Abdominal wall1,2

The muscles of the abdominal wall fall into two groups, the ventrolateral and the sublumbar. The sublumbar group are more properly considered part of the girdle division of hind limb musculature. The muscles within the ventrolateral group comprise the flank and abdominal wall muscles which are of more immediate relevance for abdominal hernias.

The intrinsic musculature of the flank and abdominal wall comprises three broad, fleshy sheets that lie one on top of the other, with contrasting orientation of muscle fibers. Ventrally each is continued by means of an aponeurotic tendon that has a main insertion along the linea alba. A fourth muscle, the rectus abdominis, lies along the ventral abdominal wall to either side of the linea alba, where it is ensheathed by these aponeurotic tendons.

The external abdominal oblique (EAO) muscle has its origin over the lateral surfaces of the ribs and the lumbar fascia from where the bulk of the fibers run caudoventrally, radiating outwards slightly. The aponeurosis of the EAO divides into two crura, with the larger medial crus terminating on the linea alba after passing around the rectus abdominis. The smaller lateral crus attaches to fascia over the iliopsoas, and the pubic brim lateral to the insertion of the rectus abdominis.

The internal abdominal oblique (IAO) muscle arises predominantly from the tuber coxa, with smaller points of origin from the lateral crus of the EAO, thoracolumbar fascia and the transverse processes of the lumbar vertebrae. The fibers of the IAO run largely caudoventrally, and the central fascicles insert via an aponeurotic tendon to the linea alba after passing round the rectus abdominis. Towards the midline insertion there is usually some interchange of fibers between the aponeurosis of the IAO and EAO. The IAO has a free caudal margin, which is important with regard to the formation of the inguinal canal. In the dog, the cremaster muscle is formed from a caudal slip of muscle from the IAO, but in the cat the levator scroti muscle replaces the cremaster, which is absent. The levator scroti muscle is formed from fibers of the caudoventral border of the external anal sphincter and the pars caudalis of the external anal sphincter.³

The transversus abdominis (TAB) arises from the inner surfaces of the last ribs and the transverse processes of the lumbar vertebrae. The fibers run transversely internal to the rectus abdominis to their aponeurotic insertion at the midline.

The rectus abdominis (RA) is a broad band of muscle on either side of the linea alba that has a segmental appearance caused by irregular transverse septae. It takes its origin from the ventral surfaces of the rib cartilages and inserts at the pelvic brim via the strong prepubic tendon in dogs. It has been speculated that in cats the strong attachments of the crura of the superficial inguinal ring on the iliopubic eminence, plus the EAO muscle aponeurosis to the medial thigh may serve the same function as the prepubic tendon does in the dog.4,5 Despite the anatomic difference, it is standard practice to refer to a prepubic tendon in cats (Fig. 25-1).

The rectus sheath is the term used for the anatomic arrangement of the aponeuroses from the IAO, EAO and TAB around the RA. The EAO aponeurosis always lies external to the RA, but the IAO aponeurosis may lie either internal or external, depending on the portion of the abdominal wall studied. In the cranial third of the abdomen, fibers pass both internal and external to the RA, but from the umbilicus caudally all fibers pass external. All fascial structures lying external to the RA are termed the external sheath or external leaf, while those lying internal to the RA are termed the internal sheath or leaf. The internal rectus sheath is lined by a thin layer of transversalis fascia and peritoneum.

The linea alba is the ventral midline fibrous zone where the right and left abdominal walls meet. In cats the linea alba is a broad white band, typically up to 3–4 mm wide, whereas in the dog the linea alba is a shallow trough rarely more than 2 mm wide.

The inguinal canal is located adjacent to the groin, between the fleshy part of the IAO on one side and the lateral crus of the EAO on the other. Other than where the external pudendal vessels, the genitofemoral nerve, and the spermatic cord (male) or vaginal process (female) pass through the canal, it contains only fatty tissue. In male dogs the peritoneum evaginates through the inguinal canal, to form the vaginal tunic around the spermatic cord and testis while in the bitch it envelops the round ligament of the uterus and is called the vaginal process. Recent dissection of eleven embalmed adult female cats, however, failed to identify any evidence of either a vaginal ring or process.⁶ Similar anatomic information is not currently available in the literature for male cats; however, anatomic dissection of three male cadavers identified a consistent vaginal tunic, with no evidence of a vaginal process (personal communication, Joshua Milgram; also Fig. 25-2).

The deep inguinal ring is a slit-like entrance to the abdomen along the free caudal edge of the IAO, while the superficial inguinal ring is contained between the two crura of the EAO aponeurosis.

