Thrombosis of the Brachial Artery.

Thrombosis of the brachial artery or of its principal branches is of
very rare occurrence in horses.

Etiology.--Partial or complete obstruction of arteries (brachial or
others) occurs as the result of direct injury to the vessel wall from
compression and tension of muscles and resultant arteritis; lodging of
emboli; and parasitic invasion of vessel walls causing internal
arteritis.

Symptomatology.--If sufficient collateral circulation exists to supply
the parts with blood, no inconvenience is manifested while the subject
is at rest. Where the lumen of the affected vessel is not completely
occluded, there may be no manifestation of lameness when the ailing
animal is moderately exercised. Consequently, the degree of lameness
depends upon the extent of the obstruction to circulation; and,
likewise, the course and prognosis depend upon the character and extent
of such obstruction.

In severe cases, lameness is markedly increased by causing the animal to
travel at a fast pace for only a short distance. There are evinced
symptoms of pain, muscular tremors and sudation, but the affected member
remains dry and there is a marked difference of temperature between the
normal areas and the cool anemic parts. When the subject is allowed to
rest, circulation is not taxed, and there is a return to the original
and apparently normal condition, only to recur again with exertion. This
condition characterizes thrombosis.