An unusual cause of low back pain in an adolescent

CASE

A 14-year-old male presented to our institution with back pain that had persisted for the past month. He denied trauma, recent strenuous activity, night pain, lower extremity dysesthesias, or bowel/bladder dysfunction. The physical examination revealed an apparent spinal asymmetry, a limitation in forward flexion of about 50%, tenderness over the thoracolumbar region, and intact motor and sensory function in the bilateral lower extremities. Plain films showed a low magnitude left-sided thoracic scoliosis but no evidence of vertebral wedging or osseous abnormalities.

DISCUSSION

Pediatric back pain The initial diagnosis was low back pain (LBP) and scoliosis. Back pain in children is considered unusual, but the prevalence is higher (20.5%) than has been previously assumed.¹ Some studies have suggested that the condition may be caused by using a backpack, sedentary time spent watching television, and the increase in pediatric obesity (39% of overweight children complain of LBP).^1,2 Other causes of the condition in children include spondylolysis, spondylolisthesis, Scheuermann's kyphosis, infection, neoplasms, and lumbar disk herniations. Back pain and scoliosis are not commonly associated, and their appearance together in our patient raised our suspicions and led us to investigate further. We were also concerned about the left-sided curve, which is atypical in scoliosis.

Plain films Typically, anteroposterior and lateral plain radiographs are obtained during a first visit for pediatric back pain. If spondylolysis or dysplastic spondylolisthesis is suspected, oblique views can help in making the diagnosis. Plain films of the spine have been reported to definitively determine the diagnosis 13.7% of the time, but most pathologies that are positively identified only resulted in a diagnosis of spondylolysis.³

MRI Magnetic resonance imaging and bone scans are usually considered for the next level of evaluation in back pain. Bone scans have been found to have low sensitivity for detecting pathology in LBP. However, this modality is sensitive for spondylolysis, osteoid osteomas, and osteoblastomas.⁴ We chose MRI for our patient because this test can evaluate the spinal cord and soft tissue structures for anomalies as well. The patient's initial MRI findings were normal. A course of physical therapy was completed without any relief of his symptoms.

The patient failed to follow up for close to a year, but eventually he returned with increasing pain and significant weight loss (70 lb). Repeat plain films showed bony changes from the prior films (see Figure 1). A repeat MRI was subsequently obtained and showed a 3.823.422.9-cm bone lesion in the posterior elements of L1 with spinal cord displacement (see Figure 2). Fluid-fluid levels in the mass most likely suggest an aneurysmal bone cyst, although other tumors commonly associated with the posterior spine include osteoid osteoma, osteoblastoma, or osteochondromas.⁵

CT Once the pathology was identified in the bone, CT was ordered to further assess the lesion and plan for surgical intervention. The CT scan confirmed the presence of an expansile mass in the posterior arch of L1 (see Figure 3). The adjacent three-dimensional reconstructions give a good conceptualization of the anatomic deformation present, which helps with further operative planning and perspective.

Biopsy An incisional biopsy of L1 was performed to definitively identify the type of mass prior to surgical excision. A fine needle aspiration, which analyzes fluid and cells obtained, was not chosen because of its low diagnostic yield, although the morbidity associated with this test is significantly lower than with other forms of biopsy. A true cut biopsy (or punch biopsy) provides higher rates of true positive results, but the risk of injury is significant around the spinal cord without direct visualization. Histologic examination of the biopsy specimen in our patient revealed a layer of normal osteoblasts lining simple, woven bony trabeculae contained in a loose fibrovascular matrix. This is characteristic of an osteoblastoma.

DIAGNOSIS AND TREATMENT

Osteoblastomas are rare, benign primary bone neoplasms. They are similar to osteoid osteomas and often thought of as large (greater than 1.5 cm) osteoid osteomas. They represent only about 1% of all bone lesions and affect the spine 40% of the time.^6,7 Unlike other bony lesions, such as osteoid osteomas, osteoblastomas produce a low-grade pain that does not intensify at night. Adolescents and young adults account for most cases of osteoblastoma, with a 2:1 male predominance. Although osteoid osteomas are responsive to NSAIDs, osteoblastomas are not and often require curettage with bone grafting or resection.^6,7 MRI is the best first choice for extended evaluation of bony tumors, but osteoid osteomas are reported as a normal finding on these radiographic examinations 10% of the time.^7,8 The reason may be that a large viewing window is obtained, which minimizes the characteristic bone marrow edema necessary to secure the diagnosis.⁸

Our patient underwent resection of the osteoblastoma after embolization of the lesion to decrease bleeding. Under direct visualization, the mass also involved both T12 and L2, requiring lamina excision of both adjacent levels. Resection extended into the L1 bilateral pedicles, transverse processes, and part of the posterior vertebral body. An en bloc (single piece) excision is preferable to prevent recurrence but could not be performed because of the proximity of the spinal cord. After complete resection with clean margins was attained, a posterior spinal fusion from T9 to L3 was performed to stabilize the spinal segment. In the immediate postoperative period, our patient reported complete relief of his back pain with the exception of some incisional pain. Osteoblastoma recurrence rates in the spine have been reported at about 11%.^6,7 JAAPA

Julie Vajnar, PA-C, RT, department editor

REFERENCES