|Important key words or phrases.|
|Important concepts or main ideas.|
Tufts Cummings School of Veterinary Medicine
Anatomic Pathology Rotation
Richard M. Jakowski, DVM, PhD, DACVP
Tufts OpenCourseWare 2012
Throughout the course of your education at Tufts Cummings School of Veterinary Medicine, many lectures, especially those given by pathologists, have relied on photographs demonstrating disease lesions. In most instances, such photographs were made during autopsies of animal patients from our teaching hospitals. This fact illustrates one of the more important reasons for performing diagnostic autopsies in a veterinary teaching hospital—the autopsy is in essence a teaching experience. Everyone learns: the animal owner, the referring clinician, the pathologist, and, students on the fourth year pathology rotation.
Additionally, long after these animals have died and been autopsied in the Section of Pathology, high quality images, taken at the time of autopsy, continue to educate veterinary students, interns, and residents when they are used in lectures and clinical presentation. One of the most important educational experiences you will have during veterinary school is the opportunity to participate in diagnostic autopsies during your three week path rotation.
What will I learn in this rotation?
Every autopsy you participate in provides an opportunity to learn basic pathophysiologic mechanisms. This is the chief value of this rotation. Additionally, there are a few things you should know when the rotation is completed.
A systematic method of performing an autopsy. This is to insure all important organ systems are properly examined so you are able to understand why the animal died or was showing clinical signs while living.
How to collect tissues for histologic, microbiologic or toxicologic evaluation.
Learn some of the common post mortem changes that occur after death in all species and that often confuse fourth year veterinary students, pathology residents, and occasionally even experienced pathologists!
Learn to describe gross lesions with clear, concise sentences. For some students this seems to be one of the most difficult aspects of the pathology rotation.
Autopsy or Necropsy?
Debate continues and veterinarians often disagree on which term is most appropriate. Most veterinarians favor "necropsy" with an implication that "autopsy" should be exclusively reserved to describe a post mortem examination on humans. Yet I recall reading a letter to the editor of the New England Journal of Medicine from a physician pathologist who justified preference for "necropsy" which literally means "to view the dead." The author of this letter believed since literal translation of "autopsy" meant to perform a post mortem examinationon oneself as such was totally ludicrous.
The most compelling argument for autopsy was made by an esteemed colleague, Dr. Arthur Hertig, former Chairman of the Pathology Department at Harvard Medical School. He said the classic definition of autopsy should simply be translated as "I saw it myself." Since then I have preferred "autopsy" because it accurately defines the essence of this gold standard diagnostic procedure, in other words, to see for oneself the cause of death or disease through observation of structural abnormalities in the deceased - animal or human. Furthermore, when I talk to the non-medically trained they usually know what "autopsy" means. If I use the word"necropsy," I oftenhave to explain what the wordmeans – an unnecessary waste of time in my view.
Note: For a more recent justification of "Autopsy" over "Necropsy" see: Necropsy or Autopsy? It's all about communication! Veterinary Pathology 48: 271-272 (20120).
Note to students regarding autopsy technique:
During the course of your anatomic pathology rotation you will work with several pathologists. Similar to hospital clinicians, pathologists are trained at different institutions and exposed to various autopsy techniques which they adopt in performing autopsies. Students sometimes become frustrated when they learn an autopsy technique from one pathologist and then see another technique being used by another pathologist. This should not be cause forconcern. The most important thing is for you to examine all of the important anatomic regions and appropriately sample all the major organs. Additionally, in performing and autopsy it is also very important to learn how to discern a real gross lesion and a post mortem change.
The following protocol is provided.
General overview of the body and preliminary incisions
Holding the cadaver in dorsal recumbency, make deep incisions through the pectoral and serratusventralis muscles, then reflect the forelimbs away from the thorax. Disarticulate the coxofemoral joints.
Make a continuous, mid-ventral, skin incision from the mandibular to pubic symphyses: reflect the skin from the ventral aspect of the cervical, thoracic, abdominal and inguinal regions of the cadaver.
Assess the subcutaneous tissue for color, amount of adipose tissue and presence or absence of edema, hemorrhage, contusions, etc.
Open the abdominal and pleural cavities; when removing the ventral aspect of the rib cage also remove the ventral cervical musculature attached to the cranial portion of the sternum exposing the cervical portion of the trachea.
