Ventral slot is a surgical procedure primarily indicated for the ventral decompression of the spinal cord, typically caused by intervertebral disc disease, specifically protrusions or extrusions, or disc-associated cervical spondylomyelopathy (also known as "wobbler syndrome"). While it can be combined with ventral fixation using pins or screws and polymethylmethacrylate (PMMA), it is generally not an ideal technique for approaching spinal tumors due to the limited visualization it offers.

Here's a detailed overview of the ventral slot procedure based on the sources:

Preparation for Surgery

Patient Positioning: The dog is positioned in dorsal recumbency. Proper, symmetrical positioning is crucial to prevent the slot from being lateralized, which could increase the risk of injury to the venous sinus and lead to significant hemorrhage. Troughs or vacuum positioning pads are helpful for maintaining spinal alignment. The thoracic limbs are tied caudally, and tape is used over the mandible and thoracic regions to secure the patient. Towels may be placed underneath the region to be approached to achieve mild cervical extension, especially in the caudal cervical area of deep-chested, large-breed dogs. It is vital to evaluate positioning before draping, as incorrect positioning often results from cervical and thoracic spinal rotation.
Clipping and Sterilization: Clipping should extend from the mandible to the cranial to mid-thoracic area, and the surgical field is aseptically prepared.
Visualization: Magnification (×2.5–3.5) and focused bright lighting are highly recommended to facilitate ventral slot procedures, particularly in large dogs where the depth of the slot can hinder visualization. For deep-chested dogs, a right-handed surgeon positioned on the patient's left side may improve visualization of the C6–C7 disc space during drilling.

Surgical Approach

Two main approaches are described for the ventral slot: the median and the paramedian.

Midline Skin Incision: A ventral midline skin incision is made from the larynx to the manubrium of the sternum.
Median Approach: In this approach, the paired sternocephalicus and sternohyoideus muscles are identified and divided along the midline. The trachea is then identified beneath the sternohyoideus muscles. Digital and scissor dissection is used to expose and retract the trachea, recurrent laryngeal nerve, esophagus, and left carotid sheath to the left, while the right carotid sheath is retracted to the right. Care must be taken to avoid damaging these vital structures, especially the recurrent laryngeal nerve, which runs on the lateral side of the trachea. Blunt self-retaining retractors, such as Balfour retractors, are used to maintain retraction, with moist towels or sponges placed underneath to protect tissues.
Paramedian Approach: After identifying the sternocephalicus and sternohyoideus muscles, the right sternocephalicus muscle is separated from the right sternohyoideus muscle. The sternohyoideus muscles are then retracted to the left along with the trachea, esophagus, and carotid sheath. This approach reduces the risk of hemorrhage from the right caudal thyroid artery and protects vital structures like the trachea, right recurrent laryngeal nerve, vagosympathetic nerves, and right carotid sheath, while offering increased exposure to the caudal cervical vertebrae.
Disc Space Identification: Identifying the correct disc space is crucial. Key landmarks include the large transverse processes of the C6 vertebra and the prominent ventral process of C1. The ventral tubercles of the vertebral bodies should also be palpated. It is important to remember that there is no intervertebral disc between C1 and C2.
Longus Colli Muscle Dissection: Once the disc space is identified, the tendinous insertions of the longus colli muscles are sharply transected. Hemostasis with bipolar cautery is important at this stage. Subperiosteal elevation of the longus colli muscle cranial and caudal to the ventral tubercle and disc space follows, and retraction is maintained using Gelpi retractors. Partial resection of the longus colli muscle at C6 may be necessary for adequate visualization due to its long, ventrally positioned transverse process.

