This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
Much disturbance of the nervous system sometimes follows the full action of iodine. It is marked at first by excitement, with restlessness, tremor, anxiety, and insomnia; but this state is liable to be succeeded by feebleness and depression. Toxic doses have caused violent headache, and sometimes convulsion. Rilliet described neuralgia, tinnitus, disturbed intellect, and convulsion, as prominent symptoms in some cases of iodism. Altered vision and paralysis were noted by Brodie. "Occasional hyperaesthesia and temporary palsy of lower extremities" occurred in a man who was taking very large doses (90 gr. thrice daily) of iodide of potassium (S. A. Lane: Lancet, ii., 1873). Such symptoms, however, must be considered rare. IT. Wood states that he has only seen the nervous system affected once, in his experience, even "with enormous doses," and then the patient, who had been taking 270 gr. daily, became "intensely sleepy and stupid," as if under the influence of bromide.
More complete observations have been made upon the action of iodoform on the nervous system. Maitre compared the effects to those of alcohol. After moderate doses - 1/2 to 1 gramme - a dog either lay at rest, or, if made to rise, staggered and fell; next day it seemed well. After 3 or 4 grammes, intense excitement set in, with quickened circulation, convulsive contraction of limbs, and opisthotonos like that of strychnine. These are symptoms as of iodine in the circulation, and the odor of this substance was strongly marked in the breath. It would seem that, when not dissolved in the blood, iodoform acts as an irritant on the nervous system, but when completely soluble it induces muscular relaxation with insensibility (acting like a narcotic). Maitre was one of the first to record its power of relieving nerve-pain (Bouchardat: Ann., 1857). Righini, considering its chemical relations with chloroform, argued that it should possess anaesthetic power, and proved that it did so to some extent; but its local effect, when directly applied, is much more marked than its general effect when taken or inhaled. Twenty grains placed in the rectum are said to destroy sensibility in the sphincter, so that defecation is not felt (Moretin).
Franchino corroborated the fact of local anaesthetic action, and produced some amount of similar effect on the general system in dogs, birds, and rabbits, by making them breathe 2 grammes of iodoform vaporized by means of bellows in a closed chamber: a stage of excitement with muscular contraction was followed by sedation and anaesthesia for five or ten minutes; then gradual recovery. Binz, however, could not obtain so marked a result as this, which he attributed in part to the carbonic acid confined in the chamber (Archiv fur Exper. Pathologic Klebs, vol. viii., 1877). McKendrick, comparing the drug to chloral, found that 10 gr. dissolved in about 1 dr. of alcohol, and injected under the skin of a rabbit, produced profound sleep for four hours, and 12 gr. destroyed life; but, again, Binz failed to verify this result, and. attributed the sleep mainly to the alcohol. In his own experiments, 2 grammes in oily solution, administered subcutaneously to dogs and cats, produced but moderate sleep in the course of an hour, and 3 grammes impaired the functions of the brain and spinal cord, without being necessarily fatal. He concludes that moderate doses exert some narcotic effect, especially on dogs and cats, but not so much as former observers thought, and that toxic doses kill by general paresis, with lowering of temperature (Edinburgh Medical Journal, 1874).
Hogyes, from recent observations made in order to reconcile the discrepancies in the above statements, states that large doses cause marked drowsiness in the dog and cat, not in the rabbit; also that during the somnolence reflex irritability is not much interfered with. Toxic doses cause death by gradual paresis of circulation and respiration (Medical Record, May, 1879).
Binz experimented also with the iodate of sodium, and found that in rather large doses this salt caused narcosis in animals. It proved especially poisonous to the respiratory and cardiac centres, and he suggests that both this salt and iodoform are decomposed and liberate iodine in the brain and cord.