3 Circulatory disturbances

Due to the high demand for oxygen and nutrient, skeletal muscle is a relatively well vascularized tissue (as you will find out when you cut a muscle belly during surgery!). This makes it somewhat resistant to vascular injury, as there is substantial collateral circulation: each myofiber is supplied by 3-12 capillaries. Thus, for the most part, ischemic damage to muscle is due to pressure over large surfaces of the body. Thromboembolic occlusion of a large vessel, most commonly seen at the aortoiliac junction in cats (and horses), can also lead to ischemic necrosis of large areas of muscle.

3.1 Compartment syndrome

Compartment syndrome occurs most commonly in chickens (and humans). As discussed in the review of muscule anatomy, muscles are covered by various layers of connective tissue. Some muscles – the supracoracoid muscles in chickens, for example – are surrounded by inelastic tissues. In the case of the supracoracoid, the muscle is contained by the breastbone on one side, and a relatively rigid outer sheath on the other, forming an inelastic or fixed “compartment”. When a chicken participates in short, vigorous exercise (wing flapping), there is a significant increase intramuscular vascular pressure that collapses veins and decreases venous outflow. On top of this, muscle contraction can increase the diameter of myofibers by 20%, and in a restricted compartment, this contributes to further venous collapse. Increased muscular activity also increases arterial blood flow. The net result is increased arterial flow with decreased venous outflow leading to ischemic necrosis, all caused by a muscle held captive by rigid surrounding tissues. Damage to vessels subsequent to the necrosis can lead to interstitial edema, further increasing tissue pressure, and exacerbating the condition. Treatment (generally for humans) can involve a fasciotomy to relieve the pressure.

3.2 Downer syndrome

Downer syndrome refers to ischemic myopathy caused by long periods of recumbency, leading to undue pressure on the dependent muscles. Size and weight of the animal are important factors: larger, well muscled animals are more prone to this syndrome than are thin animals. Cows are most frequently affected, while cats seem to be exempt.

The pathogenesis is more complicated than initially meets the eye. The increased pressure on muscles caused by prolonged recumbency initially collapses veins, causing congestion and ischemia. Eventually arteries are affected as well, further contributing to the ischemic damage. If and when the animal changes positions and relieves the pressure, arteries are the first to recover, and the increased arterial pressure in the face of decreased venous outflow leads to edema, which itself can cause increased pressure and decreased venous outflow, further compounding the problem. Reperfusion injury also likely contributes to muscle damage.

3.3 Postanesthetic myopathy of horses

Horses that undergo prolonged anesthesia are prone to ischemic myopathy, especially if inadequate padding is used. Pressure on the muscle capillaries, caused by the weight of the horse, exceeds the pressure of perfusion, resulting in ischemia. The muscles affected depend on the position of the horse. Predisposing factors include intraoperative hypotension and the presence of comorbities, especially PSSM and HYPP.