Atlanto axial. Atlantoaxial instability (subluxation) in dogs

(Atlanto-axial instability/C1-C2 instability in toy breeds of dogs)

Doctor veterinary sciences Kozlov N.A.

Gorshkov S.S.

Pyatnitsa S.A.

Abbreviations: AAN - atlanto-axial instability, AAS - atlanto-axial joint, AO ASIF - international association of medical traumatologists and orthopedists, C1 - first cervical vertebra (atlas), C2 - second cervical vertebra (epistrophy), Malformation - developmental defect, ZOE – odontoid process of the epistrophy (blue tooth of the second cervical vertebra), CT – computed tomography MRI – magnetic resonance imaging, PS – spinal column, KPS – dwarf dog breeds OA – general anesthesia, PMM – polymethyl methacrylate

Introduction

Atlanto-axial instability- (syn. atlanto-axial subluxation (subluxation), dislocation (luxation)) - represents excessive mobility in the atlanto-axial joint, between C1 - the first and C2 - the second cervical vertebrae, which leads to compression spinal cord in this area and, as a consequence, manifests itself in varying degrees of neurological deficit. AAN is one of the anomalies (malformation) spinal column.(R.Bagley,2006) This pathology typical for dwarf breeds dogs (DeLachunta.2009), but also occurs in large breeds (R. Bagley, 2006).

Anatomical features

The atlantoaxial joint provides rotation of the skull. In this case, vertebra CI rotates around the odontoid process of CII. There is no intervertebral disc between CI and CII, so the interaction between these vertebrae is carried out mainly due to the ligamentous apparatus. In dwarf dog breeds, congenital instability of the connection of the first and second cervical vertebrae is explained by for the following reasons(DeLachunta.2009):

— Underdevelopment of the ligaments that hold the epistrophy tooth.

— Absence of a tooth in the second cervical vertebra, associated with its postnatal degeneration, malformation or aplasia.

According to Dr. DeLachunta and a number of colleagues, the epistrophy tooth undergoes degeneration in the first months of the animal’s life. This degeneration process is similar to the mechanism of development of such pathology as aseptic necrosis of the head femur(Legg-Calvé-Perthes disease), which is also typical for dwarf dog breeds (De Lachunta, 2009).

The completion of the process of ossification of the tooth epistrophy occurs at the age of 7-9 months. (DeLachunta.2009).

The absence of the odontoid process and/or its underdevelopment occurs in 46% of cases. Rupture of the ligamentous apparatus - in 24% of cases (Jeffery N.D, 1996.) These anomalies in the development of the spinal column are congenital, but injuries to this area can force the appearance of clinical symptoms of the disease (Ellison, 1998; Gibson K.L, 1995).

Predisposition

Yorkshire Terrier, Chihuahua, miniature poodle, Toy Terrier, pomeranian spitz, Pekingese

Etiology. Pathogenesis

It was proposed to distinguish 2 main forms of AAN (H. Denny, 1998):

— Congenital atlantoaxial dislocation (primary).

The pathology is typical for dwarf dog breeds. The basis is a minor injury, a jump from hands, a sofa, etc.

— Acquired atlantoaxial dislocation(directly traumatic).

Occurs suddenly as a result of severe trauma, for example in a traffic accident or fall. It can occur in any animal, regardless of breed and age. More often, acquired atlantoaxial dislocations are very severe, which is associated with sudden simultaneous and massive compression of the spinal cord by the epistrophic tooth and displaced vertebral arches.

Often, animals that have received minor trauma have a more severe degree of neurological deficit than those that have been exposed to moderate or significant trauma.

This depends on how long the transverse ligament of the epistrophic tooth can withstand and resist the dorsal displacement of the tooth of the second cervical vertebra towards the spinal canal directly during trauma (DeLachunta.2009).

Also, atlantoaxial dislocation can be acute or chronic.

Acute– often provoked by injury (falling from one’s hands, jumping from a sofa). Chronic- develop unnoticed, gradually, without obvious motivating reasons, with a minimal degree of neurological deficit. If a relapse occurs, after treatment of AAN with a similar course, the clinical symptoms are more significant and the treatment is more difficult.

