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Injuries of the Head Jim Schumacher University of Tennessee Wounds of the head involving only skin usually heal rapidly. They are slow to become infected because of the excellent blood supply of the head, which causes bacteria in a head wound to multiply slowly to the critical, infective concentration of 105 organisms per gram of tissue. Many cutaneous wounds of the head can be sutured successfully days after injury. Wounds of the head of horses are often more than just cutaneous, however, and are accompanied by serious damage to deep structures. An injury to the head may be accompanied by restricted movement of air, ocular damage, neurologic deficits, or hypovolemia from severe hemorrhage. Fracture of the facial bones A horse that has suffered crushing of the nasal and maxillary bones may require temporary tracheotomy because this injury is also frequently accompanied by crushing of the nasal septum and conchae, resulting in restriction of movement of air through the nasal cavities. A healed fracture of one or both nasal bones is commonly accompanied by thickening of the nasal septum, and so, the status of the nasal septum of a horse that has suffered a fracture of a nasal bone should be evaluated after sufficient time has elapsed for the fracture to have healed. The owner should be forewarned, at the time of injury, that the horse may later require resection of its nasal septum. A fracture of a facial bone can usually be identified and assessed more accurately by visual inspection and palpation than by radiographic examination of the skull, especially if the injury has just occurred. Fracture of a facial bone can be easily overlooked during radiographic examination of the head, because multiple radiographic projections are often required to observe the facture. A depression fracture that may be obvious at the time of injury may become less obvious when a hematoma forms between the fracture fragment and the periosteum from which the bone has separated, temporarily restoring normal contour to the damaged area. The owner becomes aware of the fracture when the hematoma resolves to reveal the depression, by which time the fracture may be difficult to reduce. A depression fracture of a facial bone overlying the paranasal sinuses can often be reduced with a bone hook or a finger after positioning the hook or finger beneath the fracture fragment through a trephine hole created into paranasal sinuses adjacent to the fracture. Interdigitation of the reduced fragments creates stability. The straight incision created to expose bone for trephination is closed with staples. Another method of elevating a fracture fragment is to drill a hole into the fragment with a drill bit or a Steinmann pin and then to insert small Kirshner wire with a 90o angle into the hole to elevate the fragment. If the fracture cannot be reduced using either of these methods, the fracture site should be exposed through a large cutaneous flap. Reduced fragments that are not stable can be stabilized with sutures or one or more reconstruction plates. A fracture of a supraorbital process can usually be confirmed by inserting a gloved and lubricated finger over the dorsum of the globe to palpate the ventral surface of the supraorbital process. A depression fracture of the supraorbital process can be reduced, usually with the horse sedated, by using a bone hook positioned in the conjunctival fornix beneath the fragment. Anesthetizing the supraorbital and auriculopalpebral nerves may facilitate the procedure. The reduced fracture is usually stable. Fluorescein dye placed in the conjunctival sac can be used to evaluate the integrity of the lacrimal and nasolacrimal ducts. Discharge of dye from the nasal opening of the nasolacrimal duct indicates that the ducts are intact. A small, full-thickness defect into a paranasal sinus or nasal cavity caused by a comminuted fracture may heal by contraction and epithelialization, but when the defect is large, the skin heals to the mucosal layer of the sinus or nasal cavity, forming a completely epithelialized surface that prevents the wound from closing. A full-thickness defect into a frontal sinus can be closed using a temporalis muscle transposition flap, and defect into a maxillary sinus can be repaired using a levator labii superioris muscle transposition flap. Full-thickness defects anywhere over the paranasal sinuses or nasal cavities can be closed using periosteal flaps. The cambium layer of the transposed periosteum forms new bone over the defect. Rotational or free skin grafts are used to cover the transposed muscle or periosteum. Lacerations of the lip Full-thickness lacerations of the lip rarely become infected because of the good blood supply to the lip and can be repaired at any time. Sutured lacerations of the lip frequently dehisce because of tension caused by movement, however, unless they are anchored to the. After the fullthickness laceration to a lip is closed in 3 layers, it should be anchored to the mandible or premaxilla with stainless steel wire. To anchor the lip, tunnels are drilled through the lip and the bone at the gingival margin of the incisors with a small Steinmann pin or a 3.2 mm drill bit mounted in a handchuck. The pin enters the oral cavity slightly caudal to the incisors. Eighteengauge wire is passed through the tunnels in the lip and bone in a horizontal mattress pattern. A button or piece of rubber or plastic tubing is placed beneath each horizontal arm of the suture to prevent the wire from cutting through the skin and mucosa. Lacerations of the tongue Lacerations of the tongue may occur during recovery from general anesthesia, but are most are caused by the horse rearing backwards, with a bit in its mouth, while tied with the reins. Most tongue lacerations are located in the free portion of the