Here Kitty Kitty... Exploring Feline Dissection and Lesion Recognition
NECROPSY PROCEDURE FOR MAMMALS
Prepared and compiled by
Lois Ridgway, RVT, Technical Supervisor, Necropsy Laboratory, PDS, Inc.
Dr. A. Allen, BA, DVM, MVetSc, PhD, Professor, Department of Veterinary Pathology WCVM
Angela Turner, RVT, Department of Veterinary Pathology WCVM
The ability to perform a necropsy is an important diagnostic skill. A standardized necropsy technique should be adopted for all species, thereby facilitating a thorough necropsy examination. The following dissection protocol documents the procedures required to facilitate a complete examination of all organ systems. As a general rule, the examination proceeds methodically from an external to internal and cranial to caudal perspective. It is also helpful to approach a necropsy event from the perspective of it being a series of small procedures rather than one large one.
I External Examination
An external examination of the carcass is required to evaluate bodily condition, hydration status and conformation abnormalities. Palpation and manipulation of the limbs may reveal musculo-skeletal abnormalities such as fractures or swollen joints.
The body surface must be thoroughly washed to facilitate an examination of the skin and hair coat. Petechiation, ecchymoses, cyanosis, parasitic infestations and vesiculation are noted.
The external openings of the body are then examined, starting with the oral cavity, eyes and ears, the external genitalia and the mammary gland, if present, and finally the anus. The presence of exudates is noted.
Identifying marks, color patterns, tags, brands or tattoos are described and documented. Body weight is determined, if necessary.
II Opening
a) Skin and Limbs
The carcass of a monogastric species is placed in right lateral recumbancy with the abdomen facing the prosector. Mature ruminants are placed in left lateral recumbancy.
An incision is made into the skin and musculature at a point medial to the cubital (elbow) joint. This incision is extended cranially along the ventral midline to the intermandibular space, rostrally to the intermandubular synchondrosis. The ventral extrinsic muscles of the shoulder are severed and the forelimb is reflected backwards. The limb should lie flat on the dissection table.
An incision is made into the skin to permit the severence of the adductor muscles of the hind limb. The coxofemoral joint is exposed; the capsule and femoral ligament are severed and the synovial fluid examined. The pelvic limb is then reflected distally until it also lies flat on the dissection table. The skin is now reflected distally from the cervical and intermandibular regions and the skin that remains intact over the abdomen and thorax is now incised along the ventral midline. Care must be taken to avoid opening the abdominal cavity at this time. In the adult female, the mammary glands are now removed.
b) Tongue, Esophagus and Trachea
An incision is made through the sublingual muscles into the buccal cavity on both medial aspects of the mandibles. Both incisions should extend from the mandibular ramus to the symphysis. The tongue is then drawn through one of the incisions in the intermandibular space.
The tongue is reflected caudally toward the pharynx. The hyoid bones and their articulations are located at both lateral aspects of the pharyngeal area; they are severed at one articulation on both sides freeing the larynx, trachea and esophagus. Dissection of the esophagus and trachea continues to the thoracic inlet. Do not enter the thorax at this time.
c) Abdominal and Thoracic Cavity
The peritoneal cavity is now opened carefully to avoid lacerating viscera. The first incision is made through the abdominal musculature caudal to the last rib and must extend from the xiphoid process (ventrally) to the lumbar transverse process (dorsally). The incision continues caudally across the lumbar area, passing through the para lumbar fossa, and then should extend ventrally toward the inguinal area and linea alba. [When an intact male is necropsied, the inguinal canal and vas deferens must be located and the abdominal musculature incised cranial to their location.] The abdominal wall “flap” is then reflected toward the prosector.
The abdominal cavity is now visually examined for the presence of abnormal fluids and/or displacement of viscera.
The tonus and doming of the diaphragm are evaluated and the presence of negative pressure in the thoracic cavity is confirmed by either making a small hole in the diaphragm and listening for the sound of inrushing air or noting the sagging of the diaphragm when it is punctured.