Sometimes, as a result of chronic dislocation, atrophy of the dorsal (upper) arch of the atlas gradually develops from constant pressure, which is clearly visible on an x-ray in the form of the absence of the dorsal part of the atlas.

Clinical symptoms

Clinical signs of this pathology can vary from a slight pain reaction in the neck to tetraparesis of the limbs. Symptoms may also be as follows:

• Pain syndrome in the cervical region. The dog cannot jump onto a chair or sofa; it keeps its head down; turning the head, flexing, and extending the neck are painful, and the dog may yelp if the movement is awkward. Often owners simply notice only soreness of unknown origin. The dog reacts to touch, pressure on the stomach, and lifting. In such cases, when promptly contacting a doctor who does not specialize in neurological diseases, the latter draws incorrect conclusions based on the owners’ story, makes an incorrect diagnosis and carries out treatment or further diagnostics, which lead to loss of time and late diagnosis. (Sotnikov V.V. .2010)
• Paresis or paralysis. Motor deficits can manifest in both the pelvic and all four limbs. Tetraparesis of the limbs is often observed. Neurological disorders may vary. For a more objective assessment of the severity and prognosis of spinal cord injury, many gradations have been proposed. Most often in veterinary practice, the severity rating system for spinal cord injury is used according to Griffits, 1989. Usually, with timely treatment, grades 1, 2, and 3 of neurological deficit are noted. The prognosis for proper treatment of a “fresh” dislocation is rather favorable.
• Neurological syndromes that are associated with the manifestation of the syndrome intracranial hypertension, which appears as a result of a block of the cerebrospinal fluid tract by the tooth of the second vertebra. This manifests itself as a variety of different neurological symptoms. The dog cannot stand on its paws, falls on its side, randomly beats its paws, twists its head sharply to the side and, following its head, turns 360 degrees and can continue to tumble like this until it is stopped. Small breeds of dogs are prone to developing hydrocephalus, which is often asymptomatic, and if a dog has hydrocephalus, it can be dramatically worsened by blocking the cerebrospinal fluid pathways and increasing pressure in the ventricles of the brain. Sharp rise pressure in the brain leads to the development of intracranial hypertension syndrome.

The most common clinical signs of pathology:

1) acute pain syndrome- which manifests itself when turning or raising the head in the form of a loud “squeal”;

2) ventroflexion– forced position of the head and neck no higher than the level of the withers;

3) proprioceptive deficit thoracic limbs;

4) tetraparesis/tetraplegia.

Symptoms of brain damage may also be seen, which may be a consequence of impaired cerebrospinal fluid circulation and the development or progression of hydrocephalus, which is often present in 95% of toy dog ​​breeds (Braun, 1996) but without clinical signs. In animals, hydrocephalus may also be accompanied by syringo(hydro)myelia.

Compression of the basilar artery by the odontoid process of the epistrophy can cause symptoms such as disorientation, behavioral changes, and vestibular deficit.

Diagnostics

Differential diagnosis of this pathology includes (H. Denny):

Tumors of the PS and spinal cord

Herniated discs

Discospondylitis

With a similar clinical picture may occur:

Spinal fractures

Herniated discs Hansen type 1

Hypoglycemia is a common pathological condition in Yorkshire terrier puppies and other miniature dogs.

Visual diagnostics includes data from the following studies:

• x-ray examination cervical region PS in lateral projection
• X-ray contrast study (myelography). To exclude other pathologies - CT scan
• Magnetic resonance imaging
• Ultrasound of the atlanto-axial joint

An X-ray allows one to clearly visualize the area of ​​the AA joint, mainly in dwarf dog breeds, due to the very small thickness of the vertebrae (the average thickness of the dorsal arch of the atlas in the period from 1-3 months is 1-1.2 mm (McCarthy R.J., Lewis D.D., 1995)) . Also, using an X-ray image, you can evaluate the increase in the distance between the C1 and C2 vertebrae.

It is recommended to take the photo without general anesthesia, since relaxation and withdrawal pain syndrome(if any) will worsen damage to the spinal cord, which can, due to ascending edema, lead to paralysis of the respiratory center and death.

However, spinal cord compression cannot be judged in any way based on an x-ray. (Sotnikov V.V., 2010.) To do this, you need to perform a CT or MRI.