tongue at right angles to the tongue’s long axis. They extend dorsal to ventral through the tongue, often leaving only the ventral mucosa intact. If left to heal by second intention, the laceration eventually becomes covered with mucosa, but a gap at the site of laceration persists. Regardless, the function of the tongue is usually preserved. Partial amputation of the tongue (i.e., partial glossectomy) at or rostral to the attachment of the frenulum seems to cause no impairment to the function of the tongue. Acute lacerations can be sutured with the horse sedated and the tongue desensitized by infiltrating the margin of the laceration with local anesthetic solution. The laceration is debrided and cleansed, and the wound is closed, from ventral to dorsal, with successive rows of simple interrupted, synthetic, absorbable sutures. The lingual mucosa is closed with absorbable, vertical mattress sutures, and the sutured laceration is spanned with vertical or horizontal mattress tension sutures. Some dehiscence should be expected. Lacerations of the ear An ear with a full-thickness laceration left unsutured tends to curl, worsening an already obvious disfigurement. Full-thickness lacerations of the ear are usually closed in 2 layers, one layer being the skin on the concave surface of the ear and the other layer being the skin on convex surface. The cartilage is not penetrated with the suture. Splinting the ear with rolled gauze or x-ray paper, placed on the concave surface of the ear, may be helpful in diminishing the likelihood of dehiscence. The rolled gauze is maintained with elastic, adhesive tape, and x-ray paper is maintained with sutures placed through the full-thickness of the ear. A large, cutaneous, avulsion wound on either the concave or convex surface of the ear that cannot be sutured should be covered with a skin graft because a wound left open to heal by second intention may deform the ear when the wound contracts. Acceptance of the graft on a fresh wound halts contraction, but contraction of a granulating wound may not be diminished, even if the graft is accepted, and so, a long period of splintage may be required to prevent deformity. Lacerations of the eyelid Wounds of the eyelid should be closed primary, even when contaminated, because scaring associated with healing by secondary intention is unsightly and often results in complications such as chronic epiphora, chronic conjunctivitis, keratitis from corneal exposure or abrasion, and entropion or ectropion. Fortunately, lacerations of the eyelid, even contaminated lacerations, rarely become infected because of the good blood supply to the eyelid. Only nonviable tissue should be excised, and the edges of the laceration should not be trimmed. The first suture should approximate the lid margin and is the most important suture in the repair. This suture is placed in a figure-of –eight fashion (i.e., an inverted cruciate suture) or as a mattress suture using 4-0 or 5-0 soft, absorbable or non-absorbable suture. The suture pierces the meibomian glands on the margin of the lid and the tarsal plate. The conjunctiva need not be sutured unless it is avulsed from the stroma of the lid. The rest of the skin sutures are simple interrupted. An eyelid laceration can be sutured with the horse sedated using regional blocks to desensitize the affected lid. The nasal portion of the upper eyelid and conjunctiva can be desensitized by injecting local anesthetic solution into the supraorbital foramen to anesthetize the supraorbital nerve. Injecting local anesthetic solution subcutaneously over the foramen provides some loss of motor function to the upper eyelid by anesthetizing terminal branches of the auriculopalpebral nerve. The supraorbital foramen is located by placing the thumb at one canthus and the middle finger at the other canthus. The index finger falls on or near the foramen. The lateral canthus and temporal 25% of the upper lid can be desensitized by anesthetizing the lacrimal nerve at the orbital rim just nasal to the lateral canthus. The nasal portion of the upper and lower eyelids can be desensitized by anesthetizing the infratrochlear nerve at its location in a notch in the upper portion of the orbital rim near the nasal canthus. Sensory innervation to the temporal portion of the lower eyelid can be desensitized by placing the index finger on the ventral rim of the orbit tightly against the lateral canthus and injecting local anesthetic solution close to the orbital rim, medial to the finger to anesthetize the zygomaticofacial nerve. Lacerations of the parotid salivary gland The parotid salivary gland and its duct are commonly lacerated because of their superficial position. The duct is especially vulnerable to injury at the caudal or ventral border of the mandible. Laceration of the gland or duct should be suspected if a large volume of fluid discharges from the wound, especially when the horse is fed. A wound to the gland is not as serious as a wound to the duct, because leakage of fluid from a lacerated gland tends to gradually diminish and then cease completely. If discharge from a wound over the parotid gland continues unabated, laceration of one of the several large radicles that form the parotid salivary duct should be suspected. Horses with a lacerated parotid salivary duct are often presented for treatment after the epithelium of the duct has healed to cutaneous epithelium, causing a salivary fistula. Deferring treatment may allow continuity of the duct to spontaneously re-establish, especially if the duct has not been completely transected, but if the fistula persists, continuity of the duct must be reestablished, or the gland must be destroyed. Excising the parotid gland is nearly impossible because critical vessels and nerves are embedded deep within its parenchyma. To re-establish continuity of a severed duct, one end