Congestion of the encephalon is treated here as a pathological entity, though it cannot always be distinguished clinically from some forms of vertigo on the one hand and from the milder types of apoplexy or encephalitis on the other. It has been divided into passive or venous hyperæmia and active or arterial hyperæmia.
Passive hyperæmia, as shown under vertigo and apoplexy is a common result of a tight collar, a tight strap used for cribbiting, a too short bearing rein, dilation or valvular disease of the right heart, or disease of the lungs, violent efforts in running, draught, etc. It tends to be associated with arterial anæmia on the principle that the closed cranial cavity can only admit a certain amount of blood and if an excess accumulates in the veins and capillaries, this must be compensated first by the movement backward to the spinal canal of the cerebro-spinal fluid, and second by the diminution of the blood in the cerebral arteries.
Active hyperæmia may be brought about by any excitement which especially affects the brain. This has been already noted in connection with insolation (sun-stroke). It may result from severe exertion during hot weather, in a violently contested race, in drawing a heavy load up hill, or in harsh training. Violent exertion just after a meal is especially injurious. Also the excitement of travelling by rail, or that caused by proximity to locomotives, to discharges of firearms and to other causes of great fear; encreased blood tension in the cerebral vessels in connection with hypertrophy of the left ventricle, or obstruction in other vessels (of the limbs) so as to direct the force of the current into the carotids, the expulsion of blood from the splanchnic cavities by gastric or intestinal tympany, or overloading of the paunch, and irritation of the brain by ptomaines and toxins in certain infectious diseases (rabies, canine distemper, etc.). In the same way vegetable narcotics (opium, etc.) produce congestion. Among the most common causes of congestion are lead, poisoning by lolium temulentum, partially ripened lolium perenne, millet, Hungarian grass, and partially ripened seeds of the leguminosæ (chick vetch, vicia sativa.). Other causes are the presence of tumors (cholesteatoma) and parasites (cœnurus, cysticercus) in the brain.
Symptoms. Cerebral hyperæmia, like other brain disorders may give rise to a great variety of symptoms, according to the condition of the animal and the susceptibility of its nerve centres. Some cases have the characteristic seizures of vertigo, others the manifestations of heat stroke, and others, epileptic explosions or apoplectic symptoms. For these see under their respective headings. In other cases the symptoms are those of encephalo-meningitis but moderate in its type and often tending to a transient duration, or to prompt resolution and recovery.
Horse. There is manifest change of the nervous and intellectual conditions, which may show itself by irritability or restlessness, by pushing against the wall, by hanging back on the halter, by trembling, shaking the head, neighing, pawing and, in exceptional cases, by rearing, biting or kicking. The pulse is hard and full, the heart’s impulse strong, the beats in the carotids and temporal arteries being especially forcible, and the buccal, nasal and orbital mucosæ are strongly congested. Heat of the head is usually a marked feature. While usually very sensitive to touch, noise or light, the animal may be dull or drowsy, and in spite of its marked sensitiveness, it is then inert or lethargic and indisposed to any active exertion. Freidberger and Fröhner say that the habitual comatose condition alternates at intervals with periods of violent excitement during which the animal pushes or dashes against the wall, grinds the teeth, rears, paws, kicks, bites, etc., and then relapses into the state of coma. When the disease reaches this stage it may be questioned whether we are not dealing rather with acute encephalitis.
In active congestion the symptoms are always aggravated by whatever tends to increase the vascular tension in the brain. Active exertion, draught, the pendent position of the head, the recumbent position on the side with the head as low as the body or lower, aggravate all the phenomena and render the animal more helpless.
The following table slightly modified from Spitzka serves to point out the distinctions between anæmia and hyperæmia:
| Symptoms. | In Cerebral Anæmia. | In Cerebral Hyperæmia. |
|---|---|---|
| Pupils. | Usually dilated and mobile. | Usually small or medium. |
| Respiration. | Often interrupted by a deep breath or sigh, even when at rest. | Normal or nearly so. |
| Activity. | Lassitude. | Restless, but indisposed to exertion. |
| Temperament. | Lethargic with exceptions. | Irritable with exceptions. |
| Intelligence. | Senses impaired. | Impaired. |
| Elevation of head. | Aggravates symptoms. | No effect, or improvement. |
| Recumbent, dependent head. | Amelioration. | Aggravation. |
| Straining. | Not necessarily aggravated. | Aggravated. |
Cattle show the same general congestion and heat of the head, ears and horns, congested mucosæ, fixed eyes, and pupils, indisposition to follow the herd, irritability, and dulness with often a disposition to lie down. This may go on to violent bellowing, pushing against the wall, grinding of the teeth, working of the jaws, rolling of the eyeballs, and violent dashing in different directions regardless of obstacles.
Dogs show the same restlessness and excitability, congested head, eyes and nose, frequent movement from place to place, a desire to wander off, and it may be spasms. If there has been any gastric disturbance vomiting usually supervenes. As in the larger animals the disease may go on to more violent symptoms, and the animal howls, rushes in different directions, and may snap at imaginary objects, or at any one who interferes with him. His movements are liable to be unsteady, uncertain and swaying.