Diagnosis
3 principle diagnosis questions:
1. Is this a snakebite, or some other condition
2. If it is a snakebite, is there significant envenoming present and to what extent
3. Lastly, what type of snake was responsible?
Steps that get you to the diagnosis questions above:
1. The patient’s history and account of the injury
2. Observation of fang marks
3. Snake identification (if possible)
4. Progressive symptoms of envenomation all point to poisonous snakebite
Confirm that it’s a poisonous snake bite?
1. Puncture marks (usually on limbs). These may be difficult to see, and may consist of a single or double puncture or scratch marks or multiple punctures. They may be bleeding or oozing
2. Regional tender lymphadenopathy (NB this may also be present after bites from non-venomous snakes, and is not by itself an indication for antivenom)
Presentation of envenomation may include:
headache, nausea, vomiting
abdominal pain
collapse, unconsciousness, coma
painful, tender muscles
blurred vision
irritability, confusion
dark urine (myoglobinuria, haematuria)
weakness/paralysis
respiratory failure (neurotoxicity)
hypotension
cardiorespiratory arrest
Differential diagnosis
Of venomous snakebite
· non-venomous snakebite - leaves scratches, not punctures
· bite or sting by other venomous creature (arthropod)
· ascending neuropathy e.g. Guillain-Barre Syndrome
· AMI
· anaphylaxis
· hypoglycaemia/hyperglycaemia
· drug overdose
· closed head injury
Laboratory Investigations
Samples for venom detection and for pathology obtained to :
· Identify the genus of snake as the composition of particular venoms influences the clinical presentation of particular snakebites.
· Give the appropriate antivenom.
· Alert clinicians to particular features characteristic of envonomation.
However, fang marks alone are not an indication for the use of antivenom ( some snakes don’t give out enough ). This is when laboratory investigations come in handy.
* Antivenoms should not be used unless there is evidence of systemic envenomation
Although lab tests are of little value in the diagnosis of snake envenomation, nevertheless they are useful for monitoring the patient and deciding about specific interventions and prognosis.
They should include:
· a full blood count
· electrolytes
· glucose
· creatinine
· serum amylase
· creatinine phosphokinase (CPK)
· prothombin time (PT)
· partial thromboplastin time (PTT)
· fibrinogen and fibrin degradation products (FDP's).Looking into the tests…
Specific investigations
(a) The 20-min whole blood clotting test (20 WBCT): The 20 WBCT is a simple bedside test of coagulopathy to diagnose viper envenomation and rule out elapid bite. It requires a new clean, dry test tube made up of simple glass that has not been washed with any detergent. A few milliliters of fresh venous blood is drawn and left undisturbed in the test tube for 20 min; the tube is then tilted gently. If the blood is still liquid after 20 min, it is evidence of coagulopathy and confirms that the patient has been bitten by a viper. Cobras or kraits do not cause antihemostatic symptoms.
(b) Enzyme linked immunosorbent assay (ELISA): ELISA tests are now available to identify the species involved, based on antigens in the venom. These tests, however, are expensive and not freely available and thus have limited value in diagnosis; at present, they find use mainly in epidemiological studies.
Non-specific investigation
· Hemogram: The hemogram may show transient elevation of hemoglobin level due to hemoconcentration (because of the increased capillary leak) or may show anemia (due to hemolysis, especially in viper bites). Presence of neutrophilic leucocytosis signifies systemic absorption of venom. Thrombocytopenia may be a feature of viper envenomation.
· Serum creatinine: This is necessary to rule out renal failure after viper and sea snake bite.
· Serum amylase and creatinine phosphokinase (CPK): Elevated levels of these markers suggests muscle damage (caution for renal damage).
· Prothrombin time (PT) and activated partial thromboplastin time (aPTT): Prolongation may be present in viper bite. (repeated within 12hrs)
· Fibrinogen and fibrin degradation products (FDPs): Low fibrinogen with elevated FDP is present when venom interferes with the clotting mechanism. (repeated within 12hrs)
· Arterial blood gas and electrolyte determinations: These tests are necessary for patients with systemic symptoms.
· Urine examination: Can reveal hematuria, proteinuria, hemoglobinuria, or myoglobinuria. (Arterial blood gases and urine examination should be repeated at frequent intervals during the acute phase to assess progressive systemic toxicity).
*Include free protein, haemoglobin, myoglobin
*Arterial blood gas tested if an signs of respiratory compromise are evident
· Electrocardiogram (ECG): Nonspecific ECG changes such as bradycardia and atrioventricular block with ST-T changes may be seen. (suggested for patients >50 and patients with history of heart disease)
· Electroencephalogram (EEG): Recently, EEG changes have been noted in up to 96% of patients bitten by snakes. These changes start within hours of the bite but are not associated with any features of encephalopathy. Sixty-two percent showed grade I changes, 31% cases manifested grade II changes (moderate to severe abnormality), and the remaining 4% showed severe abnormality (grade III). These abnormal EEG patterns were seen mainly in the temporal lobes.
Blood changes include anaemia, lecuocytosis (raised white cell count) and thrombocytopenia (low platelet count). The peripheral blood film may show evidence of haemolysis especially in viperine bites. Clotting time and prothrombin time may be prolonged and a low fibrinogen may be present.
Blood should be typed and crossmatched on the first blood drawn from the patient, as both direct venom and anti-venom effects can interfere with later cross matching. Some specialised centers can identify species of snake involved.
Commonly hyperkalaemia and hypoxaemia with respiratory acidosis may be seen, particularly with neuroparalysis.
Imaging Studies
Baseline chest radiograph in patients with pulmonary edema
Plain radiograph to rule out retained fang(s)
References
Springhouse 2005, Professional Guide to Diseases (Eighth Edition), Lippincott Williams & Wilkins.
Springhouse 2003, Handbook of Diseases, 2003 Lippincott Williams & Wilkins.
M. William Schwartz MD; et al 2008, The 5-Minute Pediatric Consult , Lippincott Williams & Wilkins.
http://www.toxinology.com/fusebox.cfm?staticaction=snakes/ns-sndiag01.htm
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2700615
http://www.aafp.org/afp/20020401/1367.html
http://emedicine.medscape.com/article/168828-diagnosis
http://www.emedicinehealth.com/snakebite/page4_em.htm
http://www.nda.ox.ac.uk/wfsa/html/u16/u1606_02.htm
http://www.wrongdiagnosis.com/s/snake_bite/diagnosis.htm
http://www.wrongdiagnosis.com:80/s/snake_bite/book-diseases-7a.htm
http://emedicine.medscape.com/article/168828-diagnosis
http://www.avru.org/compendium/biogs/A000065b.htm
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