Pelvic diaphragm1–3

The perineum describes the part of the body wall that covers the pelvic outlet, and surrounds the anal and urogenital canals. The sacrotuberous ligaments in the dog form the lateral borders; cats, however, do not have a sacrotuberous ligament.³ The principal structure contained within the perineum is the pelvic diaphragm. This is a muscular structure consisting of the coccygeal and levator ani muscles, together with their internal and external fascial coverings. These muscles are anchored to the caudal vertebrae and pelvis. The ischiorectal fossa is a wedge-shaped depression formed between the external anal sphincter, levator ani and coccygeal muscles medially, the internal obturator muscle ventrally, and the superficial gluteal muscle laterally. The pudendal nerve and internal pudendal artery and vein pass into the fossa on the ventrolateral surface of the coccygeus before passing across the dorsal surface of the internal obturator muscle.

Anatomical location

Describing the anatomical locations is probably the most common way of classifying hernias, e.g., umbilical, inguinal, or perineal. External abdominal hernias are defects in the external abdominal wall that allow protrusion of abdominal contents; internal abdominal hernias are those that occur through a ring of tissue contained within the abdomen, e.g., hiatal hernia.

Congenital or acquired

A congenital defect is present from birth and is usually due to failure of embryonic signaling and fusion, such as with certain umbilical hernias. Although the defect or hernia ring is present from birth, herniation may not necessarily manifest until later in life. An acquired hernia occurs sometime after birth, and is usually traumatic or iatrogenic in origin, although some may be degenerative.

Contents

A hernia is reducible when its contents lack adhesions, are freely moveable and can be readily manipulated back into the anatomic cavity from whence they came. If hernia contents cannot be replaced readily then it is considered non-reducible. Incarceration occurs when adhesions form between the hernia contents and surrounding structures, such as between intestinal loops and the body wall in inguinal hernias. Strangulation occurs when vascular supply to structures within the hernia is compromised. This can be from direct compression on viscera/vessels; torsion of a long vascular pedicle; or contraction of the hernia ring around the contents.

Physical examination

Initial observation should cover the animal’s demeanor, mentation and gait, hydration, and nutritional state. A thorough systematic examination of all core elements should always be performed. Gentle abdominal palpation may assist in identifying organomegaly, displacement of organs, or localizing pain, although in painful or obese patients palpation may be unrewarding. If there is pain in response to palpating the hernia this can suggest ischemic damage or excessive pressure accumulation, while color changes over the skin surface may reflect deeper vascular compromise or inflammation secondary to fat or other tissue necrosis. Gastrointestinal signs such as anorexia or vomiting may develop with entrapment of intestinal loops, or urinary tract obstruction with post-renal azotemia where the bladder is involved. Rectal examination is essential if there is any suspicion of perineal hernia.

Further comments specific to each type of abdominal hernia follow later in this chapter.

Laboratory tests

Laboratory tests will not specifically diagnose the presence of a hernia, but they may form an important part of assessment and planning or provide a baseline against which to assess postoperative progress. The extent of testing required preoperatively will obviously depend both on the patient and the presenting complaint. A young animal in good health undergoing an elective surgery will require less testing than one due to undergo a more complex procedure, or an emergency procedure.⁷

Diagnostic imaging

For patients with uncomplicated, small, reducible hernias, routine abdominal imaging is unlikely to be of significant benefit unless there is a specific indication, e.g., checking for peritoneopericardial hernia with a supraumbilical defect. If the history and/or physical examination are suspicious for entrapment or strangulation of organs within hernia sacs, then ultrasound examination is most likely to be helpful. Plain and contrast radiography may be useful in specific cases, such as where bladder involvement is suspected (Fig. 25-3). Where traumatic body wall ruptures are encountered, further imaging is always warranted to investigate not only the abdominal injury but also concurrent pathologies such as possible pulmonary contusions, pneumothorax or orthopedic injuries. Advanced imaging modalities such as CT or MRI can provide excellent levels of detail, and a CT scan is strongly recommended for investigation of traumatic abdominal wall hernias in humans.⁸ These modalities are unlikely to be necessary for routine investigation of abdominal hernias in veterinary patients unless the hernia is traumatic and concurrent neurological injury is suspected.

Physical findings

Umbilical hernias are usually identified soon after birth, unless the defect is very small. A very small hernia ring that contains only falciform fat is unlikely to cause major problems in the short term, allowing repair to be postponed until the patient is older (Fig. 25-4). If, however, the defect is large enough to allow herniation of intestinal loops or this is already happening then the hernia should be addressed regardless of age. In dogs there is a known association between umbilical hernia and cryptorchidism; similar information is not available for cats but the presence of other concurrent congenital anomalies should be carefully evaluated as these may affect prognosis as well as an owner’s willingness to treat.

Treatment and surgical techniques

It is recommended not to correct umbilical hernias in puppies less than 6 months of age as there is potential for delayed closure up to this time; no similar recommendations are available for kittens.¹⁴ The surgical technique is described in Box 25-1.

Aftercare and prognosis

Following repair of an uncomplicated umbilical hernia, the prognosis is excellent, with recurrence being uncommon and aftercare being as for any routine elective abdominal surgery. Where there is an incarcerated or strangulated hernia, then the aftercare and monitoring required may be intensive and the prognosis accordingly will be more guarded.