The purely toxic cases are more clearly defined and temporary so that they may be eliminated from consideration at present, yet their possible occurrence must always be borne in mind by the practitioner especially when called to pronounce upon cases of vertigo in connection with veterinary legal questions. The cases that are due to a persistent neurosis, or to circulatory troubles may well be placed in a list by themselves, yet in their legal relations it is highly important that the practitioner should as far as possible discriminate among these as well.
Susceptible Animals. Vertigo undoubtedly exists among all domestic animals. The symptoms by which it is recognized have been noted especially in the horse and much less frequently in ox, dog, pig and sheep.
Among horses it especially attacks the mature or aged, and family harness horses, pampered and irregularly exercised; (saddle horses rarely suffer); it is more likely to appear for the first time in spring though when established it happens at all seasons; it may come on when a horse is driven in blinders and fail to appear in the absence of these.
Causes. 1st. Compression of the jugular veins by a too tight collar is the cause of one of the simplest forms of vertigo and is observed, in growing or fattening animals in which the neck has become gradually too large for the collar. The supply of a larger and well fitting collar will soon confirm the diagnosis by a complete and permanent removal of the trouble. In other cases the veins may be compressed by undue flexion of the head, the chin being drawn toward the breast, or by a throat latch buckled too tightly. The substitution of an overdraw check rein, or a loose throat latch will show the true source of the trouble.
2nd. Disease of the valves of the heart or their insufficiency from cardiac dilatation is a common cause of vertigo, and may be recognized by auscultation and by the general symptoms of chronic heart disease.
3d. Disease of the lungs interfering with the flow of blood through the right heart and more distantly with the return of blood from the brain. It further effects the brain functions through the circulation of a highly carbonized blood, which fails to maintain the normal functions of the ganglia.
4th. Disease of the blood vessels, it may be by emboli washed on from clots in the pulmonary veins or the left heart and arrested in the vessels of the brain; it may be by aneurism of the anterior aorta as reported of a horse (Lustig); it may be by phlebitis and thrombosis of the jugulars; it may be by adjacent tumors pressing on the vessels.
5th. Gastric Vertigo, Abdominal Vertigo, is a complication of gastric or hepatic disorder with giddiness and unsteady movement. The abdominal disorder may be at once a cause and result of the vertigo and it is not always easy to decide which predominates. The unsteady movements in certain cases of overloaded stomach, in the horse are illustrations of purely abdominal vertigo, while on the other hand in vomiting animals nausea, retching, emesis, and other gastric disorders promptly attend on the primary cerebral disorder. There is also a special tendency to vertigo in the fat, idle, gorged horse and in those with torpor or other disorder of the liver occurring in pampered horses in spring and early summer.
6th. Optic vertigo is a reflex disorder, determined in the excitable nerve centres by the visual influence. Thus it has been seen in horses and sheep from the intense glare of the sun’s rays, reflected from a lake or river or from white snow or ice, or even from the glistening inner surface of the blinds. The effect is intensified if the animal has just emerged from a dark stable or a darker mine. The overdraw check may be a factor by reason of its turning the eyes upward and exposing them continuously to the full glare of the sun. The sense of motion conveyed through the eyes contributes to bring on giddiness and a sense of swimming. In man this is notorious, the sense of nausea and vertigo being precipitated by looking at the nearby, moving objects in cabin or on deck, while it may be retarded by directing the eyes to steady distant objects. As dogs, horses and other animals suffer from seasickness, and even railroad sickness, this attendant factor may be logically accepted. The mere limitation of the field of vision, by the use of blinds, and the disappearance in rapid succession of near objects behind this narrow screen probably has an influence similar to the visible motions in the ship between decks, in cases in which these portions of the harness are manifest factors.
7th. Aural Vertigo is determined by irritations of different kinds affecting the external, middle or internal ear. Experimental sections show that this is especially due to injuries of the semicircular canals. If the horizontal canal is divided there are pendulum-like movements of the head alternately to the right and left, also lateral rolling of the eyes. If the posterior canal is cut there is a vertical movement, or nodding of the head and vertical rolling of the eyes. If the superior vertical canals are injured there are pendulum-like vertical movements of the head and the animal tends to fall forward. Injury to the anterior canal causes diagonal rolling of the eyeball. In destruction of all the canals various pendulum-like movements are performed, and standing often becomes impossible. Stimulation of one auditory nerve is followed by rotation of the eye and rotation of the body on its axis toward the injured side. The passage of a galvanic current through the head between the mastoid processes, or from one external auditory meatus to the other, causes rolling of the eyeballs. Injection of water violently into a rabbit’s ear, or of iced water or of a rhigolene jet, causes rolling of the eyes, and rotation of the body toward the side operated on. Dr. Weir Mitchell had a similar experience in his own person. If the injections are repeated a permanent vertiginous condition is induced, and the rabbit or Guinea pig, which has been kept in darkness for a few hours and is then suddenly exposed to sunlight, is unstable on its limbs for a few seconds. Lucæ found that with perforation of the membrana tympani, an ear air douche, at 0.1 atmospheres caused abduction of the eyeball, dyplopia, giddiness, sense of darkness, and disturbed respiration. Vulpain found that a 25 per cent. solution of chloral hydrate dropped into the ear of a rabbit caused vertiginous movements. McVey records the case of a music teacher who had intense vertigo induced by the low bass notes of a piano. Crum Brown noticed that if a person with bandaged eyes, is rotated for some time as on a potter’s wheel, he can at first estimate the degree of rotation, but after a time he fails to do so, and the rotation may be stopped, without checking his sense of whirling. The familiar method of subduing an intractable or vicious horse by running him rapidly around in a very narrow circular course, or by tying head and tail together and letting him circle around until he staggers or falls, is another manifest example of this aural vertigo. Rabbits and dogs suffering from acariasis of the external ear move around in a circle, or even turn somersaults tending toward the affected side. Trasbot has found larvæ of insects (simulium cinereum?) in the ears of vertiginous horses, which he successfully treated with injection of chloroform. Even hard pellets of wax pressing on the tympanic membrane have been found to give rise to vertigo.