Assess the pleural and abdominal cavities for excessive fluid or exudate, also assess for normal anatomic relationship of the organs.
Examination of the abdominal cavity
Remove and examine the spleen and greater omentum.
Assess the common bile duct for patency then separate and uncoil the small and large intestine by cutting it from the mesenteric attachment.
Sever the esophagus at the stomach junction; retract the colon cranially, and sever it at the pelvic inlet.
Open the entire GI tract with a continuous, longitudinal incision (cut the stomach along the greater curvature). Examine the mucosal surfaces and contents of the stomach, intestine and cecum.
Remove and examine the liver.
Remove and examine both adrenal glands and both thyroid glands.
Remove the kidneys, ureters and urinary bladder as a single unit. Be sure to sever the urethra distal to the prostate gland and remove this organ with the urinary bladder.
Examination of the brain and spinal cord
Disarticulate the head at the atlanto-occipital junction, This is best accomplished from a ventral approach.
Reflect the skin and remove the temporalis muscles; carefully cut the calvarium with a Stryker saw. Remove the brain and pituitary gland.
Note: The entire brain should be placed in 10% formalin. If a rabies test is to be performed, remove appropriate samples before fixation. Note: Brain tissue sent for rabies examination should always be refrigerated, never frozen or fixed in 10 % formalin.
Several techniques are used in removing the spinal cord. The on-duty pathologist should be consulted for this procedure.
Miscellaneous organs and tissues
The sciatic nerve is located by deeply dissecting between the semimembranosus and semitendinosus muscles to the region of the posterior, mid-shaft of the femur.
Bone marrow should be sampled from the mid-shaft region of a femur. Use the large rib cutters to remove a 1 inch segment of bone and marrow; place this entire specimen in 10% formalin.
Enucleated eyes should be placed, intact in 10% formalin similar to the brain.
Commonly observed post mortem changes
Everyone is familiar with this term which literally means "stiffness of death." It is the result of post mortem contraction of skeletal muscle and gives rise to the slang term of "stiff" for a dead body. Ambient air temperature profoundly influences the time of onset of post mortem rigor; warm temperature hastening the onset of and cold delays it. Malnutrition and chronic debilitating disease will also slow the development of rigor and in some instances, completely prevent it from occurring.
A few additional points may be helpful in interpreting the effects of rigor in other organs:
Cardiac muscle, like skeletal musclealso undergoes post mortem rigor. This accounts for the commonly observed, relatively empty left ventricle associated with a seemingly distended right ventricle. The increased strength of the left side of the heart during the process of cardiac muscle rigor results in blood being forced back through pulmonary and systemic vasculature into right heart chambers. Obviously, this occurs only if the blood has not coagulated.
Smooth muscle rigor is most apparent as unusual color patterns on the mucosa of tubular or hollow organs such as the GI tract and urinary bladder. The mucosal surface usually becomes corrugated with linear intervals of blanched and red discoloration. The pattern, sometimes called "zebra stripes," has often been considered a lesion by the inexperienced.
Gravitational hypostasis (livor mortis)
Cessation of blood flow in cadavers results in an uneven redistribution of blood. Animals that die in lateral recumbency will show asymmetric color pattern of paired organs. Organs on the "down side" are darker than those of the contralateral side. Animals euthanized during surgery and in dorsal recumbency often show dark red dorsal lung lobes.
In the liver, gravitational settling of blood in addition to uneven compression of different lobes by gas distended segments of the GI tract or compression of the chest/diaphragm muscles undergoing rigor mortis result in a mottled pattern of the capsular surface. The so called "mottled pattern" of the liver commonly described in many autopsies is usually nothing more than a post mortem change.
Is there blood flow in a dead body?
Superficially, this question seems ridiculous. The heart ceases to function after death and blood pressure goes to zero. But does this mean there really is no blood flow? Contraction of skeletal muscle of the limbs tends to force blood out of these locations and into the large, central veins. Later, when post mortem gas begins to form in the GI tract, blood in the post cava is forced into the thoracic region. This results in blood moving into the only remaining expansile reservoirs that remain in the body – the heart and lungs. Cardiac muscle rigor further reduces this space to the right ventricle since the left ventricular muscles are much stronger significantly reducing the size of the left ventricular space. As a result of minimal but very significant pressures, after death there is a massive volume of blood forced into the lungs and right ventricle of the heart resulting in an impression that pathological conditions may be present in these areas. One of the most common mistake of students and even some pathologists is to interpret this change as "pulmonary congestion."
