# Internal Medicine/Abdominal Pain
## Some Mechanisms of Pain Originating in the Abdomen
### Inflammation of the Parietal Peritoneum

Pain originating from the inflammation of the parietal peritoneum is characterized by a steady and aching quality. This type of pain is localized directly over the inflamed area and can be traced back to somatic nerves supplying the parietal peritoneum. The intensity of the pain hinges on the type and quantity of material that comes into contact with the peritoneal surfaces over a given period. For instance, a sudden release of a small amount of sterile acidic gastric juice into the peritoneal cavity can cause more pain than the same quantity of neutral feces. The pain associated with peritoneal inflammation is often accentuated by pressure or changes in tension of the peritoneum, and its character is transmitted through somatic nerves, allowing for its accurate localization.

### Obstruction of Hollow Viscera

Intraluminal obstruction is responsible for causing abdominal pain with distinct characteristics. Classic presentations involve intermittent or colicky abdominal pain that is not as precisely localized as the pain stemming from parietal peritoneal irritation. However, it's important to recognize that the absence of cramping discomfort in cases of hollow viscus distention can be misleading, as these instances may also give rise to steady pain.
Small-bowel obstruction, for instance, can manifest as poorly localized, intermittent periumbilical or supraumbilical pain. As the intestines continue to dilate and lose their muscular tone, the colicky nature of the pain may decrease. In cases of superimposed strangulating obstruction, pain may radiate to the lower lumbar region due to traction on the root of the mesentery.

### Vascular Disturbances

Abdominal pain originating from intraabdominal vascular disturbances can vary widely in its presentation. Common misconceptions about this type of pain include the belief that it's always sudden and catastrophic. While some disease processes, such as embolism or thrombosis of arteries, can indeed be accompanied by diffuse and severe pain, others may be characterized by seemingly insignificant discomfort. For instance, a patient with occlusion of the superior mesenteric artery might experience mild continuous or cramping diffuse pain for a few days before vascular collapse or signs of peritoneal inflammation become evident.
Interestingly, the pain in such cases is often caused by hyperperistalsis rather than peritoneal inflammation. It's noteworthy that the absence of tenderness and rigidity in the presence of continuous, diffuse pain ("pain out of proportion to physical findings") may indicate occlusion of the superior mesenteric artery. Additionally, certain cases of vascular rupture or occlusion can present with pain that radiates to specific regions such as the sacral area or the genitalia.

## Referred Pain in Abdominal Disease
### Thoracic and Extraabdominal Sources

Pain originating from thoracic and extraabdominal sources can sometimes mimic abdominal emergencies, necessitating careful consideration and evaluation. Diseases of the upper abdominal cavity, such as acute cholecystitis or perforated ulcer, may be associated with intrathoracic complications. Therefore, it's crucial to consider the possibility of intrathoracic disease in every patient with abdominal pain, particularly when the pain is situated in the upper abdomen.
Systematic questioning and examination can provide valuable insights into the presence of intrathoracic complications causing abdominal pain. Clinicians must actively explore the patient's history and perform a comprehensive physical examination to identify potential sources of referred pain. This thorough assessment helps in establishing the correct diagnosis and determining the appropriate course of action.

## Metabolic Abdominal Crises
### Variability and Complexity

Abdominal pain originating from metabolic causes presents a complex and diverse set of challenges for diagnosis. Metabolic diseases can manifest as abdominal pain that closely mimics other intraabdominal conditions. One example is hyperlipidemia, where pain mechanisms are characterized by severe hyperperistalsis. This phenomenon leads to abdominal pain that can be difficult to distinguish from the pain of intestinal obstruction. Similarly, conditions like C1 esterase inhibitor deficiency and porphyria can also cause abdominal pain with distinctive features.
Metabolic diseases add an extra layer of complexity to diagnosis due to their variable manifestations. In some instances, metabolic diseases may coexist with other conditions, further complicating the diagnostic process. A high level of suspicion and a comprehensive evaluation are crucial to accurately identify and differentiate these conditions from other causes of abdominal pain.

## Immunocompromise
### Complexities in Diagnosis

Immunocompromised patients, including those who have undergone organ transplantation, receive immunosuppressive treatments, or are affected by conditions such as AIDS, present unique challenges in diagnosing abdominal pain. In these cases, normal physiological responses may be absent or masked, leading to atypical presentations. Furthermore, unusual infections caused by pathogens like cytomegalovirus, mycobacteria, protozoa, and fungi can contribute to abdominal pain by affecting various gastrointestinal organs, including the gallbladder, liver, and pancreas.
Immunocompromised patients are at an increased risk of developing conditions that may cause abdominal pain. For instance, splenic abscesses due to Candida or Salmonella infection are considerations when evaluating patients with left upper quadrant or left flank pain. Acalculous cholecystitis, often observed in immunocompromised patients or those with AIDS, may be associated with cryptosporidiosis or cytomegalovirus infection.