To open the thoracic cavity, the costal cartilages are first cut immediately dorsal to the ventral midline. The thoracic wall is then elevated to permit the use of rongeurs to sever the ribs adjacent to the vertebral column. The thoracic wall is then removed.
The thoracic cavity is visually inspected for abnormal pleural or pericardial fluids and/ or visceral displacement.
To prevent spillage and contamination of organs from stomach or intestinal contents, ligatures are now placed in strategic locations:
- The esophagus – one tie is made cranial to the diaphragm.
- The duodenum – two ties are made, the first is placed approximately 6 cm caudal to the pyloric valve; the second is made two inches caudal to the first. A portion of pancreas tissue is included in both ties. (In monogastric species with gallbladders, the objective is to place the ties below the bile duct entrance into the duodenum. In ruminants, the objective is to place the ties
- anterior to the bile duct entrance into the duodenum.
- The rectum – two ties are made, approximately two inches apart.
*the tissue is then severed between the ties.
III Removal of Organs
a) the Pluck
The pluck, which includes the tongue, esophagus, larynx, trachea, lungs, thymus, heart, thyroid and parathryoid glands, is now removed. Grasp the tongue and continue dissecting caudally along the spinal column through the esophageal and aortic (mediastinal) attachments to the level of the diaphragm. The aorta and vena cava should be severed just cranial to the diaphragm. Beginning at the cranial ventral aspect of the thoracic inlet and extending to the diaphragm, cut through the pericardial sac attachments to the ventral thoracic midline. Thymus tissue may be present in this area in the juvenile animal. The pluck should be set aside for further examination.
b) the Liver, Stomach and Proximal Duodenum
Prior to the removal of any abdominal organs, the adrenal glands should be both located and removed.
The spleen and associated omentum and ligaments are now removed in monogastric species. In the ruminant species, the greater and lesser omentum is removed from around the intestines and from the border of the abomasum; the spleen is left in situ.
The diaphragm dissection is now completed and the portion of mesoduodenum adhering the duodenum to the cranial abdominal wall is severed to facilitate removal of the liver, stomach and upper duodenum as one unit.
c) the Intestinal Tract
The remaining portion of duodenum and the entire jejunum, ileum, cecum and large colon are removed as a unit by cutting through the duodenocolic fold and the mesoduodenum (root of the mesentery). In some species this unit will include the spiral colon.
d) the Urogenital System--Opening the Pelvis
The urogenital system is removed in conjunction with the terminal large intestine and the anus. This system should be examined in situ and all components located. The kidneys are elevated and the renal arteries and veins severed. Each kidney and ureter is now dissected caudally towards the urinary bladder.
In the intact female, the ovaries and the horns of the uterus are dissected from their attachments–the suspensory ligament and the broad ligament are cut close to the parietal peritoneum.
In the intact male the testes and vas deferens should be removed from the scrotum and inguinal canals respectively and moved into the abdomen. The penis is then dissected from its ventral (caudal, in domestic feline) location toward its ligamentous attachment at the caudal aspect of the pelvis.
The pelvis is now opened. One of two methods may be used:
- Two parallel saw cuts are made, each lateral to the symphysis pubis through the obturator foramen. The loose symphysis portion is then removed. OR
- One cut is made through the symphysis pubis and another is made through the wing of the ilium. The loose segment of pelvic bone is then removed. (this method is often preferred as it creates the largest pelvic opening).
The urogenital system components along with the rectum is now drawn caudally through the space in the pelvis and detached. In the female this required a subcutaneous incision around the anus and vulva. In the male, the two ligaments (crura) that hold the penis to the ischial arch must be severed carefully to avoid cutting into the urethra; a subcutaneous incision is made around the anus.
IV Examination of the Remaining Carcass
Several lymph nodes should be examined. Again, use a cranial to caudal approach beginning with cervical nodes, retropharyngeal, prescapular, mediastinal, inguinal, femoral and popliteal.
