Feline Diaphragmatic Hernia Study Guide

Overview and Clinical Importance

Diaphragmatic hernia is the disruption of diaphragmatic continuity allowing abdominal viscera to herniate into the thoracic cavity. In cats, this condition can be traumatic (acquired) or congenital (most commonly peritoneopericardial diaphragmatic hernia or PPDH). This represents a significant category on the NAVLE due to its multisystemic presentation, requiring integration of respiratory, cardiovascular, and gastrointestinal knowledge.

The condition compromises respiratory function through pulmonary compression and ventilation-perfusion mismatch, while potentially causing cardiovascular compromise through reduced venous return. The liver is the most commonly herniated organ in cats (greater than 90% of cases), followed by intestines, stomach, and omentum.

Diaphragmatic Anatomy and Embryology

The diaphragm is a musculotendinous dome-shaped partition separating the thoracic and abdominal cavities. It consists of a central tendon (trifoliate in shape) surrounded by three peripheral muscular portions: the pars lumbalis (forming the crura), pars costalis (attaching to the costal arch), and pars sternalis (attaching to the xiphoid process).

Embryological Development

The diaphragm develops from four embryonic sources: the septum transversum (forming central tendon), pleuroperitoneal membranes (forming lateral portions), the dorsal mesentery of the esophagus, and muscular ingrowths from the body wall. During normal development, the pericardial cavity separates from the pleuroperitoneal cavity before the pleural and peritoneal cavities separate. Failure of this process results in PPDH.

Classification of Diaphragmatic Hernias

Etiology and Pathophysiology

Traumatic Diaphragmatic Hernia

The mechanism involves a sudden increase in intra-abdominal pressure against a closed glottis, creating a large pleuroperitoneal pressure gradient. The diaphragm, being the thinnest and weakest muscle in the abdominal compartment, ruptures at its weakest points (muscular portions). Common causes include motor vehicle accidents (85%), falls from height (high-rise syndrome), dog attacks, and other blunt abdominal trauma.

Peritoneopericardial Diaphragmatic Hernia (PPDH)

PPDH is the most common congenital anomaly of the diaphragm and pericardium in feline species, with a reported prevalence of 0.062-0.59% in domestic cat populations. It results from failure of fusion between the pericardium and ventral diaphragm during embryonic development, creating a stoma immediately dorsal to the xiphisternum. Associated midline defects may include umbilical hernia, sternal deformities, and pectus excavatum.

Pathophysiologic Consequences

• Respiratory compromise: Pulmonary compression, atelectasis, reduced lung volume, ventilation-perfusion mismatch
• Cardiovascular effects: Decreased venous return, reduced cardiac output, potential cardiac tamponade (PPDH)
• Gastrointestinal compromise: Organ strangulation, vascular compromise, gastric dilatation
• Hepatic involvement: Hepatic congestion, hydrothorax from venous occlusion of entrapped liver lobes

Clinical Signs and Physical Examination

Clinical presentation varies dramatically based on chronicity, severity, and organs involved. Acute traumatic cases typically present with respiratory distress, while chronic cases (including PPDH) may be subclinical or show intermittent vague signs.

Clinical Signs by Presentation Type

Exam Focus: Key physical exam findings include muffled heart/lung sounds, borborygmi auscultated in the thorax, and an abdomen that feels empty on palpation. In PPDH, concurrent sternal abnormalities or umbilical hernia may be present as associated midline defects.

Diagnosis

Radiographic Findings

Thoracic radiography is the primary diagnostic modality. At least two orthogonal views are essential as some findings may only be visible in one projection. Studies show radiographs reveal evidence of diaphragmatic hernia in approximately 66% of affected animals.

