Introduction

Normal platelet count in small animals ranges between 200–800×10⁹/L. Thrombocytopaenia occurs when platelet numbers are less than 200×10⁹/L and numbers below 50×10⁹/L are classed as severe thrombocytopaenia.

Immune-mediated thrombocytopaenia (IMTP) is the most common cause of low platelet counts seen in dogs. Increased destruction of the platelets occurs when immunoglobulin antibodies attach to the platelet surface, causing macrophages to respond and initiate phagocytosis. IMTP can be classified as a primary or idiopathic disorder if the production of the antibodies is truly autoimmune with all underlying causes excluded. Classification as a secondary disorder can result from an infectious agent, such as viral, bacterial, or parasitic organisms, neoplasia, or drug induced. Platelet production can also be affected by various means, such as drug administration, radiation treatment, bone marrow neoplasia or infection. Thrombocytopaenia can also be caused by increased consumption and/or sequestration attributable to disseminated intravascular coagulopathy (DIC), splenic torsion, sepsis, or neoplasia.

Certain breeds are pre-disposed to IMTP, such as cocker spaniels, miniature and toy poodles, golden retrievers, German shepherds and old English sheepdogs. Females are twice as more likely to be affected than males with the median age for presentation between 4–8 years.

Symptoms and Diagnosis

These patients may present with lethargy, weakness, anorexia, and mild pyrexia. Signs of platelet dysfunction can be seen on physical examination such as petechiae of the skin or mucous membranes, ecchymosis on the skin, ocular haemorrhage, or bleeding at the gum margins. The owners may also report a history of haematuria, malaena, epistaxis, haematochezia, or fresh blood in the stools. It is important to obtain a thorough history for these patients to rule out underlying causes such as recent travel history, drug administration or possible toxin ingestion.

To confirm the diagnosis of thrombocytopenia, technical or laboratory error should be ruled out as analysers may miscount large platelet precursors and platelet clumps. A manual platelet count performed on a blood smear is an easy and accurate method to estimate platelet numbers and should be performed before any further invasive procedures are carried out. Peripheral blood smears should contain 8–15 platelets per high power field, and further evaluation of the smear also allows for a review of platelet size.

Further diagnostics that should be performed include:

• Full haematology, including PCV, to check for infection or concurrent anaemia.
• Biochemistry panel to check general health.
• Coagulation profile to rule out coagulopathies and DIC.
• Thoracic radiographs and abdominal ultrasound to rule out neoplasia (bone marrow sampling may be indicated if neoplasia is suspected).
• Tick-titre and infectious disease panels can also be performed as well as immune system function tests such as Coombs test, antiplatelet antibody, and antinuclear antibody test.

If the platelet numbers are known to be low, a buccal mucosal bleeding time (BMBT) should not be performed due to the risk of increased bleeding from the site.

Treatment

The treatment choice depends largely on the cause of the thrombocytopaenia. Secondary IMTP generally resolves once the underlying pathology has been treated or removed. Primary IMTP requires immunosuppressive drugs to stop the destruction of self-antigens. These include corticosteroids, azathioprine, and cyclosporine. Vincristine, which is traditionally used as a chemotherapy drug, is known to stimulate the production of platelets and can benefit the recovery of patients with acute IMTP. Other treatments that can be considered include human immunoglobulins or a splenectomy in those patients that fail to respond to medical therapies. Blood transfusions can be administered for symptomatic support if there has been significant blood loss rather than for treatment, as most transfusions supply little, if any, viable platelets to the patient. Platelet transfusions may be indicated where life-threatening bleeding occurs because of thrombocytopaenia. Options for platelet transfusions include fresh whole blood, platelet-rich plasma, platelet concentrate, cryopreserved platelets and lyophilised platelets. These products provide a temporary increase in platelet numbers, but unfortunately, not all products are currently available in the UK.

Nursing Considerations

Nursing patients with IMTP can be challenging and is aimed at minimising trauma as uncontrollable bleeding could prove fatal. The accommodation provided should be suitably padded and the environment needs to be quiet and calm to encourage the patient to settle. Frequent patient observations are required to monitor for signs of anaemia, dyspnoea, worsening petechia or ecchymosis or hypovolaemia secondary to gastrointestinal losses. The jugular vein should not be used for sampling and all blood samples should be collected from a peripheral vein which is at lower pressure and on which a pressure dressing can be easily applied. The use of intramuscular or subcutaneous injections should also be avoided as large haematomas can develop at the injection site. Procedures such as cystocentesis, urinary catheterisation or placement of a feeding tube should be avoided as these could result in trauma and bleeding. These patients should be handled gently and any stress minimised. A harness should be used when walking to prevent pressure and bruising around the neck and soft food should be offered to avoid trauma to the gums. Due to the treatment, these patients are immunosuppressed and susceptible to hospital borne infections so should be reverse barrier nursed. Side effects of the corticosteroids can include polydipsia, polyuria, and polyphagia. Therefore, its vital that these patients have access to water, are walked regularly and fed little and often to help satiate the patient. If the patient has received a cytotoxic agent such as azathioprine or vincristine, the necessary care should be taken when handling urine, faeces, saliva, and blood.

Prognosis

Patients usually remain hospitalised until a response to treatment is witnessed and the platelet count is greater than 50x10⁹/L. The prognosis for primary IMTP is fair to good with a long-term recurrence rate of 25%. Those patients with neoplasia or DIC have a guarded to poor prognosis.