A 38-year-old man comes to the office due to a 2-year history of gradually worsening low back pain, bilateral buttock pain, and stiffness. Symptoms are worse in the morning and relieved with stretching and hot showers. The patient also has fatigue but no fever, chills, night sweats, or weight loss. Vital signs are normal. The patient appears healthy but has a stooped walking posture. Deep palpation over the lumbar spine at the midline and both sacroiliac joints elicits tenderness. Erythrocyte sedimentation rate is 75 mm/hr. Which of the following pathologic findings is most likely responsible for this patient's symptoms?
Ankylosing spondylitis | |
Inflammatory |
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Examination |
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Pathophysiology |
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Laboratory |
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CRP = C-reactive protein; ESR = erythrocyte sedimentation rate; HLA-B27 = human leukocyte antigen B27. | |
This patient with chronic back and buttock pain associated with stiffness has ankylosing spondylitis (AS). The symptoms are characteristically worse with rest (eg, in the morning, overnight) and improve with stretching and warm showers. AS is most common in men age <40, and inflammatory markers (eg, erythrocyte sedimentation rate [ESR]) are usually elevated.
AS is characterized by simultaneous erosion of bone and new bone formation, unlike rheumatoid arthritis in which only erosions occur. The initial pathogenesis is driven in part by inflammatory cytokines (eg, tumor necrosis factor, IL-17), which cause activation of osteoclast precursor cells and bony erosions. This occurs primarily in the vertebral bodies and results in destruction of the trabecular microarchitecture, increasing the risk for secondary osteoporosis and compression fractures.
Once inflammation subsides, the reparative process leads to excessive new bone formation, especially in areas where fat metaplasia fills previously eroded sites; in contrast to erosion, bone formation occurs primarily at the periosteum-cartilage junction and manifests as bridging syndesmophytes in the vertebral column. This leads to spinal rigidity, postural alterations, and increased risk of fracture.
(Choice B) Paget disease of bone is characterized by excessive and disordered bone formation, with focal thickening of trabecular and cortical bone. Features include bone pain, fractures, and arthritis of adjacent joints. However, it is uncommon at age <50, and this patient's inflammatory back pain is more consistent with AS.
(Choice C) Fibrous dysplasia (FD) is a benign tumor characterized by focal bone dysplasia (ie, failure of normal differentiation) and shows irregular trabeculae of immature bone in a fibrous stroma with spindle cells. It is typically asymptomatic and detected incidentally on imaging (eg, ground glass lesion), commonly in the femur and tibia. FD is usually seen in children and is not an inflammatory condition (ie, normal ESR).
(Choice D) Intervertebral disc protrusion and thickening of the ligamentum flava can lead to spinal stenosis. The pain in this disorder typically radiates to the posterior thighs and legs and is relieved by sitting and flexion of the spine, not activity. Spinal stenosis is usually seen at age >60, and ESR is normal.
(Choice E) Multiple myeloma is characterized by plasmacytic infiltrates in the bone marrow. Patients often have lytic bone lesions throughout the axial skeleton and back pain at night and during rest, but stiffness is less prominent. Multiple myeloma is uncommon before age 50.
Educational objective:
Ankylosing spondylitis is an inflammatory spondyloarthropathy characterized by simultaneous erosion of bone and new bone formation. Bone erosions occur primarily in vertebral bodies; however, new bone formation typically occurs at the junction of the periosteal margin and adjacent cartilage, leading to bridging syndesmophytes and ankylosis. This causes spinal rigidity, postural alterations, and increased risk of fracture.