Anatomy
The main group of unusual animals targeted here belongs to the order rodentia. The members of this group, such as rats, gerbils, prairie dogs, hamsters, etc., possess continuously growing incisors. They rarely cause problems as patients except for occasionally requiring incisor trimming. Some of the rodents (caviomorphs) closely resemble lagomorphs in the fact that all their teeth continuously grow, not just the incisors. Their teeth are classified as aradicular hypsodont - no roots and long crowns. The progenitor cells at the apical end continue to produce crown throughout the life of the patient. The caviomorph dentition is not identical to the lagomorph; lagomorphs possess a second set of maxillary incisors (peg teeth) and several more premolars (lagomorphs possess 5 to 6 maxillary cheek teeth and 5 mandibular cheek teeth. Caviomorphs possess 4 maxillary and 4 mandibular cheek teeth). Because of this anatomic anomaly, caviomorphs and lagomorphs suffer from specific dental conditions.
Equipment
These patients are small and own even smaller oral cavity openings. The cheek teeth are far back in the mouth making them hard to visualize and to access. In order to be able to efficiently examine and treat rodent and lagomorph dentitions, one requires a set of instruments designed for that purpose. The set includes large otoscope and/or small bivalve speculum (Welch Allyn #26038a) and/or endoscope, incisor Luxator (Cislakb), dental Luxator (Cislak), oral speculum (Cislak, IM3), various cheek dilators (IM3c), a straight cone mounted on a slow speed handpiece, and dental burs. Two types of burs are available; cutters and grinders. The cutters are quicker but can cause serious trauma to the soft tissues. The grinders are somewhat slower but are also gentler to the soft tissues of the oral cavity. The author's favorite grinder is a tubular shaped carbide bur (Brasselerd), its size and shape allows one to reduce the crowns on a whole quadrant in one go. Brasseler also makes longer shanked burs that are quite useful when working on bigger rabbits. A good source of light, coffee stir sticks and a dental x-ray unit are also essential. The x-ray unit is to be used with films or digital sensors. Size 2 and size 4 films are most commonly used. Size 4 or occlusal films are the size of a credit card; they allow the operator to obtain a view of the whole head on one film. Sensors only come in smaller sizes 0 or 2, but software allows the operator to superimpose the images to create a panoramic view.
Positioning techniques vary from the ones used in dogs and cats; extraoral instead of intraoral techniques are employed. Four views are standard: lateral, dorsoventral, diagonal, and rostrocaudal. The first three are similar to the ones used in horses; they allow a good look at the alignment of the teeth and at the anatomy of the radicular ends. The last position allows a good view of the sinuses and of the temporomandibular joints. Nail clippers are not recommended; they are even viewed as detrimental, as they can cause diagonal fractures causing pulp exposure, pulpitis, pulp necrosis, periapical granuloma, and even osteomyelitis and sepsis. Rasps are not on the list either, as they are inefficient and often cause trauma to the soft tissues of the oral cavity.
Analgesia
Even the simplest procedure, occlusal equilibration, can be painful; thus it is imperative to use peri- and postoperative analgesia. Metacam is given subcutaneously at 0.2 mg/kg toward the end of the procedure. Buprenorphine, at a dose of 0.03 mg/kg, is also often given subcutaneously, at the same time. The patient is sent home with more buprenorphine given per OS, every 8 to 12 h, at a dose of 0.03 mg/kg. Analgesia is provided for 3 to 5 days depending on the severity of the procedure.
Anesthesia
Multiple protocols are in use: preanesthetics range from medetomidine (100 to 500 mcg/kg SC or IP), to midazolam (1 to 2 mg/kg IM, IV, IP), ketamine/medetomidine (15 mg/kg + 0.25 mg/kg IM), ketamine/midazolam (15 mg/kg + 0.2 mg/kg IM), ketamine/acepromazine (10 mg/kg + 0.05 mg/kg IM). Premed with glycopyrrolate 0.1 mg/kg IM, SC is recommended with this drug combination), Domitor (0.1 to 0.5 ml/kg IM, SC. Antisedan reversal 0.2 to 0.4 ml/kg IM). At present, we induce rabbits and guinea pig patients with a mixture of Dexdomitor (150 µg/kg) and ketamine (15 mg/kg) given subcutaneously. With chinchillas, we decrease the dosage of the mixture to Dexdomitor 100 µg/kg and ketamine 10 mg/kg. Wait 10 to 15 minutes or until the animal is unable to maintain its righting reflex. If the patient needs more injectable drugs during the procedure, we use ketamine (5 to 10 mg/kg) and Valium (0.2 mg/kg) intravenously.
Once the patient has been sedated, an intravenous catheter is placed in an ear vein. The ear is taped to the contralateral one or to a syringe case to prevent its bending and resulting kinking of the catheter. The use of a small extension set connected to the catheter makes injecting more comfortable. The patient is then loaded with subcutaneous fluids (10 ml/kg). If necessary, the patient is intubated or masked. The mask is placed over the nose while keeping the mouth accessible. As those patients are nose breathers, it is sufficient to maintain anesthesia. Intubation is reputed to be difficult, but with a bit of practice, it is actually not that hard. It is usually reserved for larger rabbits (> 2 kg). Four techniques are used; blind, guide-assisted intubation, with the use of a modified stethoscope, or with the use of an endoscope.
