thoracic spine10.
Signs and symptoms: Unlike ventral cord herniation, the majority of spinal arachnoid cysts present with back pain, with only a minority causing focal neurology12. Development of Brown-Sequard syndrome is very rare12. Average length of symptoms is reported to be longer with spinal arachnoid cysts than cord herniation12.
Management: As with arachnoid webs, asymptomatic lesions can be safely monitored while those associated with pain and/or neurological deficit are managed with surgical decompression.