Differential Diagnosis

The patient's presentation of back pain, sciatic pain, and peripheral neuropathy, along with the laboratory and imaging findings, suggests an autoimmune or inflammatory process. Here is a categorized differential diagnosis:

• Single Most Likely Diagnosis
  • Rheumatoid Arthritis (RA): The patient is ANA positive, has an elevated RH factor, and high levels of immunoglobulin A, which are indicative of an autoimmune process. The presence of peripheral neuropathy and the patient's symptoms are consistent with RA, especially given the absence of diabetes, which could otherwise explain the neuropathy.
• Other Likely Diagnoses
  • Lupus: Given the patient's positive ANA, lupus is a consideration, although the specific pattern of symptoms and other lab findings (like the high RH factor and specific immunoglobulin levels) might not fully align with systemic lupus erythematosus.
  • Sjögren's Syndrome: This autoimmune disorder can cause neuropathy and has a association with positive ANA and elevated immunoglobulins, but typically presents with dry eyes and mouth.
  • Mixed Connective Tissue Disease (MCTD): Overlapping features of different autoimmune diseases (like lupus, scleroderma, and RA) could be considered given the broad range of autoantibodies and symptoms.
• Do Not Miss Diagnoses
  • Multiple Myeloma: Although less likely, the presence of back pain, neuropathy, and elevated immunoglobulins (especially IgA) warrants consideration of multiple myeloma, a plasma cell disorder that can be devastating if not diagnosed early.
  • Vasculitis: Conditions like granulomatosis with polyangiitis or eosinophilic granulomatosis with polyangiitis could present with neuropathy and systemic symptoms, and are critical to diagnose due to their potential for severe organ damage.
• Rare Diagnoses
  • Paraneoplastic Syndromes: These are rare disorders triggered by an immune response to a cancer, which can cause a variety of neurological symptoms, including neuropathy.
  • Amyloidosis: A condition characterized by the deposition of amyloid proteins in various tissues, which can cause neuropathy among other symptoms, and is associated with plasma cell dyscrasias or chronic inflammatory diseases.

Each of these diagnoses requires careful consideration of the patient's clinical presentation, laboratory findings, and potentially additional diagnostic testing to confirm the diagnosis.