Arachnoiditis Ossificans

Kevin M. Rice, MD
9 hours ago
4 min read

An 74-year-old male with chronic low back pain and progressive lower extremity weakness. What is the diagnosis? • Xray of the Week

Figure 1. Non-contrast CT of the lumbar spine. What is the diagnosis?

Arachnoiditis Ossificans CT Scan Annotated

Figure 2. Non-contrast CT of the lumbar spine (axial, sagittal, and coronal reconstructions): Central pattern of arachnoiditis ossificans demonstrating dagger-like ossification within the central spinal canal at the L5 level (red arrows). The linear, hyperdense bony bar courses longitudinally along the central canal, causing focal narrowing and potential cauda equina compression. No peripheral or circumferential involvement is seen, consistent with the central ossification pattern.

Diagnosis

Arachnoiditis ossificans.

Discussion

Arachnoiditis ossificans is a rare end-stage manifestation of chronic adhesive arachnoiditis, characterized by ossification of the arachnoid membrane within the spinal canal.[1][2] It typically occurs in the thoracolumbar region and is associated with prior spinal surgery, trauma, infection, subarachnoid hemorrhage, or oil-based myelography.[3][4] Patients often present with progressive myelopathy or radiculopathy due to nerve root compression or tethering.[5] The condition represents metaplastic ossification of inflamed arachnoid tissue, leading to intrathecal calcified or ossified plaques that can encase the spinal cord or cauda equina.[6]

Imaging Findings

Computed tomography (CT) is the modality of choice for diagnosing arachnoiditis ossificans due to its superior depiction of ossified structures, with sensitivity for detecting intrathecal ossification approaching 100% in symptomatic cases.[2][7] On CT, hyperdense bone attenuation plaques are seen within the thecal sac, with patterns varying by type of ossification [2]:

Central pattern: Linear or dagger-like hyperdense ossification centered within the spinal canal, often appearing as a solitary midline bony bar or rod on axial views. (Figs. 1,2)
Nerve root encasing pattern: Tubular or circumferential hyperdense ossifications surrounding individual or clumped nerve roots (cauda equina), with roots appearing embedded or passing through the ossified tissue.
Weblike pattern: Interlacing, mesh-like hyperdense strands or trabeculae filling or crisscrossing the thecal sac, creating a reticular appearance between nerve roots.
Peripheral pattern: Rim-like or discontinuous hyperdense plaques along the inner walls of the thecal sac, often circumferential or partially encircling, potentially narrowing the canal without central involvement.

These patterns frequently coexist, leading to thecal sac deformity, nerve root clumping, and varying degrees of central canal stenosis. Multiplanar reconstructions (sagittal/coronal) help confirm the distribution and extent. Patterns of intrathecal ossification can guide prognosis and surgical planning.

Management and Prognosis

Management is primarily conservative, focusing on pain control and physical therapy, as surgical decompression carries high risks of reossification or worsening symptoms.[3][5] Laminectomy with duraplasty may be considered for severe cord compression, with variable success rates (30-50% improvement in symptoms).[4] Prognosis is guarded, with many patients experiencing progressive disability; early detection via imaging can facilitate timely intervention to prevent irreversible myelopathy.[6]

Key Learning Points

CT is essential for identifying hyperdense intrathecal ossifications in arachnoiditis ossificans, outperforming MRI for bony detail.
Look for circumferential or patterned ossified plaques encasing nerve roots on multiplanar reconstructions to differentiate from mimics like dural calcifications.
Radiology plays a critical role in classifying ossification patterns, which influence surgical feasibility and prognosis.
Prompt recognition on CT can guide multidisciplinary management and prevent complications like syringomyelia—always correlate with clinical history of prior spinal insult.

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References

Frizzell B, Kaplan P, Dussault R, Sevick R. Arachnoiditis ossificans: MR imaging features in five patients. AJR Am J Roentgenol. 2001;177(2):461-464. doi:10.2214/ajr.177.2.1770461
Thejeel B, Greditzer-Sobeck C, Ciacci J, Siddiqi I. Patterns of intrathecal ossification in arachnoiditis ossificans: a retrospective case series. AJNR Am J Neuroradiol. 2023;44(2):228-234. doi:10.3174/ajnr.A7764
Donalisio M, Cadosch D. Arachnoiditis ossificans. Skeletal Radiol. 2024;53(5):1019-1021. doi:10.1007/s00256-023-04465-7
Junewick JJ. Clinical image. Arachnoiditis ossificans in a pediatric patient. Pediatr Radiol. 2010;40(2):228. doi:10.1007/s00247-009-1350-2
Jaspan T, Preston BJ, Mulholland RC, Webb JK. The CT appearances of arachnoiditis ossificans. Spine (Phila Pa 1976). 1990;15(2):148-151. doi:10.1097/00007632-199002000-00022
Chan CC, Lau PY, Sun LK, Lo SS. Arachnoiditis ossificans. Hong Kong Med J. 2009;15(2):146-148. https://pubmed.ncbi.nlm.nih.gov/19342743/
Kumaran SP, Gupta K, Maddali A, Viswamitra S. Post traumatic arachnoiditis ossificans. J Emerg Trauma Shock. 2012;5(3):250-252. doi:10.4103/0974-2700.99701
Sefczek RJ, Deeb ZL. Case report: computed tomography findings in spinal arachnoiditis ossificans. J Comput Tomogr. 1983;7(3):315-318. doi:10.1016/0149-936x(83)90099-1

Kevin M. Rice, MD is the president of Global Radiology CME and is a radiologist with Cape Radiology Group. He has held several leadership positions including Board Member and Chief of Staff at Valley Presbyterian Hospital in Los Angeles, California. Dr. Rice has made several media appearances as part of his ongoing commitment to public education. Dr. Rice's passion for state-of-the-art radiology and teaching includes acting as a guest lecturer at UCLA. In 2015, Dr. Rice and Natalie Rice founded Global Radiology CME to provide innovative radiology education at exciting international destinations, with the world's foremost authorities in their field. In 2016, Dr. Rice was nominated and became a semifinalist for a "Minnie" Award for the Most Effective Radiology Educator. He was once again a semifinalist for a "Minnie" for 2021's Most Effective Radiology Educator by AuntMinnie.com. He has continued to teach by mentoring medical students interested in radiology. Everyone he has mentored has been accepted into top programs across the country, including Harvard, UC San Diego, Northwestern, Vanderbilt, and Thomas Jefferson.