Cast Nephropathy and Back/Flank Pain
Cast nephropathy itself does not directly cause back or flank pain—the pain in multiple myeloma patients comes from the underlying bone disease (lytic lesions), not from the renal tubular obstruction caused by light chain casts. 1
Understanding the Clinical Presentation
The key distinction here is recognizing that multiple myeloma presents with the "CRAB" criteria (hyperCalcemia, Renal impairment, Anemia, and Bone lytic lesions), and back pain is a manifestation of the "B" component—bone disease—not the "R" component (renal impairment from cast nephropathy). 1
What Cast Nephropathy Actually Causes
Cast nephropathy is a tubular obstructive process where monoclonal free light chains interact with Tamm-Horsfall protein in the loop of Henle, forming casts that obstruct tubules and cause acute kidney injury. 1, 2 The pathophysiology involves:
- Tubular obstruction leading to rupture and immune-mediated tubular injury 1, 2
- Direct proximal tubular cell injury through hydrogen peroxide production and activation of inflammatory pathways (NF-κB, MAPK) 1, 2
- Progressive interstitial inflammation and fibrosis 1, 2
The clinical manifestations of cast nephropathy are biochemical and systemic—elevated creatinine, decreased eGFR, uremia symptoms, and potential dialysis dependence—not localized pain. 1
Why Multiple Myeloma Patients Have Back Pain
Back or flank pain in multiple myeloma patients is caused by osteolytic bone lesions, pathologic fractures, or vertebral compression fractures—not by the renal pathology. 1 The bone disease in myeloma is a separate myeloma-defining event that frequently coexists with cast nephropathy but has a distinct pathophysiology involving plasma cell infiltration and osteoclast activation.
Clinical Pitfall to Avoid
Do not attribute back pain to cast nephropathy when evaluating a multiple myeloma patient. 1 If a patient presents with both renal failure and back pain:
- The renal failure may be from cast nephropathy (requires serum free light chains >80-200 mg/dL, abnormal κ:λ ratio, and ideally biopsy confirmation) 1, 3, 4
- The back pain requires skeletal imaging (plain radiography, whole-body MRI, or PET/CT) to identify lytic lesions or fractures 3
- These are concurrent but independent complications requiring separate therapeutic considerations 1
When to Suspect Cast Nephropathy
Cast nephropathy should be suspected when a multiple myeloma patient presents with:
- Acute kidney injury (serum creatinine >2 mg/dL or eGFR <40 mL/min/1.73 m²) 1, 2
- Elevated serum free light chains (particularly >150 mg/dL with urine M-spike >200 mg/day) 3, 4
- Abnormal κ:λ ratio (normal 0.26-1.65; in CKD stage 5, up to 0.34-3.10) 3
- Absence of other causes of AKI (dehydration, hypercalcemia, nephrotoxic drugs) 1, 4
This is a medical emergency requiring immediate bortezomib-based chemotherapy to reduce free light chain production, with a goal of >50-60% reduction by day 12 of treatment. 1, 4, 2