Pathological features in ischemic bowel disease-

Gross Features:
In acute ischemia bowel wall is edematous and dusky in colour. Mucosa is necrotic and has a nodular surface due to excessive submucosal hemorrhage.
Mucosal surface may be covered by patches of white slough.
Necrosis and gangrene may affect part or all of bowel wall.
In some cases only mucous membrane is affected especially in nonocclusive type of ischemia.

Microscopic features:

I Early mucosal lesion :

- Swelling of lamina propria. Hemorrhage, vascular dilation and  submucosal edema may or may not be present.

- Emigration of neutrophils into the lamina propria and epithelium.

- Crypt and villous atrophy .

- Adhesion between adjacent villi in some cases.

- Destruction of epithelium.

II Later stage:

- Surface membrane of mucus, fibrin, RBCs.

-  Prominent areas of hemorrhage and edema in submucosa.

- Fibrin thrombi in mucosa and submucosal capillaries.

- Muscle layer is characterized by  loss of nuclei and pale staining in the early stage. In the late stage there is lysis, separation and thinning of the muscle layer.

-  Infarction (hemorrhage in the submucosa, intravascular thrombosis, mucosal ulceration.)

-  Bacteria produce gangrene and perforation develops within days.  

- Infection may spread to mesentery leading to thrombosis and secondary arteritis of mesenteric vessels.

(Note: Microscopic study of mesenteric vessels is important to rule out vasculitis.)

CHRONIC ISCHEMIA:
Segmental or patchy distribution.
Mucosal inflammation, submucosa filled with granulation tissue, ulceration, patchy  fibrosis, stricture.

PREDISPOSING CONDITIONS;

1. Arterial occlusion:  Atherosclerosis, stenosis of celiac axis, dissecting aneurysm, angiography, surgery .

2. Pathology in mural vasculature:
Vasculitis-(Collagen vascular disorder, polyarteritis nodosa) . Compression -( Volvulus, hernia).
Amyloidosis.

3. Venous occlusion: Cirrhosis, sepsis, hypercoagulable states, surgery, trauma.

4. Nonocclusive ischemia:  Cardiac failure, shock, dehydration, vasoconstrictive drugs.

Lesions causing small bowel obstruction and bleeding  ; Meckel's Diverticulum.