Leucostasis and hyperviscosity

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Leukostasis and paraprotein hyperviscosity share microvascular consequences but require different removal techniques. Leukostasis is caused by circulating cells and is most likely with monocytic or myelomonocytic AML, other AML, ALL and blast-phase CML. The count alone is insufficient: AML blasts can cause symptoms below 100 × 10⁹/L, while CLL may exceed that level without leukostasis. Paraprotein hyperviscosity is caused by plasma protein, most often IgM, and responds to plasma exchange rather than leukapheresis.

Leukostasis is a clinical diagnosis of compatible acute organ dysfunction in a high-risk cellular context. CT may show bilateral infiltrates or cerebral haemorrhage but can be initially normal. Blood gases drawn through high-cell-count samples may show artefactual pseudohypoxaemia because leukocytes consume oxygen in the syringe; the patient's pulse oximetry and physiology remain essential. Similarly, pseudohyperkalaemia from fragile cells must be reconciled with ECG and promptly handled plasma, while true tumour lysis remains common.

Immediate management minimises viscosity and lysis. Avoid dehydration but give fluid with repeated respiratory and renal assessment. Start allopurinol or rasburicase according to tumour-lysis risk before cytoreduction. Hydroxycarbamide can lower myeloid counts while definitive induction is organised. Leukapheresis produces a rapid temporary fall but has line, anticoagulation, platelet and delay hazards and no role as routine prophylaxis. APL is a special case: ATRA and aggressive fibrinogen and platelet support begin on suspicion, with leukapheresis generally avoided.

In paraprotein hyperviscosity, fundal signs and neurological symptoms outweigh a single viscosity number because individual thresholds differ and sample processing can delay results. Exchange of about one to one-and-a-half plasma volumes rapidly removes intravascular immunoglobulin, but rebound occurs unless lymphoma or plasma-cell treatment follows. Rituximab may transiently raise IgM, so exchange and treatment sequence are planned. Red cells are given only when immediately necessary and preferably after viscosity reduction.

Key points

Hyperleukocytosis is a numerical finding, commonly leukocytes above 100 × 10⁹/L; leukostasis is clinical organ dysfunction and can occur at lower AML counts.
Key leukostasis organs are lung and brain: dyspnoea, hypoxia, confusion, headache, focal deficit, seizure and retinal change with blast-rich disease require emergency cytoreduction.
First-line tests are urgent FBC and film, coagulation and fibrinogen, renal, calcium, phosphate, potassium, urate, LDH, cultures, group sample and leukaemia flow and molecular samples.
Start tumour-lysis prevention and definitive leukaemia treatment immediately; hydroxycarbamide is a rapid oral bridge when full induction cannot begin at once.
Leukapheresis is not routine for an asymptomatic high count and does not replace systemic treatment; consider it only in selected severe symptomatic cases through the apheresis and leukaemia teams.
In suspected APL, start ATRA immediately and correct coagulopathy; avoid leukapheresis because line placement and extracorporeal circulation can worsen catastrophic bleeding.
Paraprotein hyperviscosity classically causes visual change, mucosal bleeding and neurological symptoms; urgent therapeutic plasma exchange is first-line and must not wait for a universal viscosity threshold.
Defer non-essential red-cell transfusion until the cellular or protein burden falls because increasing haematocrit can worsen microcirculatory flow.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Blast-rich myeloid leukaemia

AML and blast-phase CML most often cause leukostasis because large poorly deformable myeloid cells interact with activated endothelium.

Lymphoid hyperleukocytosis

ALL can cause leukostasis at high counts, whereas mature CLL lymphocytes remain deformable and symptoms are exceptionally rare despite striking numbers.

Paraprotein excess

Pentameric IgM, polymeric IgA or very high IgG increases plasma viscosity, usually from Waldenström macroglobulinaemia or plasma-cell disease.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Cellular sludging and adhesion
High blast burden, reduced deformability and endothelial adhesion obstruct small vessels and cause local hypoxia, inflammation, haemorrhage and tissue oedema.
2
Protein-dependent rheology
Large or polymerising immunoglobulins increase plasma resistance to flow, especially in retinal, cerebral and mucosal microcirculation.
3
Transfusion viscosity effect
Red-cell transfusion increases haematocrit and whole-blood viscosity, potentially worsening cellular or paraprotein sludging before the primary burden is reduced.
4
Cytoreduction lysis risk
Rapid treatment releases potassium, phosphate and nucleic acid, so kidney injury and fatal tumour lysis can accompany otherwise effective emergency cytoreduction.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Pulmonary leukostasisRed flag

Tachypnoea, hypoxia, diffuse infiltrates and respiratory failure with blast-rich hyperleukocytosis can mimic infection or oedema and demand parallel treatment.

Neurological leukostasisRed flag

Headache, confusion, ataxia, focal deficit, seizure or reduced consciousness may reflect microvascular ischaemia or haemorrhage.

