Investigation and Diagnosis

5.4 Detailed clinical assessment and species diagnosis

5.4.1 History

A precise history of the circumstances of the bite and the progression of local and systemic symptoms and signs is very important. Three useful initial questions are:

"In what part of your body have you been bitten?" The doctor can see immediately evidence that the patient has been bitten by a snake (eg fang marks) and the nature and extent of signs of local envenoming.

"When were you bitten?" Assessment of the severity of envenoming depends on how long ago the patient was bitten. If the patient has arrived at the hospital soon after the bite, there may be few symptoms and signs even though a large amount of venom may have been injected.

"Where is the snake that bit you?" If the snake has been killed and brought, its correct identification can be very helpful. If it is obviously a harmless species (or not a snake at all!), the patient can be quickly reassured and discharged from hospital.

Early clues that a patient has severe envenoming: Snake identified as a very dangerous one Rapid early extension of local swelling from the site of the bite Early tender enlargement of local lymph nodes, indicating spread of venom in the lymphatic system Early systemic symptoms: collapse (hypotension, shock), nausea, vomiting, diarrhoea, severe headache, "heaviness" of the eyelids, inappropriate (pathological) drowsiness or early ptosis/ophthalmoplegia Early spontaneous systemic bleeding Passage of dark brown urine

Patients who become defibrinogenated or thrombocytopenic may begin to bleed from old, partially-healed wounds as well as bleeding persistently from the fang marks.

The patient should be asked how much urine has been passed since the bite and whether it was a normal colour.

An important early symptom of sea snake envenoming that may develop as soon as 30 minutes after the bite is generalised pain, tenderness and stiffness of muscles and trismus.

5.4.2 Physical examination

This should start with careful assessment of the site of the bite and signs of local envenoming.

5.4.2.1 Examination of the bitten part

The extent of swelling, which is usually also the extent of tenderness to palpation, should be recorded. Lymph nodes draining the limb should be palpated and overlying ecchymoses and lymphangitic lines noted.

A bitten limb may be tensely oedematous, cold, immobile and with impalpable arterial pulses. These appearances may suggest intravascular thrombosis, which is exceptionally rare after snake bite, or a compartmental syndrome, which is uncommon. If possible, intracompartmental pressure should be measured (see Annex 5) and the blood flow and patency of arteries and veins assessed (eg by doppler ultrasound).

Early signs of necrosis may include blistering, demarcated darkening (easily confused with bruising) or paleness of the skin, loss of sensation and a smell of putrefaction (rotting flesh).

5.4.2.2 General examination

Measure the blood pressure (sitting up and lying to detect a postural drop indicative of hypovolaemia) and heart rate. Examine the skin and mucous membranes for evidence of petechiae, purpura, ecchymoses and, in the conjunctivae, chemosis. Thoroughly examine the gingival sulci, using a torch and tongue depressor, as these may show the earliest evidence of spontaneous systemic bleeding. Examine the nose for epistaxis. Abdominal tenderness may suggest gastrointestinal or retroperitoneal bleeding. Loin (low back) pain and tenderness suggests acute renal ischaemia (Russell’s viper bites). Intracranial haemorrhage is suggested by lateralising neurological signs, asymmetrical pupils, convulsions or impaired consciousness (in the absence of respiratory or circulatory failure).

5.4.2.6 Examination of pregnant women

There will be concern about fetal distress (revealed by fetal bradycardia), vaginal bleeding and threatened abortion. Monitoring of uterine contractions and fetal heart rate is useful. Lactating women who have been bitten by snakes should be encouraged to continue breast feeding.

5.4.3 Species diagnosis

If the dead snake has been brought, it can be identified. Otherwise, the species responsible can be inferred indirectly form the patient’s description of the snake and the clinical syndrome of symptoms and signs (see above and Annex 1 & 2). This is specially important in Thailand where only monospecific antivenoms are available.

5.5 Investigations/laboratory tests

5.5.1 20 minute whole blood clotting test (20WBCT)

This very useful and informative bedside test requires very little skill and only one piece of apparatus - a new, clean, dry, glass vessel (tube or bottle).

20 minute whole blood clotting test (20WBCT) Place a few mls of freshly sampled venous blood in a small glass vessel Leave undisturbed for 20 minutes at ambient temperature Tip the vessel once If the blood is still liquid (unclotted) and runs out, the patient has hypofibrinogenaemia ("incoagulable blood") as a result of venom-induced consumption coagulopathy In the South East Asian region, incoagulable blood is diagnostic of a viper bite and rules out an elapid bite Warning! If the vessel used for the test is not made of ordinary glass, or if it has been used before and cleaned with detergent, its wall may not stimulate clotting of the blood sample in the usual way and test will be invalid If there is any doubt, repeat the test in duplicate, including a "control" (blood from a healthy person)

5.5.2 Other tests

Haemoglobin concentration/haematocrit: a transient increase indicates haemoconcentration resulting from a generalised increase in capillary permeability (eg in Russell’s viper bite). More often, there is a decrease reflecting blood loss or, in the case of Indian and Sri Lankan Russell’s viper bite, intravascular haemolysis.

Platelet count:this may be decreased in victims of viper bites.

White blood cell count: an early neutrophil leucocytosis is evidence of systemic envenoming from any species.

Blood film: fragmented red cells ("helmet cell", schistocytes) are seen when there is micro-angiopathic haemolysis.

Plasma/serum may be pinkish or brownish if there is gross haemoglobinaemia or myo-globinaemia.

Biochemical abnormalities: aminotransferases and muscle enzymes (creatine kinase, aldolase etc) will be elevated if there is severe local damage or, particularly, if there is generalised muscle damage (Sri Lankan and South Indian Russell’s viper bites, sea snake bites). Mild hepatic dysfunction is reflected in slight increases in other serum enzymes. Bilirubin is elevated following massive extravasation of blood.Creatinine, urea or blood urea nitrogen levels are raised in the renal failure of Russell’s viper and saw-scaled viper bites and sea snake bites.Early hyperkalaemia may be seen following extensive rhabdomyolysis in sea snake bites. Bicarbonate will be low in metabolic acidosis (eg renal failure).

Arterial blood gases and pH may show evidence of respiratory failure (neurotoxic envenoming) and acidaemia (respiratory or metabolic acidosis).

Warning : arterial puncture is contraindicated in patients with haemostatic abnormalities (Viperidae)

Desaturation: arterial oxygen desaturation can be assessed non-invasively in patients with respiratory failure or shock using a finger oximeter.

Urine examination: the urine should be tested by dipsticks for blood/haemoglobin/myoglobin. Standard dipsticks do not distinguish blood, haemoglobin and myoglobin.Haemoglobin and myoglobin can be separated by immunoassays but there is no easy or reliable test. Microscopy will confirm whether there are erythrocytes in the urine. Red cell casts indicate glomerular bleeding. Massive proteinuria is an early sign of the generalised increase in capillary permeability in Russell’s viper envenoming.

Taken From :
http://www.searo.who.int/EN/Section10/Section17/Section53/Section1024_3900.htm