of a catheter is inserted through the fistula into the distal portion of the duct, and the other end of the catheter is inserted through the fistula into the proximal portion of the duct. The ends of the duct are then anastomosed over the catheter using fine, absorbable suture. The catheter is withdrawn through the mouth, and the cutaneous wound is sutured or left open to heal by second intention. Destroying the gland is far easier and more practical than reconstructing the gland’s duct, and no complications have been associated with loss of one parotid salivary gland. The parotid salivary gland can be destroyed by instilling a destructive chemical into the gland or by ligating the duct to cause its physiological death. Formalin is the chemical most commonly used to destroy the parotid salivary gland, and 35 to 40 mL of it are infused into the gland by inserting a catheter into the proximal aspect of the duct at the fistula. The solution is maintained within the gland for about 90 seconds before it is allowed to drain. Secretion of saliva is eliminated usually within 3 weeks. To cause physiological destruction of the parotid gland, the duct is ligated between the gland and the fistula. A catheter is advanced into the proximal portion of the duct through the fistula, and an incision is made over the catheter, which can be palpated beneath the skin. Several sutures are placed around the catheterized duct and tied after the catheter is withdrawn. The duct should be ligated with heavy sutures because if fine suture is used for ligation, the suture may cut through the duct causing the duct’s lumen to be re-established. If the wound is fresh, making locating the severed end of the duct difficult, the duct can be located and ligated where it crosses the tendon of insertion of the sternomandibularis muscle. To locate and ligate the duct where it crosses the tendon, an incision is made caudal to the vertical ramus of the mandible over the longitudinal axis of the tendon. The duct can be located by palpation through this incision. Because a radicle may occasionally exit the gland distal to the duct at this location, the duct should also be ligated further distally. To locate the duct further distally, the lumen of the duct distal to the ligatures is catheterized, and a second incision is made over the catheter 3 to 4 cm distal to the first incision, and several sutures are placed around the portion of the catheterized duct exposed in the second incision and tied after the catheter is withdrawn. The cutaneous incisions are sutured. Wounds of the temporomandibular joint Wounds to the region between the base of the ear and the eye may involve the temporomandibular joint (TMJ) and may be accompanied by fracture of the articulation, or septic arthritis of that joint. The TMJ is difficult to image radiographically because of the complexity of the region and because other regions of the skull are superimposed over it. Ultrasonographic examination of the area may provide useful information. Contrast radiographic examination of the TMJ can be performed after introducing a contrast medium into the joint, either through the wound or, if possible, at a site remote from the wound to confirm communication between the joint and the wound. Horses with septic arthritis of a TMJ may respond favorably to systemic or intra-articular administration of antimicrobial drugs alone or to administration of antimicrobial drugs coupled with surgical excision of septic tissue, even if sepsis has extensively destroyed the articular surfaces of the affected TMJ. If conservative treatment fails to resolve pain associated with the TMJ, removing the mandibular condyle may bring relief from pain and restore normal mastication. Fractures of the mandible or premaxillae Fractures of the premaxillae or the incisive portion of the mandible are common and most often involve just the alveolar plate or the alveolar plate and the body of the mandible or premaxillae. These types of fracture most commonly occur when the horse’s teeth are caught on a solid object. They can usually be evaluated without the use radiography and most can be reduced and stabilized with interdental wiring. Unilateral fractures of the incisive portion of the mandible and bilateral fractures of the rostral portion of the mandible or maxilla/premaxillae in the interdental space are usually transverse and most commonly occur from kicks from other horses. Common methods of repair of these fractures include application of an intraoral or extraoral acrylic prosthesis, dynamic compression plating, and lag-screw fixation. Performing surgery of the head with the horse standing Many of the surgeries described above can be performed with the horse standing, provided that the horse is compliant. Horses are generally more tolerant of surgery of the head than of surgery of other parts of the body, and administering regional or local anesthesia and a sedative aides compliance. To perform surgery of the head with the horse standing, the horse can be sedated with detomidine (0.01 – 0.02 mg/kg IV or 0.03-0.04 mg/kg IM) and butorphanol tartrate (0.02 – 0.05 mg/kg, IV) and re-sedated during surgery, if necessary, with xylazine (0.5 mg/kg, IV). Constant-rate infusion of detomidine (0.02 mg/kg/hour) and butorphanol (0.012 mg/kg/hour) can be administered to provide a prolonged, constant state of sedation, after first administering a loading dose of detomidine (0.008 mg/kg, IV) and butorphanol (0.02 mg/kg, IV). Structures of the face can be desensitized by administering an infraorbital or maxillary nerve block, and structures of the lower jaw can be desensitized by administering a mental, mandibular alveolar, or mandibular nerve block. Even with these nerve blocks, however, skin surrounding the wound may need to be desensitized by subcutaneous infiltration of local anesthetic solution.