arachnoiditi

Arachnoiditi is a rare radiologic phenomenon in which a portion of the arachnoid membrane lies between the liver and the right hemidiaphragm. It is typically an incidental finding on chest or abdominal imaging and is usually not symptomatic.

Most cases are asymptomatic and discovered incidentally. When symptoms occur, they are nonspecific and may include abdominal discomfort, chest pain, dyspnea, or back pain. Symptoms can mimic thoracic or abdominal conditions, and patients may present with concern for pneumoperitoneum or subphrenic pathology on initial imaging.

Imaging features depend on the modality. On chest radiographs, subdiaphragmatic visualization of the arachnoid membrane can appear as a linear or band-like structure over the liver, sometimes with CSF-like density variations. Computed tomography or magnetic resonance imaging confirms the interposed arachnoid membranes and helps distinguish arachnoiditi from other causes of subdiaphragmatic signs, such as free air, subphrenic fluid, or organomegaly. CT and MRI often show signal characteristics consistent with CSF for the interposed tissue.

Differential diagnoses include subphrenic free air, subphrenic abscess, hiatal hernia, or other subdiaphragmatic processes. Management is typically conservative, focusing on reassurance and avoidance of unnecessary interventions. Surgical treatment is rare and reserved for persistent, clearly attributable symptoms after thorough evaluation.

Prognosis is generally favorable, with no known long-term consequences attributable to the finding itself.