These methods are not for everyone and are often not always available, due to the insolvency of the financial situation of the animal’s owners, as well as the lack of CT and MRI machines in ordinary veterinary clinics in the Russian Federation.

In this case, as an additional method for diagnosing AAN in dwarf breeds of dogs, you can resort to ultrasound of the AA joint. This method is possible and used (Sotnikov V.V., Conference materials: Neurology of small domestic animals // St. Petersburg, 2010.)

MRI data allows us to obtain more full information about spinal cord edema, myelomalacia or syringohydromyelia (Yagnikov, 2008).

Currently, we use the following to surgically solve the problem: surgical stabilization techniques(if there are indications for surgery):

• Ventral stabilization;
• Stabilization using – 2 spokes (2 mini-screws);

Rice. 1 and 2. Intraoperative photo

• Dorsal stabilization. As a possible solution to the problem, it is possible to use a dorsal tie (Kishigami) as a fixator.

The joint between the first (atlas) and second (axis) cervical vertebrae is the most important moving part of the spine, but it has little inherent stability compared to other parts of the spine.

Atlantoaxial instability in dogs is caused by traumatic or rheumatic destruction of the ligaments that hold the odontoid process in place.

In dogs of dwarf breeds, AAN is a congenital pathology, distinctive feature which lies in the instability of the atlas in relation to the axis. It causes an abnormal bend between two bones and, as a result, compression of the spinal cord.

In most cases, congenital atlantoaxial instability in dogs makes itself felt before the age of one year, but there are also animals with this pathology older than 5 years.

Traumatic subluxation of the joint is possible in representatives of any breed and does not depend on age. The degree of damage to the spinal cord varies depending on both the severity of the compression and the duration of the condition.

Symptoms

Symptoms atlantoaxial instability in dogs are different, and their progression can increase gradually or worsen sharply.

• Neck pain is the most common symptom. Often it is the only sign of pathology. The severity of pain can be quite severe.
• Loss of coordination.
• Weakness.
• Neck drooping.
• Impaired supportability of all limbs up to complete paralysis, which can also lead to paralysis of the diaphragm, as a result of which the animal cannot breathe.
• Brief fainting (rare)

Diagnostics

The diagnosis is made on the basis of breed predisposition, medical history, clinical symptoms and the results of a neurological examination, as well as the results of an X-ray examination or MRI/CT diagnostics (depending on the clinic’s facilities).

What is the difference between these diagnostic methods? With mild instability, X-ray examination may be ineffective and often only indirectly indicates this pathology. MRI diagnostics allows you to most clearly visualize the spinal cord, the degree of its compression and swelling. CT diagnostics allows the most accurate visualization of bone structures and is more effective in cases of suspected atlantoaxial instability due to a traumatic fracture.

Treatment

Conservative treatment of atlantoaxial instability in dogs is rarely used, but may be prescribed if symptoms and compression are minor or if there are medical contraindications to surgical intervention. Conservative treatment consists of:

• Severe restriction of mobility
• Use of steroids and pain medications

At conservative treatment There is always a risk of persistence of symptoms or their progression up to sudden paralysis and death of the animal. For this reason, surgery is most often recommended to relieve spinal cord compression and stabilize the joint. The choice of technique depends on the size of the animal and the presence of associated fractures.

Forecast

The prognosis depends on the severity of the spinal cord injury and the results of neurological deficits. Animals with mild symptoms have a favorable prognosis. When paralysis is present, the prognosis is generally guarded, but significant recovery is possible if prompt surgical intervention is performed. Significantly greater success with surgical intervention is observed in younger dogs (less than 2 years old), dogs with more acute problems(less than 10 months of symptoms) and dogs with less severe neurological problems.

veterinary neurologist "MEDVET"
© 2018 SEC "MEDVET"

Atlantoaxial instability typically occurs in small breed dogs and begins clinically in young animals, although it can occur at any age. This condition can be inherited or result from injury. With atlantoaxial instability, subluxation, or displacement, of the second cervical vertebra (epistrophy) relative to the first (atlas) occurs, followed by compression of the spinal cord, which leads to severe neurological symptoms: tetraparesis, paralysis, and proprioceptive deficit. The disease may be accompanied by hydroencephaly and syringohydromyelia. Among the main causes of atlantoaxial instability are the following:

