Incisional Hernia

Incisional hernia is a postoperative complication characterized by the disruption of the abdominal wall layers at the site of a previous surgical incision, leading to the protrusion of intra-abdominal contents (omentum, intestine, or other viscera) through the defect. It is a form of ventral hernia that occurs specifically along the linea alba or paramedian incisions used for laparotomy. The hernia sac is typically lined by peritoneum and may contain omentum or bowel. Incisional hernias can range from small, asymptomatic defects to large, complicated hernias with incarceration or strangulation of viscera. They are classified based on size, location, reducibility, and the presence of complications. The condition is a significant surgical challenge due to the compromised tissue quality, previous scar formation, and potential for recurrence. Surgical repair is the mainstay of treatment, with techniques ranging from primary closure to mesh augmentation, depending on the defect size and tissue integrity.

The primary etiology of incisional hernia is the failure of the abdominal wall to heal properly after a laparotomy incision. This can result from technical errors during closure, such as excessive tension on suture lines, inappropriate suture material or pattern, inadequate tissue bites, or premature suture failure. Systemic factors that impair wound healing, such as malnutrition, hypoproteinemia, diabetes mellitus, hyperadrenocorticism, obesity, and chronic corticosteroid use, also contribute. Postoperative complications like wound infection, seroma formation, or excessive intra-abdominal pressure (from vomiting, coughing, or tenesmus) can lead to dehiscence of the muscular and fascial layers. Additionally, conditions that increase intra-abdominal pressure, such as pregnancy, ascites, or abdominal distension, can predispose to hernia formation. Iatrogenic factors, including overly aggressive tissue handling, devascularization of wound edges, or closure under tension, are also significant. In some cases, the hernia may develop weeks to months after surgery due to gradual weakening of the scar tissue.

Incisional hernias are reported in both dogs and cats, with an incidence ranging from 2% to 15% following laparotomy, depending on the study and the presence of risk factors. There is no strong breed or sex predisposition, but older animals and those with concurrent diseases (e.g., obesity, endocrinopathies) are at higher risk. Working dogs or those with high activity levels may have increased risk due to mechanical stress on the incision. Cats may have a slightly lower incidence, but similar risk factors apply. The condition is more common after emergency surgeries, contaminated procedures, or when the patient has a compromised immune system. The use of certain suture materials (e.g., rapidly absorbable sutures) and improper closure techniques increase the likelihood of hernia formation. In veterinary medicine, incisional hernias are less common than in human surgery, but they remain a clinically significant complication.

The pathophysiology of incisional hernia involves a complex interplay of impaired wound healing, mechanical stress, and tissue degeneration. After laparotomy, the healing process begins with an inflammatory phase, followed by proliferation and remodeling. If the fascial edges are not adequately apposed or if there is excessive tension, the healing tissue is weak and prone to disruption. Collagen synthesis and cross-linking are critical for tensile strength; any factor that disrupts this process (e.g., infection, malnutrition, corticosteroid use) leads to a weak scar. Over time, the scar tissue may stretch or tear, allowing the peritoneum and abdominal contents to herniate through the defect. The hernia sac is formed by the peritoneum and may contain omentum or bowel. If the hernia becomes incarcerated, the blood supply to the herniated viscera may be compromised, leading to ischemia, necrosis, and strangulation. This can result in peritonitis, sepsis, and shock. The increased intra-abdominal pressure from the herniated contents can further impair respiratory and cardiovascular function.

Predisposing factors for incisional hernia can be divided into patient-related and procedure-related factors. Patient-related factors include obesity, which increases intra-abdominal pressure and impairs wound healing due to poor vascularity of adipose tissue; malnutrition, particularly protein deficiency, which reduces collagen synthesis; advanced age, which is associated with decreased tissue healing capacity; and concurrent diseases such as diabetes mellitus, hyperadrenocorticism, or chronic kidney disease. Use of immunosuppressive drugs (e.g., corticosteroids) also impairs healing. Procedure-related factors include the type of incision (midline vs. paramedian), the length of the incision, the suture material and pattern used, the experience of the surgeon, and the presence of postoperative complications such as wound infection or seroma. Excessive tension on the closure, inadequate tissue bites, or tying sutures too tightly can cause tissue ischemia and necrosis. Additionally, early excessive activity or trauma to the incision site can disrupt the repair.