Surgical Procedure

Creating the Slot: The slot is created using a high-speed pneumatic drill. It is crucial to stay on the midline and minimize the slot's size. Due to the angulation of cervical intervertebral discs, the slot should begin in the cranial vertebral body. Ideally, the slot should not exceed one-third of the length and width of the vertebral bodies. Drilling proceeds through the outer cortex, cancellous bone, and intervertebral disc, until the inner cortex is identified. The surgeon must distinguish between the reddish, soft cancellous bone and the white, hard cortical bone. Frequent irrigation and suction are essential to remove bone debris and dissipate heat. The inner cortex is thinned to a soft cortical shell.
Spinal Canal Entry and Disc Removal: The dorsal annulus is grasped, and the remaining cortex and dorsal longitudinal ligament are excised with a #11 blade. Removal of the inner thin cortex can be achieved with bone curettes, Love-Kerrison rongeurs, or a Wayne laminectomy punch. Bleeding from the cancellous bone can be controlled with bone wax. The vertebral canal should be opened along the midline initially to avoid injuring the internal vertebral venous plexus, which can cause severe hemorrhage. Disc material is removed using blunt curved nerve hooks, fine forceps, or a malleable iris spatula. All disc material should be removed, exploring laterally, cranially, and caudally along the slot. The spinal cord should be visible and in a normal position after disc removal. Lateral inspection is crucial to ensure no residual disc material is compressing nerve roots, which could lead to severe postoperative pain.
Closure: After thorough lavage to remove debris and blood clots, surgical closure is performed. It is unnecessary and contraindicated to place any material in the slot site as it could interfere with bone fusion. Fusion of the slot site is expected within 8–12 weeks postoperatively, although not all sites fuse. The longus colli muscles are apposed with simple interrupted sutures, and the sternohyoideus muscle is apposed with a simple continuous pattern. Subcutaneous tissues and skin are closed routinely, often with tacking to the fascia below to reduce dead space and seroma risk.

Variations of Ventral Slot Procedure

Inverted Cone Technique: This modification aims to minimize bone removal, thereby reducing the risk of vertebral subluxation. The decompression window resembles an inverted cone, with its base adjacent to the ventral vertebral canal, providing maximal surgical access cranially, caudally, and laterally. The ventral decompression window is limited to 20% of the cranial vertebral body width. This technique allows more complete removal of protruded disc material with less hemorrhage risk and minimizes disc space collapse, which can prevent postoperative nerve root entrapment.
Slanted Ventral Slot Technique: This approach removes only the caudal aspect of the cranial vertebral body to access extruded disc material, making it a non-transdiscal procedure. A window is created cranial to the disc, aiming to enter the spinal canal at the dorsal portion of the annulus fibrosus. This technique provides access to the disc herniation site without removing a large portion of the annulus fibrosus, potentially preserving more stability and promoting bone healing. However, it offers limited visualization and carries a risk of damaging the vertebral venous plexus if extended too far cranially.
Video-assisted Ventral Slot: This minimally invasive technique uses an endoscopic device. A small skin incision (2.5–5 cm) is made over the targeted disc space. While the initial drilling of the ventral cortex and cancellous bone may be done without video assistance, video-assisted endoscopy is used for drilling the inner (dorsal) cortex and for disc removal. This technique is associated with limited invasiveness and faster recovery.

Complications

Ventral slot procedures can have complications ranging from minor hemorrhage to death.

Hemorrhage: This is probably the most common complication, reported in 26.7% of cases in one study. It can occur if the midline is not correctly identified, muscles are dissected at the transverse processes, or the vertebral venous plexus is damaged. Hemorrhage can be worse in dogs on aspirin or with von Willebrand factor deficiency, necessitating preoperative testing in predisposed breeds like Doberman Pinschers and Rottweilers. All bleeding should be controlled before closure to prevent hematoma formation.
Ventilatory Compromise: Ventilatory failure requiring positive-pressure ventilation has been reported in 4.9% of dogs undergoing cervical spine surgery. This is often associated with lesions between C2 and C4. Pneumonia has also been reported as a fatal complication.
Cardiac Dysrhythmias and Hypotension: The risk of cardiac dysrhythmias is higher with ventral slot surgery compared to thoracolumbar intervertebral disc decompression. This is likely due to retraction of carotid arteries, vagal nerve, and sympathetic trunks. Severe bradycardia and hypotension have been reported, sometimes leading to death.
Vertebral Instability and Subluxation: Ventral slots can alter the vertebral range of motion, leading to instability and subluxation, especially in the first few weeks post-surgery. Subluxation was radiographically documented in 8% of dogs in one study, occurring more commonly in small-breed dogs and in the caudal cervical region. Clinical signs include worsening neck pain or neurological status.

Postoperative Care

Activity Restriction: Strict cage confinement should be enforced for several weeks post-surgery for proper healing. Activity should be very limited in the first two weeks, with gradual increases in leash walks (using a body harness). Free activity is only permitted once fusion of the slot is complete or nearly complete (around 8 weeks postoperatively).
Pain Management: Pain control is crucial and can be achieved with opioid analgesics (e.g., fentanyl) and other medications.
Nutrition: Postoperative nutritional support is important, with options including assisted syringe feeding, appetite stimulants, or feeding tubes.

Other Considerations: Preoperative anti-seizure medications are continued as needed. Two-view radiographs of the entire shunt (if placed) are obtained post-surgery to serve as a baseline.