Occult Andersson Lesions in Patients with Ankylosing Spondylitis

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Occult Andersson Lesions in Patients with Ankylosing Spondylitis: Undetectable Destructive Lesions on Plain Radiographs

Introduction
Ankylosing spondylitis (AS), a chronic inflammatory disease primarily affecting the axial skeleton, is frequently complicated by Andersson lesions (ALs). First described in 1937, ALs are destructive lesions involving the discovertebral junction or vertebral bodies. These lesions manifest as osteolytic destruction with reactive sclerosis, often leading to localized pain, spinal instability, or neurological deficits. Despite their clinical significance, ALs remain challenging to diagnose due to heterogeneous terminology, overlapping features with other spinal pathologies, and limitations in imaging modalities. Plain radiography (PR), the most accessible imaging tool, often fails to detect ALs, necessitating advanced techniques like computed tomography (CT) and magnetic resonance imaging (MRI). This study introduces the concept of occult ALs—lesions undetectable on PR but identifiable via CT/MRI—and investigates their prevalence, radiographic characteristics, and clinical implications.

Methods
A retrospective analysis of 496 consecutive AS patients admitted to a single institution between 2003 and 2019 was conducted. Patients with tumors, tuberculosis, or other destructive spinal pathologies were excluded. ALs were diagnosed using predefined criteria:

  1. PR: Osteolytic destruction of vertebral endplates/vertebral bodies with sclerosis.
  2. CT: Irregular osteolysis with reactive sclerosis.
  3. MRI: Abnormal signal intensity in intervertebral discs (e.g., hypointense T1-weighted signals, mixed T2-weighted signals).

Inclusion criteria for occult ALs analysis required preoperative whole-spine PR and CT/MRI. Occult ALs were defined as lesions visible on CT/MRI but not PR. ALs were classified into five types:

  • Type 1–3: Localized lesions (central, anterior, or posterior third of discovertebral junction).
  • Type 4–5: Extensive lesions (whole discovertebral junction or entire vertebral body).

Statistical comparisons used independent t-tests for age and chi-square/Fisher exact tests for categorical variables (e.g., lesion type, distribution, imaging features).

Results

Prevalence and Demographics
Among 496 AS patients, 107 (22%) had ALs. Of these, 92 met inclusion criteria (mean age: 44.4 ± 10.1 years). Occult ALs were identified in 23 patients (25% incidence). Patients with occult ALs were younger than those with detectable ALs (40.3 ± 12.6 vs. 45.9 ± 9.1 years, P = 0.032). Sex distribution did not differ significantly.

Lesion Characteristics
A total of 111 ALs were identified: 77 detectable on PR and 34 occult.

  • Types: 21 localized (Type 1: 16; Type 2: 3; Type 3: 2) and 90 extensive (Type 4: 88; Type 5: 2).
  • Distribution: Most ALs occurred at the thoracolumbar junction (T10–L2, 62%). Occult ALs were more common in the thoracic spine (29%) compared to detectable ALs (10%, P = 0.034).

Imaging Features

  • Plain Radiography:

    • Detectable ALs exhibited osteolytic destruction with sclerosis (100%), angular kyphosis (22%), adjacent osseous bridges (86%), and abnormal intervertebral space height (84%).
    • Occult ALs showed none of these features except osseous bridges (38%) and abnormal space height (9%).

  • CT:

    • All ALs (107 lesions) showed osteolysis with sclerosis. Posterior column involvement was higher in detectable ALs (75% vs. 35%, P < 0.001). Vacuum phenomena (6 lesions) and ossification of ligaments (10 lesions) were rare.

  • MRI:

    • Discs in ALs showed hypointense T1 signals (100%) and mixed T2 signals (58% hyperintense/isointense/hypointense). Subchondral bone marrow edema (32 lesions) and fatty degeneration (21 lesions) were observed.

Extensive vs. Localized ALs
Extensive ALs (97% detectable) were more symptomatic, with 14.5% causing neurological deficits. Occult extensive ALs (15 lesions) lacked typical PR features, emphasizing the role of CT/MRI.

Discussion

Challenges in Diagnosing Occult ALs
PR’s limited sensitivity stems from:

  1. Subtle Changes: Occult ALs often lack reactive sclerosis, angular kyphosis, or significant disc space alterations.
  2. Anatomic Interference: Thoracic ALs are obscured by lung/rib projections (Figure 4).
  3. Posterior Column Predominance: Occult ALs frequently involve posterior elements (e.g., lamina, facets), poorly visualized on PR (Figure 3).

Clinical Implications

  • Missed Diagnoses: Reliance on PR risks overlooking occult ALs, particularly extensive lesions that threaten spinal stability.
  • Neurological Risks: 14.5% of extensive ALs caused stenosis or myelopathy, underscoring the need for timely CT/MRI in symptomatic patients.
  • Therapeutic Considerations: Detectable ALs often require long-segment fixation due to instability, while occult lesions may be managed conservatively if asymptomatic.

Limitations

  1. Retrospective design and exclusion of 15 patients without CT/MRI.
  2. Bone scans were not evaluated, potentially underdetecting inflammatory activity.

Conclusion
Occult ALs, present in 25% of AS patients with ALs, represent a diagnostic challenge due to their subtle radiographic features. CT and MRI are indispensable for identifying these lesions, especially in younger patients or those with thoracic involvement. Extensive occult ALs, though rare, demand vigilance due to their potential for neurological compromise. Future studies should explore advanced imaging protocols and long-term outcomes to refine management strategies.

doi.org/10.1097/CM9.0000000000001557

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