In some of the conditions where tremor occurs the influence of the will is weakened or is entirely absent. This is seen in hysterical trembling and in the tremor of old age as well as in those cases where there is general enfeeblement of the body, as in the fevers.
Trembling is connected with disease of the pyramidal tracts, because in this way the influence of the cerebral centres is withheld from the muscles. When a muscle is in a condition of tonic spasm, it is the result of the running together of very rapidly-repeated muscular contractions. It is like the contraction in a muscle from an interrupted electrical current. If the interruptions are slow, the muscular contractions are seen at intervals like a tremor; but if the interruptions are rapid from frequent vibrations of the hammer of the instrument, then the contractions in the muscle are fused together, as it were, and the muscle is in a state of tonic spasm.
It is held by some writers that tremor is caused by the want of balance between the cerebrum and cerebellum. When, for example, the control of the cerebrum is enfeebled the action of the cerebellum is so great as to bring about tremor by its uncontrolled power.
If we accept the first view, we must consider the tremor as a preliminary stage of paralysis; for the lesion, which at first is slight and causes only an interruption of the conduction of impulses from the brain to the muscles, as it becomes more extensive totally prevents conduction, and paralysis ensues.
Hughlings-Jackson's view, that general convulsions are the result of discharges from the cortex of the brain, and that the tonic contractions of tetanus are caused by discharges from the cortex of the cerebellum, may be applied to the pathology of tremor as well. When, for instance, in a disease like disseminated sclerosis a voluntary effort instead of causing a steady muscular contraction results in irregular spasmodic contractions and relaxations of the muscle, we may imagine that a series of discharges were taking place from the cortex of the cerebellum as long as the voluntary efforts were persisted in. On the other hand, in paralysis agitans it is more probable that a lack of conducting power in the pyramidal tracts prevents the influence of the centres being continuously exerted upon the muscles through their motor nerves.
It is probable that in simple tremor the lesion is situated in the spinal cord; for in this disease we seldom see any evidences of cerebral disturbance. There are no paralytic or psychical symptoms, and no vertigo. In toxic tremors the disease is no doubt located in the brain, for accompanying the trembling resulting from alcohol, opium, mercury, and other drugs are mental changes and more or less muscular enfeeblement.
TREATMENT.—Should the tremor depend upon some cause which can be discovered, of course the obvious course is to attempt to remove the source of trouble. The effort is of greater or less success in different conditions. The tremor from mercurial poisoning sometimes yields to treatment which is directed to the elimination of the mercury. The free administration of the iodide of potassium is the best means to be used, and is often successful. The same means are available in lead tremor. Of course the patient must be removed during treatment from the risk of further absorption of the poisonous substances.
In simple tremor many remedies have been recommended, but the results of treatment are not encouraging. Baths of various kinds and galvanism have been used, and many drugs are advised. Hyoscyamus and its alkaloid, hyoscyamine, have enjoyed a high reputation, and good results have been reported from their use. I have seen relief, but not cure, from their administration. Arsenic is a more reliable remedy and it may be used hypodermically. Eulenburg4 has used this method with good results. I have given arsenic per orem with beneficial effects in cases of simple tremor. In a case to which I have referred above the tremor was relieved while the patient was taking Fowler's solution, and on changing to hyoscyamus the trembling got worse. On returning to the arsenic the symptoms improved, and finally the tremor ceased after the remedy had been taken for some weeks. Hysterical tremor requires that the hysteria should be relieved. Franklinic electricity sometimes controls the tremor in these cases.
4 Ziemssen's Cyclopædia, vol. xiv. p. 392.