Retinal and priapic features

Retinal engorgement or haemorrhage, visual loss and priapism are uncommon but highly concerning microcirculatory manifestations.

Paraprotein triad

Visual disturbance, mucosal bleeding and neurological change with IgM or another large paraprotein is symptomatic hyperviscosity until treated.

Concurrent lysis or DIC

Hyperkalaemia, hyperphosphataemia, hypocalcaemia, high urate, renal injury, low fibrinogen or bleeding alters the first-hour support.

Red flags requiring action

Hypoxia, tachypnoea or bilateral pulmonary infiltrates with marked myeloid leukocytosis may be leukostasis and can worsen during early cell lysis.
Confusion, focal neurology, seizure, visual change, retinal haemorrhage or papilloedema indicates threatened cerebral or retinal microcirculation and needs immediate specialist action.
Mucosal bleeding or DIC with suspected acute promyelocytic leukaemia requires immediate ATRA and blood-product support; leukapheresis is generally avoided.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
First-line FBC and expert blood filmFirst stepFirst line
Why
Identify cell burden, lineage clues, anaemia, thrombocytopenia and an APL-like morphology.
Interpretation and limitations
Urgent flow and molecular samples classify disease, but treatment of organ dysfunction or suspected APL must not wait for the final integrated report.
02
Immediate lysis and coagulation panel
Why
Detect spontaneous lysis, DIC and treatment constraints before cytoreduction.
Interpretation and limitations
Measure potassium, phosphate, calcium, urate, LDH, creatinine, PT, APTT, fibrinogen and D-dimer repeatedly; verify suspected artefactual potassium on a rapidly processed sample.
03
Organ-directed imaging and gases
Why
Identify haemorrhage, infarction, oedema, infection and respiratory failure requiring concurrent treatment.
Interpretation and limitations
Use urgent CT brain for neurological features and chest imaging for hypoxia; recognise leukocyte oxygen consumption can make delayed laboratory PaO2 falsely low.
04
Viscosity, immunofixation and fundal examination
Why
Support paraprotein hyperviscosity and establish a baseline for exchange.
Interpretation and limitations
Send viscosity and quantify and type immunoglobulin, but do not wait for the result when retinal, bleeding or neurological findings are convincing.
05
Definitive clonal classificationDefinitive
Why
Select systemic treatment that prevents rebound after temporary cytoreduction or exchange.
Interpretation and limitations
Use flow, marrow, cytogenetics and molecular tests for leukaemia, or immunofixation, free light chains, marrow and tissue for lymphoplasmacytic or plasma-cell disease.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Sepsis and pulmonary oedema

Infection, leukemic infiltration, haemorrhage, fluid overload and pulmonary embolism can mimic hypoxic leukostasis and frequently coexist.

Stroke and intracranial haemorrhage

Conventional vascular events, thrombocytopenia and DIC require immediate neuroimaging; leukostasis remains a clinical diagnosis supported by the haematological context.

Posterior reversible encephalopathy

Hypertension, calcineurin inhibitors, renal failure and systemic therapy can cause seizures and visual symptoms with a different imaging and treatment pattern.

Other retinal or bleeding disease

Diabetes, hypertension, thrombocytopenia, coagulopathy and ocular disease can cause retinal haemorrhage or mucosal bleeding independently of viscosity.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01Suspected leukostasisCytoreduce while protecting organsFirst stepAcute pulmonary, neurological, retinal or priapic dysfunction occurs with blast-rich hyperleukocytosis.

1Call acute leukaemia, critical care and apheresis teams and obtain clonal, lysis, coagulation, culture and group samples.
2DefinitiveStart monitored hydration and tumour-lysis prophylaxis and use immediate definitive therapy or hydroxycarbamide bridging.
3Consider leukapheresis only for selected severe symptomatic disease without allowing it to delay effective systemic therapy.

02Possible APLATRA and haemostatic support firstPromyelocyte morphology and DIC suggest acute promyelocytic leukaemia.

1Give ATRA immediately on suspicion and send urgent PML-RARA confirmation.
2Maintain fibrinogen and platelets to the APL protocol with frequent coagulation monitoring and avoid invasive procedures.
3Control leukocytes with the APL risk-adapted systemic protocol; generally avoid leukapheresis because of bleeding risk.

03Paraprotein hyperviscosityExchange plasma then suppress the cloneVisual, neurological or mucosal bleeding manifestations occur with a monoclonal immunoglobulin.

1Assess fundi and neurology, send viscosity and paraprotein tests and call therapeutic apheresis immediately.
2Exchange approximately 1 to 1.5 plasma volumes and repeat according to symptoms and rebound; avoid routine pre-exchange red-cell transfusion.
3Start lymphoma or plasma-cell treatment promptly with an IgM flare plan where rituximab is used.

04High count without symptomsPrevent lysis without procedural overtreatmentMarked leukocytosis is present but no compatible organ dysfunction is found.

