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An uncommon cause of lumbar radiculopathy

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Q1: What are the features seen on the MRI scan (see p 182)?

  • The sagittal T2-weighted image shows low signal (suggestive of disc degeneration) at L4/5 and L5–S1 levels and a high intensity cystic lesion at L4/5 level with a low signal intensity rim due to the cyst wall.

  • The axial T2-weighted image shows that the cystic lesion arising from the right L4/5 facet joint and indenting the theca posterolaterally.

  • Facet joint hypertrophy/arthritis.

Q2: What is the likely diagnosis?

Facet joint cyst is the diagnosis. Facet joint cysts can be either synovial cysts or ganglia. Synovial cysts have a synovial lining and communicate with the joint whereas ganglia, in a similar site, lack a synovial lining but otherwise have similar components. Only histopathological examination can aid in differentiating between the two. Both types of the cysts arise in association with degenerative disease of spine and can cause similar symptomatology.

Q3: What is the usual line of management?

Surgical decompression and resection of the cyst has been the most effective and widely used treatment modality in symptomatic patients. Conservative management including bed rest, anti-inflammatory medications, immobilisation with a brace, and observation for spontaneous resolution can be used if symptoms are non-intrusive and acceptable to the patient. Facet joint aspiration and injection with steroids and long acting local anaesthetics may provide temporary or prolonged pain relief in patients unfit for surgery.

Discussion

Radiculopathy presenting as sciatica is a common clinical scenario. Facet joint cysts can present as radiculopathy mimicking the more common prolapsed intervertebral discs. As radiculopathy is very common in clinical practice, surgeons and physicians alike should be aware of this entity and its clinical presentation.1 The incidence of lumbar facet synovial cysts as a cause of back pain and sciatica based on MRI has been suggested to be 0.65%.2

Learning points

  • Facet joint cyst is a well recognised, yet uncommon condition presenting with low back pain and radiculopathy due to the presence of an extradural mass.

  • MRI is the technique of choice to detect and diagnose a facet joint cyst.

  • Computed tomographic arthrography by injection of air or iodinated non-ionic contrast medium may be used to confirm the diagnosis in doubtful cases and also to note whether the patient's presenting symptoms can be provoked.

  • Accurate detection and diagnosis of these cysts is important because the therapeutic decisions and surgical approaches are affected.

Facet joint cysts occur adjacent to a facet joint in the extradural space. These are most common between the fourth and fifth lumbar vertebrae (68%). Fifteen per cent occur at L5–S1, 12% at L3/4, and 5% at more proximal levels.2 The majority occur on the medial aspect of facet joints within the posterolateral aspect of the spinal canal at 2–5 o'clock (on the left) or 7–10 o'clock (on the right) positions.

The aetiology of facet joint cysts is poorly understood. The favoured theory is excess stress (motion or direct trauma) or osteoarthritic changes causing herniation of synovial tissue through a capsular defect. Other theories are (1) mucous degeneration of connective tissues, (2) proliferation of developmental synovial rests, and (3) metaplasia of pluripotential mesenchymal cells.1

These cysts present with chronic low back pain with 84% exhibiting radicular symptoms. Rarely they can be asymptomatic.2 Depending on size and location, neurogenic claudication may be the presenting symptom. Cauda equina syndrome can be caused by spontaneous or post-traumatic haemorrhage into the cyst.

With facet joint cysts, sciatica can be due to direct pressure on the nerve root or to irritation associated with inflammatory response around the cyst and often does not respond to conservative measures. Inflammatory response or ischaemia of the nerve root itself can be the causative mechanism. Spontaneous resolution also occurs and cyst rupture has been cited as an explanation for this.3 Lumbar facet synovial cysts are sometimes an incidental finding and careful consideration of clinical findings is essential when assessing the pain.

MRI provides an accurate method of diagnosis in many cases. Facet joint degeneration is an almost universal finding. Degenerative spondylolisthesis is seen in 42% to 65% of cases.2 The cyst size varies from 5–25 mm with varying degrees of nerve root and thecal compression. The wall of the cyst is best identified in T2-weighted images. Rim enhancement is a frequent finding after intravenous injection with gadolinium chelate, which is associated with histological evidence of subacute inflammatory change. Previously myelography, computed tomography, computed tomographic myelography, facet arthrography, and computed tomographic facet arthrography were used.

The differential diagnosis on MRI and computed tomograms includes sequestered disc prolapse, conjoint nerve root, an intraspinal cyst and a cystic neurofibroma.

Surgical decompression and resection of the cyst has been the most effective and widely used treatment modality in symptomatic patients with a success rate of 80%.4 Conservative management including bed rest, anti-inflammatory medications, and immobilisation with a brace can be used if symptoms are non-intrusive and acceptable to the patient. Symptomatic improvement in about 55% of patients has been reported.5 Facet joint aspiration and injection with steroids and long acting local anaesthetics may provide temporary or prolonged pain relief in patients unfit for surgery.2,6

Final diagnosis

Right sided lumbar facet joint cyst.

References

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