Journal of Medical Cases, ISSN 1923-4155 print, 1923-4163 online, Open Access
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Case Report

Volume 000, Number 000, August 2026, pages 000-000

Hyperviscosity Syndrome as a Rare Presentation of Immunoglobulin G Kappa Smoldering Multiple Myeloma Successfully Treated With Daratumumab

Figures

↓  Figure 1. Bone marrow biopsy (with Wright-Giemsa staining) at initial diagnosis of high-risk MGUS demonstrating approximately 6% plasma cells (black arrow), consistent with high-risk MGUS. MGUS: monoclonal gammopathy of undetermined significance.
Figure 1.
↓  Figure 2. Peripheral blood smear (with Wright-Giemsa staining) demonstrating well-formed rouleaux (Black arrows).
Figure 2.
↓  Figure 3. Bone marrow biopsy (with Wright-Giemsa staining) showing approximately 10% plasma cells (black arrows), meeting diagnostic criteria, and demonstrating progression to smoldering multiple myeloma.
Figure 3.
↓  Figure 4. Peripheral blood smear (with Wright-Giemsa staining) demonstrating rouleaux formation (black arrows) and red blood cell agglutination (red arrow) associated with elevated monoclonal immunoglobulin levels and hyperviscosity syndrome.
Figure 4.
↓  Figure 5. Trends in serum viscosity and IgG levels following initiation of daratumumab therapy. Trends consistent with a significant hematologic response and resolution of hyperviscosity-related symptoms. Ig:
Figure 5.
↓  Figure 6. Learning points.
Figure 6.

Tables

↓  Table 1. Chronologic Disease Progression
 
DateFindings
IgG: immunoglobulin G; FLC: free light chain.
June 2018IgG kappa paraprotein 1.18 g/dL; FLC ratio 3.39
August 2020Bone marrow biopsy: 6% plasma cells
2020–2025Progressive increase in M-protein and FLC ratio
June 2025Bone marrow biopsy: 10% plasma cells; rouleaux formation
January 2026Gross hematuria requiring bladder irrigation
February 2026Serum viscosity 5.7 centipoise
March 2026Retinal hemorrhage and epistaxis
April 2026Daratumumab initiated
June 2026Serum viscosity improved to 2.2

 

↓  Table 2. Selected Hematologic and Myeloma Laboratory Data
 
DateWBC (× 103/µL)Hemoglobin (g/dL)Platelets (× 103/µL)IgG (mg/dL)M-protein (g/dL)
WBC: white blood cell; IgG: immunoglobulin G.
August 20203.213.8121—2.21
August 20233.312.6148—2.51
April 20243.513.3120—2.68
June 2025———5,4633.01
January 20262.512.2105——
April 20263.012.7114——
May 20263.113.7118—2.26
June 220265.415.21412,6531.35

 

↓  Table 3. Serum Viscosity Trend Following Daratumumab Therapy
 
DateSerum viscosity (cP)
cP: centipoise.
February 18, 20265.7
Treatment initiated on April 8, 2026
April 20, 20264.5
April 27, 20263.7
May 4, 20263.4
May 18, 20262.5
June 1, 20262.2

 

↓  Table 4. Common Causes of Hyperviscosity Syndrome
 
Underlying disorderPredominant paraprotein/causeFrequency of HVSTypical serum viscosity thresholdComments
HVS: hyperviscosity syndrome; IgG: immunoglobulin G; IgA: immunoglobulin A; IgM: immunoglobulin M; PCL: plasma cell leukemia; MGUS: monoclonal gammopathy of undetermined significance; AML: acute myeloid leukemia; CML: chronic myeloid leukemia; WBC: white blood cell; cP: centipoise; SLiM-CRAB: ≥ 60% clonal bone marrow plasma cells, light chain ratio ≥ 100, magnetic resonance imaging focal lesions, hypercalcemia, renal insufficiency, anemia, and bone lesions.
Waldenstrom macroglobulinemiaIgM monoclonal proteinMost common causeOften > 4 cP (may occur at lower levels)IgM is pentameric and remains largely intravascular, making it highly viscosity-inducing. Approximately 10–30% of patients develop symptomatic HVS during the disease course.
Multiple myeloma (IgA)IgA monoclonal proteinCommon among myeloma-associated HVSUsually > 4–5 cPIgA tends to form dimers and polymers, increasing serum viscosity despite lower concentrations than IgG.
Multiple myeloma (IgG)IgG monoclonal proteinUncommonUsually > 5–6 cPMost cases occur with very high IgG concentrations. IgG3 has the greatest tendency toward self-aggregation and hyperviscosity.
PCLHigh circulating plasma cell burden with IgG or IgA paraproteinemiaUncommon but well recognizedVariableHyperviscosity results from both markedly elevated monoclonal protein levels and circulating plasma cells. Patients often present with aggressive disease and extensive extramedullary involvement.
Smoldering multiple myelomaUsually IgG or IgAExtremely rareVariableSymptomatic HVS without SLiM-CRAB criteria is exceptional. The present case represents this rare presentation.
MGUSLow-level monoclonal proteinVery rareRarely clinically significantUsually insufficient paraprotein burden to produce symptomatic HVS.
CryoglobulinemiaCryoglobulinsUncommonNot serum viscosity dependentIncreased viscosity occurs primarily at low temperatures due to protein precipitation rather than absolute paraprotein concentration.
Polycythemia veraIncreased erythrocyte massUncommonHematocrit > 60–65%Hyperviscosity results from increased cellular mass rather than paraproteins.
Leukostasis (AML/CML)Extreme leukocytosisRareWBC typically > 100–300 × 109/LSymptoms result from impaired microvascular flow caused by circulating leukemic blasts rather than serum protein abnormalities.

 

↓  Table 5. Myeloma-Defining CRAB Criteria
 
CRAB featureDefinition (IMWG criteria)Clinical significance
CRAB: hypercalcemia, renal insufficiency, anemia, and bone lesions; IMWG: International Myeloma Working Group; CT: computed tomography; PET/CT: positron emission tomography/computed tomography.
C: hypercalcemiaSerum calcium > 11 mg/dL or > 1 mg/dL above the upper limit of normalIndicates increased osteoclastic bone resorption due to active myeloma
R: renal insufficiencySerum creatinine > 2 mg/dL or estimated creatinine clearance < 40 mL/minReflects myeloma-related kidney injury, often from light chain nephropathy
A: anemiaHemoglobin < 10 g/dL or > 2 g/dL below the lower limit of normalResults from bone marrow infiltration and impaired erythropoiesis
B: bone lesionsOne or more osteolytic lesions identified on skeletal survey, CT, or PET/CTRepresents myeloma-related bone destruction and skeletal involvement