A bone marrow biopsy is a highly valuable diagnostic procedure in veterinary medicine, particularly for investigating various hematologic disorders in dogs. It involves collecting samples of the bone marrow (the spongy tissue inside bones where blood cells are produced) for microscopic examination.
Why is a Bone Marrow Biopsy Performed? (Indications)
Bone marrow biopsies are indicated when peripheral blood abnormalities cannot be explained by other diagnostic tests, or when there’s suspicion of a primary bone marrow disorder or systemic disease affecting the marrow. Common indications include:
Unexplained Cytopenias (Low Blood Cell Counts):
Non-regenerative Anemia: When the body isn’t producing enough red blood cells, and the cause isn’t clear (e.g., chronic kidney disease).
Thrombocytopenia: Persistently low platelet counts, especially after ruling out immune-mediated causes or when other cell lines are also affected.
Leukopenia/Neutropenia: Low white blood cell counts, particularly neutrophils.
Pancytopenia: Low counts of red blood cells, white blood cells, and platelets.
Unexplained Cytoses (High Blood Cell Counts):
Leukocytosis: Persistently high white blood cell counts, especially with immature or abnormal cells (e.g., unexplained neutrophilia, mastocytosis, circulating blast cells).
Polycythemia: Rarely, to rule out primary polycythemia (polycythemia vera).
Presence of Abnormal Cells:
Circulating blast cells or other immature cells in the peripheral blood.
Unexplained presence of mast cells, atypical lymphocytes, or other neoplastic cells.
Staging of Neoplasia:
Diagnosis and staging of certain cancers, such as lymphoma, multiple myeloma, mast cell tumors, or other metastatic cancers to the bone marrow.
Fever of Unknown Origin (FUO):
To investigate potential infectious agents (e.g., Leishmania, fungal organisms) or inflammatory conditions affecting the bone marrow.
Hyperglobulinemia/Monoclonal Gammopathy:
To diagnose underlying conditions like multiple myeloma.
Suspected Myelofibrosis or Myelonecrosis:
To confirm the presence of fibrosis or necrosis within the bone marrow.
Assessment of Iron Stores:
In some cases, to evaluate iron reserves.
Contraindications
While generally safe, there are some situations where a bone marrow biopsy might be contraindicated or require careful consideration:
Severe Coagulopathy: Significant bleeding disorders (e.g., severe thrombocytopenia < 50,000/µL, prolonged PT/aPTT) increase the risk of hemorrhage. This is a relative contraindication, as the diagnostic information might outweigh the risk if managed appropriately (e.g., with platelet transfusions).
Unstable Patient: Patients in critical condition due to severe anemia or other illnesses should be stabilized first.
Osteomyelitis or Skin Infection: Infection at the biopsy site.
Severe Osteoarthritis or Neoplasia: In the chosen biopsy site.
Biopsy Sites
The most common and safest sites for bone marrow collection in dogs are:
Proximal Humerus (Greater Tubercle):
Advantages: Relatively easy to access, site is well-delineated.
Disadvantages: Requires the dog to be positioned laterally, risk of nerve damage (radial nerve, though rare with proper technique).
Iliac Crest:
Advantages: Less risk of vital structure damage, easy to position.
Disadvantages: Can be challenging to obtain sufficient sample in smaller or very obese dogs, or dogs with very hard bones.
Proximal Femur (Trochanteric Fossa/Greater Trochanter):
Less commonly used than humerus or iliac crest, but an option, especially in young pups or small breeds.
Less common sites (due to higher risk) include sternum or rib.
Procedure Overview
A bone marrow biopsy typically involves both an aspirate (for cytology) and a core biopsy (for histopathology). Both provide complementary information and are ideally performed together.
Patient Preparation:
Anesthesia: General anesthesia is usually required to ensure the patient remains still, comfortable, and to allow for proper sterile technique. Deep sedation with local anesthetic can be an option for fragile patients but is riskier due to potential movement.
ASEPTIC TECHNIQUE: The biopsy site is clipped, surgically scrubbed, and draped.
Local Anesthetic: Infiltrated into the skin, subcutaneous tissues, and down to the periosteum (bone covering) at the biopsy site.
Equipment:
Bone marrow biopsy needle (e.g., Jamshidi for core biopsy, Rosenthal or Illinois for aspiration).
Scalpel blade (size 15).
Syringes (5, 10, 20 mL) for aspiration.
Microscope slides for smears.
Formalin for core biopsy preservation.
EDTA tube (for aspirate if smears are made first, or for “line prep”).