Post mortem blood clots
Most of the blood clots observed at the autopsy are the "currant jelly" variety, which are dark red, soft and friable. They are red because fibrin in the clot has not contracted which results in squeezing the red blood cells out of the clot. Currant jelly clots are often molded to the shape of blood vessels or cardiac chambers in which they are found.
"Chicken fat" clots are also commonly seen during the autopsy. These clots are also soft and friable, however, the color is pale tan/yellow. Chicken fat clots occur in animals that are in extremis for several hours prior to death. Sluggish blood flow results in antemortem clot formation with partial retraction of the loose fibrin aggregate. This results in extrusion of red blood cells and the resulting tan/yellow color of these clots. Strictly speaking, chicken fat clots are antemortem but they usually have formed several hours prior to death in a moribund patient.
Mottled appearance of various organs
Students performing their first autopsies frequently describe various types of organ mottling, however it is almost always non-significant. Mottling or blanching of organs occurs for different reasons depending on the particular organ in question. In addition to the liver which has previously been mentioned it is occasionally seen in portions of the GI tract that distends with gas after death. Smooth muscle rigor can account for subserosal mottling of the GI tract. Similar changes can be seen in the heart due to cardiac muscle rigor.
Enteric bacteria break down hemoglobin with the resultant formation of black pigment first seen in the large intestine. As these bacteria invade other portions of the cadaver, the same change is also observed in other locations. The liver, because of it direct communication to the GI tract through the biliary system, often shows blotchy areas of pseudomelanosis.
Green staining of the distal GI tract is also occasionally observed. This is a variation of pseudomelanosis however green, rather than black hemoglobin breakdown product is formed.
Post mortem imbibition
Blood: Hemolysis occurs after death and behaves as a common dye in the cadaver. Various shades of pink or red stained aortic intima or endocardial surfaces of the heart chambers are commonly seen in animals that have been dead for several hours.
Bile: Like hemoglobin, bile can also behave as a dye in the dead animal resulting in yellow or green staining of the liver, intestine in addition to the body wall and tissues adjacent to the gall bladder.
Post mortem crepitation
Gas-forming enteric bacteria especially Clostridia speciescommonly invade tissues after death forming gas bubbles in the subcutis, lungs and liver. Crepitation is a term used to describe the crackling or "bubble wrap-like" sensation detected when these tissues are handled.
Enzymes produced in the GI tract and pancreas are capable of degrading tissues after death. In the pancreas, digestion autolysis is often observed as poorly delineated, foci of hemorrhage in the pancreas. Absence of fibrinous inflammation in these regions indicates that this change is not a result of acute pancreatitis however, microscopic tissue examination is necessary to confirm this observation. Digestion autolysis rapidly ensues in the mucosa of the upper GI tract aided, to a large extent, by bacterial action.
This represents the postmortem rotting that eventually makes it impossible to gain useful information from an autopsy. The process begins soon after death (minutes) in the GI tract and rapidly spreads to the liver and biliary system. High ambient temperatures and prolonged retention of body heat in animals with a heavy hair coat accelerate this process. Large animals (over 500 pounds) cool slower than small animals simply due to the volume of tissue and the high specific heat properties of water.
To illustrate how rapidly this process can occur it may be helpful to remember that under certain conditions, enteric bacteria have been cultured from the heart and pulmonary blood vessels 1 hour after death.
Sodium Pentobarbital Artifact
Since many autopsy cases in veterinary teaching hospitals are performed on patients that have been euthanized by intravenous injection of sodium pentobarbital it is important to remember this agent can produce a number of post mortem artifacts that often confuse students and pathologists.
Lungs: Sodium pentobarbital euthanasia results in rapid depression of respiratory centers with resultant anoxia, cardiac arrest and pulmonary edema. The lungs are uniform dark red and are firm on palpation. Cut surfaces are wet, and froth and clear fluid are sometimes found in the trachea. Unfortunately, this change mimics those seen in naturally occurring pulmonary edema.
It is important to remember that pulmonary edema can also occur in a natural death. This has been poetically emphasized by the statement that "life's candle frequently sputters out in a watery lung." Since pulmonary edema can occur naturally, its presence requires careful interpretation and a knowledge of the manner of death. It is very important in such instances to know if death occurred naturally or was assisted with barbiturate euthanasia.
