Thrombocythaemia

Primary thrombocythaemia (PT) (also known as essential thrombocythaemia) is a myeloproliferative disorder. It may be complicated by vascular occlusive and haemorrhagic events. This risk can be reduced six-fold by cytoreductive therapy (from 24% for those not given treatment to 3.6% for those receiving hydroxycarbamide). In a study where all patients received hydroxycarbamide, the incidence of major thrombotic events was 5.6%/year

An anti-aggregating agent, such as aspirin, has been shown to reduce/alleviate minor ischaemic symptoms. Therefore, except in patients with haemorrhagic symptoms, peptic ulceration or known side-effects to aspirin, the use of low-dose aspirin (75mg daily taken with food) is appropriate. The use of aspirin is not contra-indicated in patients with PT receiving anticoagulant therapy.

Haematological transitions to acute leukaemia and myelofibrosis may occur in the longer term. The incidence of acute leukaemic transformation in selected patients treated with Hydroxycarbamide is 5-10% over 4-11 years. Whilst cytoreductive agents reduce the incidence of vascular occlusion and haemorrhage, there is a risk that they may enhance leukaemogenic transformation. Their potential to delay myelofibrotic transformation is unknown.

For treatment purposes patients are divided into ‘low’ and ‘intermediate’ and ‘high risk’ of vascular occlusion.

Low-risk
Age under 40 years with no high risk features

Intermediate-risk
Age 40 to 59 years with no high risk features. Microvascular symptoms are not generally regarded as thrombotic events for the purpose of risk classification but if they are severe or not responding to aspirin the patient could be reclassified as ‘high risk’. 

High risk (any of the following features)

• Age over 60 years
• Platelet count >1500x10/⁹l (current or previous). This has been used as an indicator for cytoreductive therapy in view of the increased haemorrhagic risk. In young patients (<40 years) it may be reasonable to use a higher platelet threshold for risk classification in the absence of symptoms. 
• History of ischaemia, thrombosis or embolic events (including erythromelalgia)
• Haemorrhage considered to be related to primary thrombocythaemia (PT)
• Presence of hypertension requiring treatment or diabetes

Patients should be screened for hypertension, hyperlipidaemia, diabetes and a smoking history should be taken. Any cardiovascular risk factors should be aggressively managed.

Low risk patients (aged under 40 years with no high risk features)
Receive aspirin alone. There is evidence that in patients under the age of 40 years the complication rate is only one quarter of that seen in patients aged 40 - 59 years.

Intermediate risk patients (aged 40 to 59 years with no high risk features)
Usually are treated with either aspirin or aspirin and hydroxycarbamide.

High risk patients
Receive aspirin and hydroxycarbamide

The target platelet count for those receiving cytoreductive therapies is 200-400x10⁹/l.

Pregnancy in PT requires careful management. In general low risk patients should continue aspirin and be closely monitored by a haematologist and obstetrician. Patients being treated with hydroxycarbamide should use adequate contraception. For patients of either sex contemplating pregnancy hydroxycarbamide should be interrupted for 3-6 months beforehand and interferon alpha substituted when a cytoreductive drug is considered necessary.