Lumbar Arachnoid Cyst Presenting as Arachnoiditis on MRI: A Case Report and Diagnostic Approach.

Lumbar arachnoid cysts (ACs) and arachnoiditis may appear similar on magnetic resonance imaging (MRI) and may appear difficult to diagnose accurately. Considering that lumbar arachnoid cysts have a benign source, in contrast to arachnoiditis, surgical intervention poses a challenge. We present a case initially assumed to be arachnoiditis but later confirmed as an arachnoid cyst by MRI and computed tomography (CT) myelogram. The goal of this report was to provide a clearer understanding of the pathophysiological differences and key diagnostic features, including imaging techniques and clinical presentations that distinguish these two conditions. A 46-year-old Caucasian female presented to the emergency department with complaints of back pain and lower-extremity weakness with associated saddle paresthesia. Clinical examination revealed hyporeflexia with a diminished sensation in the left lower extremity. Accurate diagnosis using MRI and a comprehensive clinical approach is vital for early recognition and differentiation of cauda equina syndrome, spinal stenosis, arachnoiditis, spinal abscess, and epidural hematoma. In this case, the initial lumbar MRI demonstrated an empty thecal sac at L4-L5, which can be seen in cases of arachnoiditis and inflammatory arthropathy. With a negative inflammatory lumbar puncture and unresolved symptoms post methylprednisolone treatment, CT myelogram was ordered, which confirmed the presence of a lumbar arachnoid cyst at L4-L5. While arachnoiditis is primarily managed with symptomatic treatment, this patient had a spinal arachnoid cyst, which required surgical intervention to relieve compression and symptoms.