Spleen: Splenomegaly is also observed in some dogs euthanized with sodium pentobarbital. Because of the dynamic nature of the canine spleen, smooth muscle relaxation produced by pentobarbital results in a dramatic increase in the size of this organ. Cut sections are uniformly engorged with dark red, jam-like blood. This lesion was more common in the past when patients were anesthetized exclusively with sodium pentobarbital and occasional anesthetic over dosage resulted in death. It is less common today even with pentobarbital euthanasia where the injection is given in a single, rapid bolus injection.
Heart and great vessels: Bolus injection of sodium pentobarbital euthanasia solution frequently pools in the right atrium, right ventricle and pulmonary artery after the heart stops. Propylene glycol is the vehicle used to solubilize sodium pentobarbital and this chemical is highly irritating to tissue. In the living patient, intravenous injection of propylene glycol is quickly diluted in circulating blood and therefore does not result in irritation except in situations of an accidental, perivascular infusion. Concentration of euthanasia solution in the right ventricle and pulmonary arteries after death frequently cause the post mortem blood clots forming in these regions to take on a chocolate brown color with a friable, crumbly texture. Brown discoloration of the intemal surfaces of the right atrium, right ventricle and pulmonary artery can also occur. This change is the result of a chemical scald to red blood cells and tissues of these regions. Additionally, the faint alcohol-like odor of such clots in these regions will confirm that the change is a result of pentobarbital eusthanasia artifact.
Description of gross lesions
Tell me, and I forget.
Show me, and I might remember.
Involve me, and I understand.
Size of an organ or lesion
Always use a ruler to determine size of an organ you suspect as being larger or smaller than normal. The same is true in describing or the dimensions of specific lesions. This practice avoids using food or vegetables for comparison and pays dividends when you are eating dinner at some later time.
Fluid volumes within a body cavity should be quantified in conventional units (i.e., ml, liters, quarts, gallons). Be consistent throughout the autopsy but don't be too concerned about measuring every drop of fluid – when you observe fluid in the pleural cavity it's important to say 5, 50 or 250 ml were present as well as it was in the right, left or both pleural cavities.
Round, oval, linear, nodular, plaque like, well/poorly demarcated or circumscribed, sessile based, pedunculated, wart-like (verrucous) are some examples of shape descriptors.
This needs little explanation but remember that organs and lesions are rarely simply one color (i.e., red, white, yellow). Use color combinations in describing the color of an organ or lesion, it is always more accurate (i.e., red/brown, gray/tan, yellow/green).
Red....Fresh blood or fresh myoglobin
Red-orange:...Bilirubin; hemosiderin (sometimes)
Yellow....Lipid (adipose tissue; adrenal cortex; most necrosis); elastic fibers (vessels; yellow ligaments)
Blue....Something non-white seen through a reflective surface (blood in your veins through your skin; carbon pigment under the pleura; blue iris; cornea in osteogenesisimperfecta)
White....Tumor; granuloma; collagen (fibrous tissue; scar; etc.); calcium flecks
Brown....Feces; hemosiderin; lipofuscin; melanin; cytochromes (as in the liver)
Black....Carbon ("anthracotic pigment"); very abundant melanin; formalin-fixed hemoglobin turns dark brown or black
Hard, soft, firm (a short word which has more direct meaning than the often used term consolidated), rubbery, resilient, friable, viscous, gelatinous, smooth, granular or rough, dry, inspissated (dry and crumbly)
Soft:...like your earlobe
Firm:...like your strongest, leanest muscle when you flex it
Hard:...like your knuckle
Focal, multifocal, patchy, coalescing, locally extensive, diffuse, symmetric, asymmetric, bilateral, unilateral, random. Additional terms are: few, numerous, profuse, scant.
Grossly identifiable exudates
Hemorrhagic, serous, fibrinous, fibrinohemorrhagic, serosanguineous, watery (implies crystal clarity and is more suggestive of a transudate)
Degree or intensity a response
Mild, moderate, marked, severe, unusually severe (use this last term sparingly)
A few additional gross pathology terms used for common types of lesions
Necrotic, gangrenous, ulcerative, proliferative, infiltrative, congenital, traumatic, atrophic, hyperplastic, neoplastic.