Clinical signs of incisional hernia vary depending on the size and contents of the hernia. Small hernias may be asymptomatic and detected incidentally on physical examination as a soft, reducible swelling along the incision line. Larger hernias may cause visible bulging, especially when the animal stands or strains. The swelling may be reducible with gentle pressure, and a fascial defect can often be palpated. If the hernia contains omentum or bowel, the animal may show signs of discomfort, vomiting, anorexia, or constipation. In cases of incarceration or strangulation, the hernia becomes firm, painful, and non-reducible, and the animal may exhibit signs of acute abdomen, including severe pain, abdominal distension, vomiting, and shock. Fever, lethargy, and signs of sepsis may be present if strangulation has occurred. In some cases, the hernia may be associated with a seroma or abscess, leading to localized swelling and drainage.

Differential diagnoses for incisional hernia include: 1) Seroma: A fluid-filled swelling that is typically soft, fluctuant, and non-painful, without a palpable fascial defect. It usually resolves with conservative management or aspiration. 2) Hematoma: A collection of blood that may be firm and painful, often associated with trauma or bleeding disorders. Ultrasound can differentiate. 3) Abscess: A localized infection that presents with heat, pain, and purulent discharge. It may be associated with systemic signs. 4) Neoplasm: A soft tissue mass (e.g., lipoma, sarcoma) that may be firm or soft, but lacks a fascial defect. Imaging and biopsy are needed. 5) Muscle rupture: A tear in the abdominal musculature without a peritoneal defect, which may cause a bulge but is less likely to contain viscera. 6) Eventration: Protrusion of abdominal contents through a wound without a peritoneal sac, often due to complete wound dehiscence. 7) Postoperative wound dehiscence: Complete separation of all abdominal wall layers, leading to evisceration, which is an emergency. 8) Inguinal or umbilical hernia: These are located at specific anatomical sites and are not associated with a previous incision. 9) Lymphadenopathy: Enlarged superficial lymph nodes may mimic a hernia but are located in specific regions. 10) Subcutaneous emphysema: Air in the subcutaneous tissues may cause crepitus and swelling, but there is no fascial defect.

The diagnostic algorithm for incisional hernia begins with a thorough history and physical examination. Palpation of the incision site may reveal a fascial defect and reducible or non-reducible swelling. If the hernia is suspected, imaging is recommended to confirm the diagnosis and assess the contents. Abdominal radiography may show a soft tissue opacity or gas-filled loops of bowel within the hernia sac. Ultrasonography is highly useful to identify the fascial defect, the contents of the hernia (e.g., omentum, bowel), and to assess for complications such as strangulation. Doppler ultrasound can evaluate blood flow to herniated bowel. In complex cases, computed tomography (CT) may be used to provide detailed anatomical information, especially for large or recurrent hernias. If strangulation is suspected, emergency surgery is indicated without further imaging. Laboratory tests, including complete blood count, serum biochemistry, and electrolytes, are performed to assess the patient's overall health and to identify any underlying conditions that may affect surgical planning.

Laboratory findings in incisional hernia are generally non-specific unless complications such as strangulation or peritonitis are present. In uncomplicated cases, blood work may be within normal limits. If there is incarceration or strangulation, the complete blood count may show leukocytosis with a left shift, and serum biochemistry may reveal elevated liver enzymes, azotemia, or electrolyte imbalances due to vomiting and dehydration. In cases of sepsis, there may be hypoglycemia, lactic acidosis, and coagulopathy. Preoperative coagulation panel (PT/aPTT) is recommended to assess surgical risk. Inflammatory biomarkers such as C-reactive protein (CRP) and serum amyloid A (SAA) may be elevated. If the hernia is associated with an underlying disease (e.g., hyperadrenocorticism), specific tests such as ACTH stimulation test may be indicated. Synovial fluid analysis is not relevant for this condition.