Sterile gloves, drapes, antiseptic solution.
Aspiration Technique:
A small skin incision is made over the biopsy site.
The aspiration needle (with stylet) is advanced through the incision and into the cortical bone, then rotated and advanced through the cortex into the medullary cavity.
The stylet is removed.
A 5-10 mL syringe is attached, and strong negative pressure is applied for a few seconds. The goal is to collect a small amount of blood mixed with marrow spicules (small fragments of marrow). A larger volume of liquid is usually just peripheral blood contamination.
The syringe is detached, and the needle is removed.
IMMEDIATELY transfer a drop of the aspirate onto several microscope slides and create smears (using various techniques like pull smears, squash preps, or line preps) to preserve cellular integrity and prevent clotting.
If additional aspirate is collected, it can be placed into a small EDTA tube for further processing (e.g., flow cytometry, PCR, or for making “line preps” later).
Core Biopsy Technique (Ideally performed after aspiration, or at a slightly different entry point):
Using the same skin incision or a slightly different spot, the Jamshidi needle (with stylet) is advanced through the incision and into the cortical bone.
Once firmly seated in the cortex, the stylet is removed.
The needle is advanced another 1-2 cm into the medullary cavity with a rotational motion to cut a core of marrow.
Once the desired depth is reached, the needle is rotated 360 degrees in both directions to separate the core from the surrounding marrow.
The needle is then carefully withdrawn.
Using the stylet or a specific core retrieval tool, the bone marrow core is gently pushed out of the needle and IMMEDIATELY placed into a formalin-filled container for histopathological examination.
Post-Procedure Care:
Apply firm pressure to the biopsy site for several minutes to minimize bleeding.
Apply a sterile bandage.
Administer appropriate pain medication (NSAIDs or opioids) for several days, as the site can be sore.
An Elizabethan collar may be necessary to prevent the dog from licking or chewing the bandage or incision.
Monitor the site for swelling, discharge, or excessive pain.
Potential Complications
Pain: The most common complication, both during and after the procedure.
Hemorrhage/Hematoma: Especially in patients with coagulopathies.
Infection: Rare with proper sterile technique.
Fracture: Extremely rare, but possible if the bone is abnormal or excessive force is used.
Pneumothorax: A risk if sternal or rib biopsies are performed incorrectly.
Damage to Adjacent Structures: Nerves or blood vessels (rare with appropriate technique and site selection).
Dry Tap: Failure to obtain a diagnostic aspirate.
“Dry Tap”
A “dry tap” occurs when a bone marrow aspirate yields no spicules or only peripheral blood. This can happen due to:
Technical Error: Incorrect needle placement, insufficient negative pressure.
Bone Marrow Pathology:
Myelofibrosis: Marrow is replaced by fibrous tissue, making aspiration difficult.
Myelonecrosis: Death of marrow cells.
Aplasia/Hypoplasia: Markedly reduced cellularity.
Severe Neoplastic Infiltration: Dense packing of neoplastic cells.
Piecemeal Disease: Focal lesions not hit by the needle.
In case of a dry tap from an aspirate, a core biopsy becomes even more critical, as it can still provide diagnostic information in cases of myelofibrosis, necrosis, or aplasia.
Interpretation
A board-certified veterinary pathologist examines the bone marrow aspirate smears and core biopsy. They evaluate:
Cellularity: The overall density of hematopoietic cells (hypocellular, normocellular, hypercellular).
Megakaryocytes: Number, morphology, and maturation.
Erythroid Series: Number, morphology, maturation, and presence of dysplastic changes.
Myeloid Series: Number, morphology, maturation, and presence of dysplastic changes.
Myeloid:Erythroid (M:E) Ratio: The ratio of granulocytic precursors to erythroid precursors. Abnormal ratios can indicate various pathologies.
Presence of Abnormal Cells: Blast cells, neoplastic cells (lymphoma, mast cell tumor, metastatic carcinoma), macrophages containing infectious agents, or storage cells.
Evidence of Inflammation, Necrosis, Fibrosis: Especially important in core biopsies.
Iron Stores: Assessed on the core biopsy.
Architecture: The overall structure of the bone marrow (provided by the core biopsy).
Conclusion
A bone marrow biopsy is an indispensable diagnostic tool for investigating a wide range of hematologic and systemic diseases in dogs. By providing direct visualization of the hematopoietic tissue, it often yields definitive diagnoses that are unobtainable through other means, guiding appropriate treatment and prognostication. Due to the need for anesthesia, sterile technique, and proper sample handling, it is typically performed by experienced veterinarians or specialists.
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