1Assess repeatedly for subtle lung, brain, retinal and priapic symptoms and distinguish mature from blast-rich disease.
2Begin lysis prophylaxis and lineage-appropriate systemic cytoreduction according to disease tempo.
3Do not perform routine leukapheresis solely to normalise the laboratory count.

Key medicines and prescribing safety4 treatments · regimens, roles and cautions

Rapid temporary cytoreduction for hyperleukocytic myeloid disease when definitive systemic treatment cannot start immediately.

Hydroxycarbamide for emergency cytoreduction

Give 50 to 100 mg/kg/day orally in 2 or 3 divided doses under the acute-leukaemia protocol, with a maximum and dose adjustment defined by renal function and response, until definitive therapy controls the count.

Monitor FBC, lysis chemistry and renal and liver function at least daily and more often when unstable. It is not definitive therapy, can worsen cytopenia, requires safe handling and is avoided in pregnancy unless specialist life-saving benefit dominates.

First-line rapid removal of intravascular paraprotein in symptomatic hyperviscosity.

Therapeutic plasma exchange

Exchange approximately 1 to 1.5 calculated plasma volumes with the apheresis replacement protocol urgently, repeating according to symptoms, fundal findings, immunoglobulin and viscosity rebound.

Monitor vascular access, calcium, coagulation and haemodynamics. Exchange is temporary and must be followed by clone-directed treatment; avoid routine red-cell transfusion before viscosity falls.

Differentiation treatment that rapidly begins correcting the biological driver of APL coagulopathy and must not await final genetics.

Tretinoin for suspected APL

Give all-trans retinoic acid 45 mg/m²/day orally in 2 divided doses immediately when APL is suspected, continuing under the risk-adapted APL protocol while urgent PML-RARA confirmation is obtained.

Monitor differentiation syndrome, leukocytosis, liver tests, headache and pseudotumour cerebri. At suspected differentiation syndrome give dexamethasone 10 mg intravenously every 12 hours immediately until resolution under the APL protocol. Apply strict pregnancy prevention because tretinoin is teratogenic.

Rapidly converts existing uric acid during emergency cytoreduction when renal and lysis risk is high.

Rasburicase

Give 0.2 mg/kg intravenously once daily for up to 7 days for high-risk or established tumour lysis, with actual duration guided by urate, burden and response under the current BSH protocol.

Contraindicated in G6PD deficiency because of haemolysis and methaemoglobinaemia; test high-risk ancestry when time permits. Send urate samples on ice after dosing to prevent ex-vivo falsely low results.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Respiratory and neurological death

Diffuse pulmonary microvascular failure, intracranial haemorrhage, infarction, cerebral oedema and seizures are the principal immediate lethal complications.

Tumour lysis and renal failure

Spontaneous or treatment-related electrolyte release causes arrhythmia, seizures and acute kidney injury and must be prevented before cytoreduction.

Bleeding and thrombosis

DIC, thrombocytopenia and endothelial injury cause haemorrhage, while sluggish flow and the malignant clone can also promote thrombosis.

Rebound after apheresis

Leukapheresis or plasma exchange removes circulating burden but not marrow production, so recurrence is rapid without definitive systemic therapy.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

During cytoreduction, repeat neurological, respiratory and retinal assessment with FBC, potassium, phosphate, calcium, urate, LDH and creatinine at an instability-based frequency.
Monitor fluid balance closely because dehydration worsens viscosity but overhydration can worsen pulmonary leukostasis or renal failure.
Track PT, APTT, fibrinogen, D-dimer and clinical bleeding frequently in APL or DIC and replace components to the disease-specific target.
After plasma exchange, reassess symptoms and fundi, remeasure viscosity when useful and anticipate immunoglobulin rebound until systemic treatment acts.
Document the timing of red-cell transfusion, apheresis and systemic therapy so worsening physiology is interpreted against changing whole-blood viscosity.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Leukostasis is not a count

Clinical lung or brain dysfunction defines the syndrome, and AML can cause it at counts tolerated without symptoms in CLL.

Pseudohypoxaemia can mislead

Metabolically active leukocytes consume oxygen in a delayed syringe, so pulse oximetry and rapidly processed gases help resolve discordance.

Apheresis methods are not interchangeable

Leukapheresis removes cells, whereas plasma exchange removes paraprotein; neither controls marrow production.

APL is an exception

ATRA and haemostatic support start immediately, while leukapheresis is avoided because procedures can intensify fatal coagulopathy.

Red cells can worsen flow

Raising haematocrit before burden reduction can aggravate viscosity, so transfusion is deferred unless immediately necessary.

11Common pitfallsFrequent interpretation and management errors.

01
Equating every leukocyte count above 100 with leukostasis.
02
Excluding leukostasis because the AML count is below 100.
03
Waiting for viscosity before exchanging a symptomatic patient.
04
Using leukapheresis as definitive leukaemia treatment.
05
Performing leukapheresis routinely in suspected APL.
06
Transfusing red cells reflexively before reducing severe viscosity.
07
Starting cytoreduction without tumour-lysis prevention.