Imaging plays a crucial role in the diagnosis and management of incisional hernia. Radiography: Abdominal radiographs may show a soft tissue opacity or gas-filled loops of bowel within the hernia sac. In cases of strangulation, there may be evidence of intestinal obstruction, such as dilated loops of bowel. However, radiography is often inconclusive. Ultrasonography: This is the preferred imaging modality. It can accurately identify the fascial defect, the contents of the hernia (omentum, bowel, fluid), and assess for complications. The hernia sac appears as a hypoechoic or anechoic structure protruding through the abdominal wall. The contents can be identified by their echotexture and peristaltic activity. Doppler ultrasound can evaluate blood flow to herniated bowel, which is critical in determining viability. Computed Tomography (CT): CT provides excellent anatomical detail and is particularly useful for large, complex, or recurrent hernias. It can accurately measure the size of the defect, identify the contents, and assess for complications. CT is also helpful in surgical planning, especially when mesh repair is considered. Magnetic Resonance Imaging (MRI) is rarely needed but may be used to evaluate soft tissue detail in specific cases. Fluoroscopy can be used to assess dynamic changes in the hernia during straining.

Cytology and histopathology are not typically required for the diagnosis of incisional hernia, but they may be performed if there is a suspicion of an underlying neoplastic process or if the hernia is associated with a mass. Fine-needle aspiration of the hernia sac may yield fluid or cells that can help differentiate between a seroma, hematoma, or abscess. If a mass is present, biopsy and histopathology are indicated to rule out neoplasia. In cases of strangulation, histopathology of the resected bowel may show ischemic necrosis, hemorrhage, and inflammatory infiltrate. If a mesh is used, histopathology of the surrounding tissue may show a foreign body reaction. However, these are not routine diagnostic steps.

Treatment of incisional hernia is primarily surgical. Conservative management is rarely recommended due to the risk of incarceration and strangulation. Preoperative stabilization is essential, especially in cases of strangulation or sepsis. This includes fluid therapy, electrolyte correction, and broad-spectrum antibiotics. The surgical approach involves reopening the previous incision, excising the hernia sac, and identifying the fascial defect. The edges of the defect are debrided to healthy tissue. For small defects (<2 cm) with good tissue quality, primary closure with a tension-relieving suture pattern (e.g., near-far-far-near) using non-absorbable or slowly absorbable monofilament suture (e.g., polypropylene, polydioxanone) is appropriate. For larger defects or when there is excessive tension, mesh repair is indicated. Options include polypropylene mesh, expanded polytetrafluoroethylene (ePTFE), or biologic mesh. The mesh is placed either as an onlay (over the defect), inlay (within the defect), or sublay (under the rectus sheath). The mesh is secured with sutures or staples. In cases of strangulation, the herniated bowel must be assessed for viability. If non-viable, resection and anastomosis are performed. Postoperative care includes pain management, antibiotics, and activity restriction. The use of abdominal bandages may provide support but is not a substitute for surgical repair.

The prognosis for incisional hernia is generally good with appropriate surgical repair. The recurrence rate is reported to be between 5% and 20%, depending on the size of the defect, the presence of infection, and the technique used. Primary closure has a higher recurrence rate for large defects, while mesh repair has a lower recurrence rate but a higher risk of complications such as seroma, infection, and mesh migration. The prognosis is worse if the hernia is strangulated, as this is associated with a higher morbidity and mortality. Factors that negatively affect prognosis include obesity, malnutrition, chronic corticosteroid use, and postoperative wound infection. With successful repair, most animals return to normal function. However, long-term follow-up is necessary to monitor for recurrence.