### Specific Conditions

Among the specific conditions that can cause abdominal pain in immunocompromised individuals, neutropenic enterocolitis (typhlitis) stands out. This condition is often identified as a cause of abdominal pain and fever in patients with bone marrow suppression due to chemotherapy. Acute graft-versus-host disease is another significant consideration, particularly in the context of stem cell or bone marrow transplantation. Optimal management of these patients requires meticulous follow-up, including serial examinations to assess the need for further surgical intervention, such as addressing perforation.

## Neurogenic Causes
### Causalgic Pain and Nerve Injuries

Neurogenic causes of abdominal pain involve diseases that injure sensory nerves, leading to distinct patterns of discomfort. Causalgic pain is a type of neurogenic pain characterized by a burning sensation. This pain is usually limited to the distribution of a given peripheral nerve. What makes causalgic pain unique is its ability to transform stimuli that are normally not painful, such as touch or changes in temperature, into causalgic sensations. These sensations may persist even at rest, indicating the presence of an old nerve injury.
The detection of irregularly spaced cutaneous "pain spots" may be the only indication of a pre-existing nerve injury. Despite being triggered by gentle palpation, causalgic pain lacks associated rigidity of the abdominal muscles, and respiratory activity is usually unaffected. Additionally, distention of the abdomen is rare in cases of causalgic pain, and its occurrence isn't influenced by food intake.

### Functional Disorders

Functional gastrointestinal disorders can also contribute to abdominal pain of neurogenic origin. One prominent example is irritable bowel syndrome (IBS), a common functional disorder characterized by abdominal pain and altered bowel habits. IBS diagnosis relies on clinical criteria and the exclusion of structural abnormalities. The episodes of abdominal pain in IBS may be provoked by stress, and the pain's type and location can vary significantly among patients. Notably, symptoms such as nausea and vomiting are rare in IBS cases.
The mechanisms behind functional abdominal pain, including IBS, are not definitively understood. Nonetheless, recognizing the role of neurogenic factors in these conditions is essential for providing accurate diagnosis and effective management strategies.

## Approach to the Patient: Abdominal Pain
### Importance of Orderly Approach

In the evaluation of abdominal pain, the significance of an orderly approach cannot be overstated. While certain abdominal conditions may necessitate urgent operative intervention, maintaining a systematic approach remains vital for accurate diagnosis and appropriate intervention. Urgent operative procedures are rarely required, except in cases of exsanguinating intraabdominal hemorrhage, such as a ruptured aneurysm.

### Role of Examination and History

During the assessment of a patient with abdominal pain, critical inspection plays a key role in providing valuable clues. Simple observations such as evaluating facies, assessing the patient's position in bed, and observing respiratory activity can offer insights into the underlying condition. The depth and thoroughness of the examiner's approach directly impact the amount of information gleaned from the examination. Gentle and thorough examination techniques yield more accurate results and contribute to better patient care.
Attention to the patient's history is equally important. The chronological sequence of events in the patient's medical history often holds greater significance than the precise location of pain. Additionally, thorough examination of extraabdominal regions can provide essential diagnostic clues. Importantly, withholding narcotics or analgesics until a definitive diagnosis or treatment plan is formulated is unnecessary. Adequate pain relief does not obfuscate the diagnostic process.

### Diagnostic Aids

While laboratory examinations, radiography, and other diagnostic aids are valuable tools for assessing abdominal pain, they rarely serve as standalone diagnostic tools. Leukocytosis, for instance, should not be the sole criterion for determining whether surgery is indicated. Although a white blood cell count above 20,000/μL can indicate perforation of a viscus, other conditions such as pancreatitis, acute cholecystitis, pelvic inflammatory disease, and intestinal infarction can also cause marked leukocytosis.
Radiographic studies, such as plain and upright or lateral decubitus radiographs of the abdomen, have limited utility and may be unnecessary in some cases. In scenarios where surgical or medical intervention decisions are unclear, low-dose computed tomography (CT) scans offer better diagnostic accuracy compared to traditional radiography. Additionally, ultrasonography and laparoscopy play crucial roles in diagnosing specific conditions, such as ovarian cysts, tubal pregnancies, salpingitis, and acute appendicitis.

### Observation and Decision-Making

In some instances, even with the best available diagnostic aids and clinical expertise, a definitive diagnosis may elude healthcare providers during the initial examination. In such cases, an approach of watchful waiting, accompanied by repeated questioning and examination, can often lead to a clearer understanding of the patient's condition. This strategy allows for the natural progression of symptoms and signs to provide further insights into the underlying illness. Should doubts persist about the necessity of surgery or medical intervention, observation and repeated assessments may ultimately elucidate the true nature of the ailment and guide appropriate courses of action.