$$\rightleftharpoonup{xx}$$
$$\longleftharp{xx}$$,
$$\longrightharp{xx}$$,
All procedures involving animal models have been reviewed by the local institutional animal care committee and the JoVE veterinary review board.
1. Animal Preparation
- Sedate a pig by intramuscular (i.m.) injection of tiletamine (3.75 mg/kg) and zolazepam (3.75 mg/kg) and dexmedetomidine (37.5 μg/kg). Wait for it to become unconscious.
- Prepare an intravenous line by inserting a 20 G cannula into the ear vein.
NOTE: Make sure the cannula is in the vein by injecting 5-10 mL of saline through the cannula. If the vein has been missed, this will be noticeable by small edema in the ear tissue.
- Intubate the pig to ensure that the breathing rate can be regulated throughout the surgery.
NOTE: Ensure successful intubation by applying pressure on the pig's thorax and confirm that forcibly expired air is coming out of the intubation tube.
- Attach the intubation tube to a ventilator set to a breath rate of 14 breaths/min.
- Connect a pulse oximeter and cuff to the tail to monitor the heart rate (HR), blood pressure (BP), and oxygen saturation (sats). Insert a rectal thermometer to monitor the core temperature.
- Prepare an IV bag of ketamine (5 mg/kg/min), midazolam (0.25 mg/kg/min), and fentanyl (2.5 μg/kg/min), in saline and begin to infuse through the ear vein at approximately 2 drops/s.
NOTE: Throughout the surgery, the infusion rate may need to be increased or decreased based on the animal's vitals.
- With the pig in the prone position, palpate the back of the head and neck of the animal to locate and mark the occipital crest and spine of the first thoracic vertebrae and the base of each ear.
- Draw a straight line between the crest and the vertebrae along the longitudinal axis. Draw two lines from the crest to the base of each ear by following the base of the skull (Figure 1A).
- Check that the animal is in a deep sleep by carefully clamping the tail and watching for the absence of a tail reflex.
NOTE: If the animal is still reflexive, the anesthetic infusion rate should be incrementally increased until the animal no longer exhibits a reflex.
2. Surgery
NOTE: All through the surgery, it is necessary to have at least one assistant to suction the light bleeding and cauterize any severed vessels.
- Using a scalpel with a # 21 blade, make a dermal incision along the longitudinal line down to the muscle.
- Extend two perpendicular dermal incisions further along the shoulders, 10-15 cm in length.
- From the occipital crests, make dermal incisions along the line down to the base of each ear.
- Gripping the skin corners formed at the occipital crest with anatomical forceps, carefully separate the skin from the underlying muscle by lightly running the scalpel blade over the fascia, moving from the rostral to caudal. Once the skin has been resected following each of the five incisions, parts of the trapezius muscles should then be visible.
- Make a longitudinal incision with the scalpel, approximately 1 cm deep, where the trapezius comes together at the midline.
NOTE: When cutting through the muscles, there is an increased propensity for bleeding, so the cauterizer should be ready. If a larger vessel is severed, one person should quickly compress it with the gauze, while the other person uses the cauterizer.
- Using a combination of straight and curved surgical forceps, perform blunt dissection working along the longitudinal cut in the muscles. This will separate the bellies of the trapezius, as well as the underlying semispinalis capitus biventer muscle.
- Sever any persisting muscle fibers with a scalpel and continue blunt dissection until semispinalis capitus complexus becomes visible.
- Sever the origins of the trapezius and semispinalis capitus biventer muscles along the posterior aspect of the skull. Carefully separate them longitudinally with the scalpel performing blunt dissection until the semispinalis capitus complexus is fully visible.
- Retract the trapezius and semispinalis capitus biventer muscles using self-retaining retractors.
- Where the bellies of the semispinalis capitus complexus come together in the midline, make a longitudinal incision with the scalpel approximately 1 cm deep.
NOTE: Be aware for any additional bleeding here. Bleeding can be managed using a combination of cotton swabs and cauterization.
- Using surgical forceps, perform a blunt dissection working along the longitudinal cut between the muscle bellies until the dorsal aspect of the atlas (CI) is palpable.
- Sever the origins of the semispinalis capitus complexus muscles along the posterior aspect of the skull and separate it longitudinally from the underlying vertebrae by scalpel and blunt dissection.
- Retract the semispinalis capitus complexus muscles using another set of self-retaining retractors.
- Using a scalpel, carefully remove any remaining tissue overlying the region where the atlas meets the skull base.
- Placing one arm under the animal's neck and one finger at the juncture of the atlas and skull, simultaneously elevate the head and flex the neck while palpating with the finger to reveal the cisterna magna using the other hand.
NOTE: The cisterna magna is recognizable when palpating as a strong elastic structure with a small amount of rebound as pressure is released with the finger.