Postoperative follow-up for incisional hernia repair is crucial to ensure proper healing and to detect any complications. The patient should be re-examined at 10-14 days for suture removal and assessment of the surgical site. Activity restriction is recommended for 4-6 weeks to allow adequate healing. Serial ultrasound examinations may be performed at 4, 8, and 12 weeks postoperatively to assess the integrity of the repair and to detect any early recurrence. If a mesh was used, imaging may be repeated at 6 months and 1 year. The owner should be instructed to monitor for any swelling, pain, or changes in the incision site. In cases of strangulation, the patient may require longer hospitalization and more intensive monitoring. Long-term follow-up is recommended to ensure that the hernia does not recur.

Clinical Pearls: 1) Always use a tension-relieving suture pattern (e.g., near-far-far-near) for closure of midline incisions, especially in large or obese patients. 2) Use a slowly absorbable monofilament suture (e.g., polydioxanone) or non-absorbable suture (e.g., polypropylene) for fascial closure. 3) Ensure adequate tissue bites (at least 1 cm from the wound edge) and avoid excessive tension. 4) Consider using a mesh for defects larger than 2-3 cm or when there is excessive tension. 5) In cases of strangulation, always assess bowel viability and resect if necessary. 6) Postoperative pain management is essential to reduce stress and prevent complications. Pitfalls: 1) Closing the incision under tension is a common mistake that leads to tissue ischemia and hernia formation. 2) Using rapidly absorbable sutures (e.g., catgut) for fascial closure is associated with a higher risk of hernia. 3) Failing to debride devitalized tissue before closure can lead to infection and dehiscence. 4) Overlooking the presence of infection or seroma can lead to delayed healing and hernia. 5) Inadequate postoperative activity restriction can cause early disruption of the repair. 6) Not recognizing the signs of strangulation can lead to catastrophic complications.

Perioperative drug protocols for incisional hernia repair are based on Plumb's Veterinary Drug Handbook. Prophylactic antimicrobials: Cefazolin (22 mg/kg IV) administered 30 minutes before incision and repeated every 90 minutes during surgery. Postoperative antibiotics are not routinely indicated unless there is contamination or infection. Analgesics: Opioids such as hydromorphone (0.05-0.1 mg/kg IV or IM q4-6h) or fentanyl (2-5 mcg/kg IV bolus, then 2-5 mcg/kg/h CRI) are used for perioperative pain. Non-steroidal anti-inflammatory drugs (NSAIDs) such as carprofen (2.2 mg/kg PO q12h) or meloxicam (0.1 mg/kg PO q24h) are used postoperatively for 3-5 days, provided there are no contraindications. Local anesthetic blocks, such as a linea alba block with bupivacaine (1-2 mg/kg) or lidocaine (2 mg/kg), can provide additional analgesia. Muscle relaxants are not typically used. If the patient has underlying conditions such as hyperadrenocorticism, adjustments may be needed. For example, in diabetic patients, insulin therapy may need to be adjusted. In cases of strangulation, broad-spectrum antibiotics such as ampicillin (22 mg/kg IV q8h) and enrofloxacin (5-10 mg/kg IV or PO q24h) may be indicated. Fluid therapy with balanced electrolyte solutions is essential to maintain perfusion.

The veterinary literature on incisional hernia is limited, but several studies have evaluated risk factors and outcomes. A retrospective study by Coolman et al. (1999) found that the incidence of incisional hernia in dogs after laparotomy was 2.5%, with obesity and wound infection being significant risk factors. Another study by Gower et al. (2008) compared primary closure with mesh repair for large ventral hernias and found that mesh repair had a lower recurrence rate but a higher complication rate. A study by Smeak (2003) emphasized the importance of proper suture technique and material selection in preventing incisional hernias. In human surgery, the use of mesh has become the standard of care for incisional hernia repair, and this is increasingly being adopted in veterinary medicine. A recent study by Balsa et al. (2019) evaluated the use of biologic mesh in dogs and found it to be safe and effective. However, there is a need for more prospective, randomized controlled trials in veterinary patients. The ACVS and ECVS have published guidelines on abdominal wall closure and hernia repair, emphasizing the importance of tension-free repair and the use of mesh for large defects.