Therapy for nerve agent poisoning

Newmark, J. Therapy for nerve agent poisoning. Arch Neurol 2004; 61:649-652

Nerve agents include tabun (GA), sarin (GB), soman (GD), cyclosarin (GF) and VX. All evaporate and are gone within 24 hours, except VX which is oily and the only one which is persistent. VX is the most lethal substance known to man. The drugs were weaponized in Nazi Germany, used by Iraq against Iran in the Iraq-Iran war, and used in 2 subway attacks in Japan in 1994-5.

Clinical scenario-- in a shopping mall, you hear a "pop" and see smoke, your vision dims and nose runs severely. People may collapse, breathing heavily or seizing, with constricted pupils.

The mechanism is similar to organophosphate pesticides, although they are much less fat soluble and therefore more toxic. Theyinhibit AchE, acetylcholinesterase, producing a cholinergic crisis. Most exposures occur through the vapor route and act within seconds to minutes. The initial symptoms are miosis and rhinorhhea and salivation. Later, brochorrhea and bronchospasm occur. They are absorbed at the alveoli, and although they are not important clinically, the gold standard of diagnosis is RBC AChE. Circulating toxin first affects the GIT causing cramping, pain, nausea and defecation. It will affect the heart variably with unpredictable effects on BP and HR. They cause massive overstimulation of peripheral neuromuscular synapses witha progression from fasciculation to twitching that may be misinterpreted as a seizure. When ATP is depleted (a late finding) the patient will eventually have flaccid paralysis. Rapid CND cholinergic stimulation will cause LOC, seizures, inhibition of medullary respiratory center with central apnea. Death is due to respiratory failure (combined) due to bronchospasm, bronchorhhea, inadequate function of respiratorymuscles, and central apnea.

Liquid exposure has a longer slower onset and offset than vapor. Skin decontamination may not suffice if not done immediately, as nerve agent may be trapped subcutaneously and continue to penetrate.

Patients recovered frommild poisoning have a neurobehavioral syndrome including HA, personalitychange, depression, and higher order problemswith memory and reading. It may resemblePTSD or postanoxic encephalopathy. Chronic effects are few, with one report of a PN and a few of vestibular problems.

THERAPY:
Ventilatory support can be lifesaving even for someone in full arrest, if an antidote is given. Treatment has not changed since 1945 and includes an anticholinergic drug to counteract crisis, an oxime to reactivate inhibited AChE, and an AED to prevent seizures. In the field,atropine is used with i-m autoinjectorswith an initial therapy of 2,4,or 6 mg with retreatment every 5-10 minutes as needed. 0.5 or 1.0 mg ampules are available for children. In the hospital it may be given intravenously. Atropine binds to muscarinicreceptorsbut not nicotinic receptors, so twitching and dyscoordination will continue. The respiratory problems are life threatening and atropine saves lives,per the Iranian experience.

The fielded oxime is 2-pralidoximm chloride (2 PAM). The i-m dose is 600 mg per autoinjector. The upper limit is 2000 mg/hr due to the risk of sudden elevation of blood pressure. Oximes reactivate catalytic cholinesterase and simultaenously split nerve agent or organophosphate insectisides into harmless, rapidly metabolized fragments. After nerve agent binds AChE , the resultant complex spontaneously loses a side chain ("aging") rendering the remaining enzyme-inhibitor complex unable to be reactivated by oxime. Most nerve agents age slowly and the reaction can be ignores (sarin over hours, VX over days to weeks) but soman over two minutes. Once the complex has aged, the oximes are useless although not harmful to the patient.

The MARK 1 set (Meridian Medical Technologies, Columbia, MS) has a 2 mg atropine and 600 mg 2-PAM autoinjectors and is already approved by the FDA for the general use.

Standard AED's (antiepileptic drugs) are not useful for seizures due to nerve agents. Only benzodiazepines are effective. The military uses diazepam 10mg autoinjectors. Midazolam is the most effective, given i-v, in terms of speed of actions, low dose required, and broad spectrum. Epilepsy does not develop and chonic AED's are not advised.

The fourth drug available is pyridostigmine bromide, which may be useful if soldiers are concerned about a rapidly aging poison (soman) . Pyridostigmine will sequester a percentage of the patients excess AChE and render it unavailable to be irreversibly inhibited by a a nerve agent. Therefore a lethal dose becomes survivable with antidotes.

Care must be provided inthe field, not in the hospital (too late). Additional information is available at the chemical casualty division website at http://ccc.apgea.army.mil or the Army Medical Research Institute of Chemical Defense at (410) 436-3276.