Blind intubation: With the patient in lateral or sternal recumbency and its head and neck extended, deposit lidocaine at the back of the throat. After 10 seconds, the ETT is gently advanced to the back of the throat. Observe for condensation with the lumen of the ETT and listen for breath sounds. If the tube is directly over the larynx, condensation and breath will be present. Watch chest movement and time introduction of tube with inspiration. A few trials may be necessary. Proper placement in the trachea is confirmed by the appearance of condensation inside the tube. If the rabbit swallows, chews, coughs while being intubated, more sedation is required.
Modified stethoscope-aided intubation: With the patient in lateral or sternal recumbency and its head and neck extended, deposit lidocaine at the back of the throat. After 10 seconds, advance the ETT gently to the back of the throat. Attach the tube to the modified stethoscope. The stethoscope amplifies the breath sounds. Listen for breath sounds. Advance tube until breath sounds are readily audible and condensation forms inside the tube. Same as above, time the introduction of the ETT with inspiration. It may be necessary to advance and withdraw the tip of the ETT several times, all the while observing chest movement, listening for sustained breath sounds and looking for condensation within the ETT. If the breath sounds decrease in intensity, back out of the oesophagus and try again. When the sounds become very loud, stop advancing the ETT. You have successfully intubated your patient. Avoid advancing the ETT beyond the thoracic inlet. Once you are sure of your intubation, secure the tube to the patient's neck. If the rabbit swallows, chews, coughs while being intubated, more sedation is required.
Guide-assisted intubation: With the rabbit in sternal recumbency, extend the neck as you would for a cat intubation. Using an otoscope or a laryngoscope with a size 0 blade, visualize the rabbit's arytenoid cartilages on either sides of the glottis. Advance a 12-inch length of a 3.5 French plastic urinary catheter, 2 or 3 inches into the trachea to serve as a guide. Remove the otoscope or laryngoscope. Feed the ETT over the urinary catheter without letting go of the catheter guide. Continue to hold onto the catheter as the ETT is gently advanced until it is well into the trachea. Remove guide. Verify proper placement of the tube by visualizing condensation in the ETT, normal respiratory effort, and by hearing breath sounds in the ETT.
Endoscope-assisted intubation: With the rabbit in sternal recumbency, extend the neck as you would for a cat intubation. Introduce the endoscope gently over the tongue to the back of the throat. You should be able to visualize the opening of the trachea. Once the endoscope is in position, introduce the ETT alongside. Bring the tip near the trachea. When you visualize inspiration (maximum opening of the arytenoid cartilages), gently introduce the ETT in the trachea. Remove the endoscope. Secure the tube to the patient's neck. Alternatively, if using a 1.9 mm endoscope, the ETT can be slid over the endoscope. The scope is then used as a guide for intubation.
Core temperature is maintained by using a heating system such as hot water circulating blanket, forced air or electrical. If one uses a hot water bottle, make sure to place a towel around the bottle to prevent burning the patient.
Antibiotics
Here, too, the operator does not have the same array of choices that is available when dealing with cats and dogs. Many antibiotics are harmful to rodents and lagomorphs. Enrofloxacin is a great product but, unfortunately, it is a poor choice when working in the oral cavity. As far as spectrum is concerned, chloramphenicol is better but it is a bacteriostatic drug, not a bacteriocidal one. Certainly the best choice at present is an injectable penicillin G benzathine suspension (Bicillin, Duplocillin). It is readily available, relatively inexpensive and does not cause any of the dreaded side effects that can be seen when using other antibiotics on these patients. Recommended doses are 75,000 U (1/4 cc) every 48 h for patients under two kilos and 150,000 U (1/2 cc) every 48 h for patients over two kilos. In severe cases, the injections can be given every 24 h instead of 48 h. Another method to avoid causing gastroenteritis and to increase the choice of antibiotics permissible, is to place the antibiotic locally. This technique is particularly beneficial when treating large abscesses; the antibiotic powder is mixed with saline solution to create a slurry and is introduced directly into the abscess cavity. It is left in situ for 10 days to 2 weeks before being removed and replaced if necessary. Using this method, antibiotics such as doxycycline, clindamycin, tetracycline, gentamycin, and minocycline, have been safely employed. They deliver their action locally without any systemic side effects. They have been instrumental in designing new ways of treating dental abscesses in rodents and lagomorphs.
Endnotes
a. WelchAllyn, 4341 State Street Road, P.O. Box 220, Skaneateles Falls, NY, USA, 13153-0220.
b. Cislak Manufacturing Inc., 7450 North Natchez Avenue, Niles, IL, USA 60714.
c. IM3 Inc., 12119 NE 99th St., Suite 2060, Vancouver, WA, USA 98682.
d. Brasseler USA, 800 King George Boulevard, Savannah, Georgia, USA 31419.
References
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