Additional Diagnostic Modalities

• Ultrasonography: Highly useful when radiographs equivocal; can identify liver/gallbladder adjacent to heart; demonstrates diaphragmatic discontinuity
• Echocardiography: Essential for PPDH; visualizes abdominal contents within pericardium; assesses cardiac function and rules out primary cardiac disease
• Contrast studies: Positive contrast celiography (1.1 mL/kg water-soluble iodinated contrast IP) or upper GI series when radiographs inconclusive
• CT/MRI: Gold standard for complex cases; clearly delineates herniated structures and defect size
• Laboratory findings: Often unremarkable; may show elevated liver enzymes if hepatic congestion; blood gas analysis reveals hypoxemia

Treatment

Surgery is the only definitive treatment for diaphragmatic hernia. The timing of surgical intervention depends on patient stability, with the current recommendation being to operate as soon as the patient is stable for anesthesia rather than waiting an arbitrary time period.

Preoperative Stabilization

• Oxygen supplementation: Face mask, oxygen cage, or flow-by; avoid stress
• Intravenous fluid therapy: Crystalloids for shock; avoid overhydration which worsens pulmonary function
• Thoracocentesis: If significant pleural effusion or pneumothorax present
• Positioning: Sternal recumbency preferred; elevate front end if tolerated
• Analgesia: Opioids (buprenorphine, methadone) that minimally affect respiration
• Emergency gastric decompression: Percutaneous needle if stomach herniated and dilated

Anesthetic Considerations

Anesthesia requires careful planning as diaphragmatic function is necessary for spontaneous ventilation. Key principles include rapid induction with injectable agents, immediate intubation, and mandatory intermittent positive pressure ventilation (IPPV). Airway pressures should be maintained around 15 cmH2O to minimize barotrauma.

Surgical Technique

• Approach: Ventral midline celiotomy from xiphoid to pubis; allows access to entire abdominal cavity and diaphragm
• Hernia reduction: Gently replace herniated organs; break down adhesions with blunt dissection (more common in chronic cases)
• Organ assessment: Evaluate viability of herniated organs; resect necrotic tissue if needed
• Herniorrhaphy: Simple continuous or interrupted pattern using absorbable suture (2-0 to 3-0 Vicryl or PDS)
• Air evacuation: Remove air via feeding tube through defect before final closure, three-way stopcock with syringe, or thoracocentesis
• PPDH-specific: Separate pericardium from diaphragm at hernia edges; suture diaphragmatic defect without reducing pericardial size

Treatment Summary by Hernia Type

Complications and Prognosis

Re-expansion Pulmonary Edema (RPE)

Re-expansion pulmonary edema is a potentially fatal complication occurring after rapid re-expansion of chronically collapsed lungs. Cats are more commonly affected than dogs. The pathophysiology involves microvascular damage from stretching of thickened, inflexible lung parenchyma, leading to permeability pulmonary edema. Risk is greatest when lungs have been collapsed for greater than 72 hours.

Prevention of RPE

• Gradual, slow lung re-expansion over 12 hours
• Avoid rapid or forced inflation during surgery
• Do not attempt to fully reinflate collapsed lobes immediately
• Maintain airway pressures around 15 cmH2O
• Consider thoracostomy tube for gradual post-operative air evacuation

Other Complications

• Cardiac arrhythmias: Common with traumatic cases; monitor ECG perioperatively
• Pneumothorax: Most common transient postoperative complication; may require thoracocentesis
• Hemorrhage: From adhesiolysis or concurrent injuries
• Organ necrosis: If strangulation occurred; may require resection
• Ischemia-reperfusion injury: When blood flow returns to previously ischemic organs

Prognosis

Memory Aids

PPDH = "Pericardial-Peritoneal Direct Highway"

Remember: PPDH creates a direct highway between peritoneum and pericardium, while the pleural spaces stay SEPARATE (intact). This is why you see an enlarged cardiac silhouette but no pneumothorax!

"HERNIA" Mnemonic for Diagnosis:

• History of trauma (or longhair breed for PPDH)
• Empty abdomen on palpation
• Respiratory distress (dyspnea, tachypnea)
• No diaphragm visible on radiograph
• Intestinal sounds in thorax (borborygmi)
• Auscultation: muffled heart/lung sounds

RPE Prevention: "SLOW"

• Slow re-expansion (12+ hours)
• Low pressure ventilation (15 cmH2O)
• Observe for edema signs closely
• Wary of chronic cases (greater than 72 hours collapse)