The major muscles of the thoracic and pelvic limbs must be incised and examined for abnormalities.
A femur is removed for evaluation. The left femur is the standard choice as the coxo-femoreal joint as already been opened on that side. An inspection of the femoro-tibial joint is also achieved as the femur is removed.
The animal’s head is now removed. The atlanto-occipital joint can be palpated from the ventral aspect if the head is extended and the articulation is flexed. This joint is opened; spinal cord severed and cerebro-spinal fluid visually examined and/or collected, as necessary. Proceed to detach the head by inserting a knife or scalpel into the atlanto-occipital foramen and cut through the spinal cord and ligaments. This will loosen the head sufficiently to allow its removal; cut through any remaining soft tissue and skin.
At this point in the necropsy dissection several joints will have been opened including the left coxo-femoral, femoro-tibial joint and the atlanto-occipital joint. The carpal, hock and stifle joints are easily exposed from the medial aspect. Remove the skin and hair from the joint area to prevent contamination with debris and then locate the area of articulation by flexing the joint; incise the medial joint capsule and ligaments. An evaluation of synovial fluid volume, consistency and color is appropriate. Examine the articular cartilage.
V Systemic Examination of Organs
A clean table surface is essential for this procedure, otherwise changes in organs may be missed if contaminated by blood, ingesta or other debris.
Generally, tissues required for histopathology, microbiology or other diagnostic examination are collected at the time of detailed examination, in the order that follows:
| Tongue: | The tongue is examined for the presence of foreign bodies, lacerations, erosions or ulcers. The muscle should be incised longitudinally and crosswise in several locations. |
| Esophagus: | The esophagus is opened from the pharynx distally. The mucosal surface is examined closely for erosions and ulcers. |
| Trachea: | A cut is made through the dorsal midline of the larynx and down through the trachea to its bifurcation. The tracheal mucosa is examined for evidence of such events as hemorrhage or necrosis; the contents are noted and may include froth, blood or ingesta. |
| Heart: | The heart is examined before the lungs. The pericardial sac is incised and any adhesions or abnormal fluid volume, consistency or color noted. The epicardial surface is examined for evidence of petechiation or ecchymosis or other abnormalities.
The heart is opened following the course of the blood flow. The apex of the heart is first removed and the relative thicknesses of the ventricles is compared–typically it is 3:1 (L:R). The cut surface is held toward the prosector and the first incision is made, staying close to the septum, through the right ventricle wall toward the inferior and superior vena cavas. The lumen of the vena cavas and the right ventricle is examined. A second incision is made, staying close to the septum, this time extending from the cut surface toward the pulmonary artery and its bifurcation into each lung. The arteries are examined for anomalies or abnormalities and the tricuspid valve inspected. Now position the heart with the left ventricle on top, the cut section toward the prosector. A cut is made between the two papillary muscle groups starting near the apex and cutting toward the pulmonary veins. When these veins, and their exits from the lungs, has been examined, the left ventricle should be spread open, a scissor blade is postioned beneath the left A.V. valve and a cut is made into the aorta. Note: if the heart being examined is that of a fetus or neonate, the foramen ovale and ductus arteriosus must be examined. The myocardial and endocardial surfaces should be examined for necrosis, hemorrhage, etc. It is important that the heart not be removed from the lungs. Vascular continuity is necessary to demonstrate such abnormalities as transposition of the great vessels. |
| Lungs: | The lungs are gently palpated to detect abnormal consistency. Scissors are used to cut down the full length of the bronchial tree. Cross sections are cut with a knife or scalpel through those parts of the lobes that appear to feel abnormal. The entire lungs should be examined. |
| Thyroid: | The thyroid glands are located on the lateral surfaces of the larynx (or ventral midline of the trachea in porcine) and examined. Changes in their expected size are noted. |
| Spleen: | A longitudinal incision is made into the spleen to allow an examination of the parenchyma. A portion may be required for microbiological study. |
| Liver: | The liver is positioned with the diaphragmatic surface on the table. |
| Stomach: | The stomach is opened, beginning at the esophageal area and continuing around the greater curvature to the pylorus and into the duodenum to site where the duodenal tie has earlier been placed. The stomach and duodenum are spread open. Gastric contents are examined in terms of quality, consistency and quantity. The fundic and pars esophageal areas of the gastric mucosa and duodenal mucosa are examined for abnormalities.
