Causes of pain in the abdomen:

A. Inflammation:

1. Appendicitis.
2. Diverticulitis. 
3. Cholecystitis
4. Pancreatitis.
5. Pyelonephritis.
6. Intra abdominal absecess.

B. Perforation or rupture :

1. Peptic ulcer.
2. Diverticular disease.
3. Ovarian cyst.
4. Aortic aneurysm.

C. Obstruction:

1. Intestinal obstruction.
2. Ureteric colic.

D. Extra-intestinal causes of chronic or recurrent abdominal pain : 

1. Diabetes mellitus.
2.  Addison’s disease.
3. Sickle cell disease.
4. Haemolytic disorders.
5. Drugs: Corticosteriods, lead, alcohol.

E. Other (rare);

1) Retroperitoneal:  Aortic aneurysm, Malignancy, Lymphadenopathy, Abscess.

2) Psychogenic: Depression, Anxiety, Hypochondriasis, Somatisation.

3) Locomotor: Vertebral compression, Abdominal muscle strain.

4) Metabolic/endocrine: Diabetes mellitus, Addison’s disease, Acute intermittent prophyria Hypercalcaemia.

5) Drugs/toxins: Corticosteroids, Azathioprine, Lead, Alcohol.

6) Haematological: Sickle cell disease, Haemolytic disorders.

7) Neurological: Spinal cord lesions, Tabes dorsalis, Radiculopathy.

Common causes of acute upper abdominal pain:

1. Acute pancreatitis.
2. Peptic ulcer disease.
3. Acute cholecystitis.
4. Acute cholangitis. 
5. Acute Cholangitis
6. Renal colic.
7. Acute inferior MI

Differential diagnosis of upper abdominal pain:

A. GIT:

1. Peptic ulcer disease.
2. Perforation.
3. Intestinal obstruction.
4. Perforated oesophagus.
5. Ca stomach.

B. Hepatobiliary & pancreatic:

1. Cholecystitis.
2. Hepatitis.
3. Pancreatitis.

C. Cardiovascular:

1. MI
2. Aortic aneurysm.

D. Renal:

• Pyelonephritis.

E. Others:

Pneumonia.

Ruptured spleen.

Approach to a patient with abdominal pain:

History:

• History of taking NSAIDS.
• History of previous peptic ulcer disease.
• Relation of pain with eating, defaecation or micturition.
• Nausea, vomiting.
• Weight loss.
• Change in bowel habit.
• Previous jaundice.
• Recent dysentery.
• Previous abdominal surgery.
• Previous tuberculosis.
• Personal history-Diet, smoking, alcohol.

Clinical evaluation:

The pain should be evaluated by the following points-

• Site
• Radiation
• Duration
• Severity
• Nature
• Mode of onset
• Aggravating and relieving
• Associated feature

Clinical examination of abdominal Pain :

1`. General examination:

• Hippocratic face in peritonitis.
• Body built-Cachectic in abdominal TB, malignancy.
• Knee-elbow position in acute pancreatitis. 
• Anaemia-In acute or chronic Gl bleeding.
• Jaundice.
• Clubbing-In IBD.
• Koilonychia – In iron deficiency anaemia due to chronic GI bleeding.
• Pulse and BP-To assess haemodynamic status.
• Temperature – Raised in inflammatory conditions.
• Dehydration – In vomiting, diarrhoea.

2) Examination of abdomen:

Features of peritonitis: Rigid abdomen. 

Tenderness in any particular area.

Ascites.

Abdominal mass.

Organomegaly.

Investigations of abdominal pain :

1. Full blood count (leucocytosis). 
2. Urea and electrolytes (dehydration).
3. Serum amylase or lipase level (acute pancreatitis).
4. Erect chest X-ray (air under the diaphragm).
5. Abdominal X-ray (obstruction).
6. Upper Gl endoscopy.
7. Abdominal ultrasound.
8. Contrast studies for intestinal obstruction.
9. CT (pancreatitis, retroperitoneal collections or masses.
10. Aangiography (mesenteric ischaemia).
11. Multi-slice CT angiography (newer technique).

Management of Abdominal Pain :

1. . Conservative: In acute abdomen-

• Nothing by mouth. Intravenous fluid.
• Empirical intravenous antibiotic.
• Intravenous proton pump inhibitor.
• Parenteral antispasmodic.

2. Specific treatment :  According to cause.