Tophaceous gout is a common metabolic disease. The incidence of gout is estimated to be 0.2–0.4% worldwide, but in some developed countries, such as the US, its frequency is much higher, with almost 4% of the population affected6–7. Gout mostly affects joints, such as the metatarsal-phalangeal joint, elbow and knee, and the main symptoms are localized swelling and pain resulting from stimulation and compression of the tophus8. Risk factors contributing to the development of gout include hyperuricemia, local hypothermia, local low pH and the presence of a nucleating agent within synovial fluid9–11.
However, intraspinal gout is uncommon, despite the high incidence rate of gout. The etiology of axial skeleton-affected gout is not yet clear. Obesity, sedentary habits and degenerative disc disease may initiate and promote the formation of spinal tophus. Volkov A. and his colleagues suggested that intraspinal gout is associated with spinal degenerative disease because they found that damaged microcirculation leads to a lower-pH condition locally, which promotes monosodium urate crystallization and deposition12. The imaging findings of our eight patients confirmed this hypothesis, showing degeneration in the spine during the operation.
According to a review, after the first spinal gout case was described in 1950, only 133 relevant cases of spinal gout were reported by 201513,14. Two asymptomatic cases were confirmed in autopsy, which means that the number of spinal gout patients may be underrated because of nonsymptoms and signs in the early or quiescent stage of spinal gout15,16.
Nonspecific pain and neurological deficits in the lower extremities can be the only indicators of intraspinal gout, and it is difficult to differentiate gout from other diseases by routine imaging tests and laboratory tests. Adding to its rarity, spinal gout is difficult to diagnose precisely in clinical practice.
Laboratory examinations, such as C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), and blood uric acid (BUA), are nonspecific. These indicators are often in the normal range when gout is at the resting stage. Furthermore, BUA levels even remain normal during the progression of gout in some patients.
Routine radiological examinations, including MRI, CT, and DR, can hardly distinguish the tophi from other lesions, and spinal gout is easily confused with other diseases in imaging, such as disc herniation, primary or metastatic epidural tumors, and epidural infection. Both issues lead to a false diagnosis and treatment.
The clinical manifestations of spinal gout show regional-specific diversity, as all segments of the spine could be affected. The majority of tophi are located in the lumbar spine (44–54%), followed by cervical and thoracic vertebrae13,17. In a review by Toprover et al. of 131 patients with spinal gout, the most common complaint was back pain, followed by loss of sensation, motor weakness, bowel/bladder dysfunction and quadriparesis13.
Spinal gout is usually diagnosed during surgery due to inconsistencies among laboratory, radiological and clinical findings. We believe that the incidence of spinal gout is greatly underestimated, with a number of asymptomatic or misdiagnosed patients.
In fact, the first case of intraspinal gout in our group was misdiagnosed, and the patient was primarily diagnosed with epidural abscess. Intraspinal gout was not taken into consideration until an amorphous white material was observed during surgery. Subsequently, we searched the relevant literature and began to pay attention to this ignored disease.
In this article, all patients suffered from varying degrees of back pain, lower limb pain and numbness combined with intermittent claudication, which mimics the symptoms of lumbar spinal stenosis, epidural abscess and ligament ossification. By performing percutaneous transforaminal endoscopy, all eight patients were diagnosed with lumbar intraspinal gout and eventually received the proper treatment.
We reviewed previous reports on intraspinal gout. Although there are no guidelines for the treatment of intraspinal gout to date, open laminectomy and decompression is regarded as the first choice, which can avoid further neurological impairment and relieve pain and numbness rapidly18. Symptoms of nerve compression and stimulation were mostly relieved after surgery, and with the use of allopurinol, the follow-up showed no sign of recurrence. However, this kind of open surgery is often accompanied by a large incision, spinal instability, intractable back pain and other shortcomings.
Compared to other techniques, the technique of percutaneous transforaminal endoscopy has many advantages, such as less blood loss, less soft tissue disruption and better spinal stability, as well as a smaller incision, faster recovery and lower cost. After studying and discussing the first case of intraspinal gout, our team tried to introduce percutaneous transforaminal endoscopy into the diagnosis and treatment of intraspinal gout. In practice, we found that the tophus in the spine could be observed more clearly during the operation with magnified vision, and since the tophus was only slightly attached to the tissues, it could be completely removed with mild damage (Fig. 2).
Compared with the routine surgical option of open laminectomy and decompression, percutaneous transforaminal endoscopic decompression and resection has the distinct advantage of a smaller incision, less soft tissue disruption, and better preservation of spinal stability. The technique is effective in relieving neurological compressive and irritative symptoms, as well as being well tolerated by older patients due to its minimal blood loss, low postoperative pain and few complications. Moreover, all eight patients who underwent this minimally invasive surgery experienced no complications or recurrence in our follow-up. All patients’ mental and physical conditions were significantly improved after surgery, and they were all perfectly satisfied with the treatment (Table 4).
Due to the low incidence and diagnosis rate of spinal gout, we think that a large randomized study is difficult to perform. We advocate that a minimally invasive technique, percutaneous transforaminal endoscopy, could be used in the diagnosis and treatment of symptomatic spinal gout based on the satisfactory outcomes of the eight patients described in this article.