The patency of the bile duct is ascertained by locating the gallbladder and gently pressing it to cause passage of bile into the bile duct and finally into the opened duodenum. The diaphragmatic surface of the liver is now wiped clean and examined. The visceral surface is also examined and several incisions are made into the hepatic parenchyma. |
| Intestines: | The intestines are laid out on a flat surface in a parallel loop array for detailed inspection. The duodenum is examined first; then the jejunum, ileum and spiral colon (if present), cecum and descending colon are opened. Their contents are evaluated in terms of volume, color and consistency; any abnormalities are noted. The pancreas should be visually inspected and palpated. The mesenteric lymph nodes are examined and incised. |
| Adrenal: | The adrenal glands are sectioned and examined. Changes in morphology or color are noted. |
| Urinary: | The kidneys are sectioned longitudinally from one pole to the other; from cortex through to the pelvis. The capsule is carefully stripped with forceps. Normally, it will strip easily and leave behind a smooth, shiny cortical surface.
If the ureters are large enough to permit entrance of a scissor blade, they are opened. They are examined from their point of formation at the kidney to their entrance into the urinary bladder. The urinary bladder is opened; its contents and mucosal surfaces are inspected. The urethra is also opened and examined. |
| Genital: | An incision is made through the vulva, into the roof of the vagina. If the animal has not been spayed, the dissection should continue through the cervix into the body of the uterus. A cut is made with scissors, into the horns of the uterus. The mucosal surfaces of these structures is examined.
The ovaries should be palpated, visually inspected for abnormalities and stage of estrus cycle; then sectioned. |
| Bone Marrow: | Excess muscle is trimmed from the femur. Hold the femur obliquely over the table edge, hit it sharply with the back of a knife. This will fracture the femur mid shaft and provide an oblique opening of the medullary cavity facilitating a visual inspection and removal of bone marrow. A longitudinal opening, made with a saw, will facilitate examination of the epiphyses, physis and diaphysis regions as well as the medullary cavity and bone marrow. |
| Brain: | The skin and muscles overlying the frontal, parietal and occipital bones are removed from the surface of the head.
Place the head securely in a vice. Use a saw to create two lateral cuts, at 45 degree angles on the medial aspect of bothoccipital condyles. A third transverse cut is made just behind the orbits to connect the lateral cuts. Note that these cuts must not penetrate the brain parenchyma. A chisel is inserted a few millimetres into the saw cuts at various locations to pry off the bony cap and expose the meninges and brain. Scissors and forceps are used to remove the portion of meninges and tentorium cerebelli that is visible through the opening just created. Remove the head from the vice. Position the head upside down (dorsal) and use scissors to cut the cranial nerves--beginning at the foramen magnum and medulla oblongata. The brain is allowed to gradually fall into the prosector’s hand. The alternate methods of brain removal may be selected based on species anatomical differences, size and maturity of the animal. They include:
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| Nasal Cavities: | The head is now gripped firmly by the cranial vault, and a transverse saw cut is made through the maxillae at the level of the second premolar teeth, across the nasal septum. This cut permits examination of the turbinate bones for evidence of atrophic rhinitis as well as inspection of the nasal passages for discharges or neoplastic changes. |
| Spinal Cord: | The spinal column is removed from the carcass by cutting the remaining ribs and limbs away. Remove all overlying skin and as much skeletal muscle as possible from both the dorsal and ventral aspects. The size and age of the specimen and the equipment available will influence which of the following methods is used:
When the spinal canal has been opened wide enough to permit passage of the spinal cord, forceps should be used to grasp the meninges while scissors are needed to cut the nerves. |
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