1. Abnormal shape of the odontoid process or its absence
2. Underdevelopment of the odontoid ligaments
3. Post-traumatic rupture of the atlantoaxial ligaments
4. Fracture of the odontoid process due to trauma (strong flexion of the neck)

Anatomically, there are no intervertebral discs between the occipital bone, atlas and epistropheus, and these vertebrae form a flexible segment of the cervical spine, providing good mobility of the neck. The interaction between the first and second cervical vertebrae is carried out due to the articular surfaces, ligaments and the odontoid process of the epistrophe, which enters the fossa of the atlas tooth. The odontoid process, in turn, is fixed by the longitudinal and alar ligaments, as well as the transverse ligament of the atlas. The epistrophic crest is attached to the dorsal arch of the atlas by the dorsal atlantoaxial ligament.

Rice. 1 - ligamentous apparatus of the atlanto-axial joint.

Rice. 2 - congenital absence of the odontoid process, predisposing to rupture of the dorsal atlantoaxial ligament and leading to the displacement of the epistrophy dorsally, and the atlas - ventrally.
Rice. 3 - fracture of the odontoid process and rupture of the transverse atlas ligament, rupture of the dorsal atlanto-axial ligament (can occur independently of each other).

Normally, the odontoid process is fixed by strong ligaments that reliably articulate the first two vertebrae. These ligaments can be weak or underdeveloped and can be damaged by the slightest impact on the cervical spine. If the odontoid process has an abnormal shape, then the ligaments, as a rule, are torn, and the epistrophy is displaced relative to the atlas. The odontoid process may be completely absent - in this case, the vertebrae are not fixed in any way, which also leads to subluxation of the atlanto-axial joint and compression of the spinal cord. Although atlantoaxial instability is congenital disease, inherent small breeds, rupture of ligaments with subsequent displacement of the vertebrae can occur as a result of injury in any animal.

Clinically, the disease manifests itself as pain in the cervical spine, as well as partial or complete loss of sensitivity, paresis and paralysis. Proprioceptive deficits, resulting from an excessive increase in the amount of cerebrospinal fluid in the cranial cavity (hydroencephaly), are characterized by impaired motor skills and coordination of movement. Congenital atlantoaxial instability is often combined with syringohydromyelia (formation of cysts and cavities in the central canal of the spinal cord).

Some dogs with congenital AO instability also have portosystemic shunts: this may be due to the inheritance of genes that influence the development of these two diseases. Thus, if one of them is detected, it is advisable to carry out diagnostic studies, aimed at identifying (or excluding) the other.

The disease is diagnosed based on X-ray examination. On the radiograph of an animal with AO instability, sharp increase space between the epistrophic crest and the dorsal arch of the atlas, indicating a rupture of the dorsal atlantoaxial ligament. With a fracture of the odontoid process and its abnormal shape, the lower contour of the epistrophy is displaced dorsally and does not coincide with the lower contour of the atlas (the dorsal AO ligament may be intact, and the separation of the atlas from the epistrophy may not be observed).

Rice. 4 - radiographs: normal spine (A), AO instability (B). White arrows indicate an increase in the distance between the epistrophic crest and the dorsal arch of the atlas

The images are taken in a lateral projection, with the head bent at the cervical spine, which should be done extremely carefully, since excessive force directed at the damaged segment of the spine can cause damage to the spinal cord. Direct and axial views can also be useful in assessing the shape of the odontoid process. Myelography is contraindicated because it can cause unnecessary compression of the spinal cord and cause seizures.

Computed tomography provides more detailed diagnostic information than x-ray examination. However, the presence or absence of syringohydromyelia can be concluded only from the results of MRI. These diagnostic methods are associated with anesthetic risk, since the animal must be under general anesthesia at the time of the study.

Rice. 5 - computed tomograms: A - normal, B - AO instability. An asterisk indicates an abnormal odontoid process; the displacement of the lower contour of the epistrophe is indicated by a white arrow.

Treatment is mainly surgical, aimed at fixing the vertebrae with wire cerclages or bone cement. If the odontoid process has an abnormal shape, its resection is performed. If there are cysts in the central canal of the spinal cord, they are drained.

Conservative treatment is also possible, when the animal is placed in a cage and the cervical region is immobilized with a bandage. But it is ineffective and is mainly used as a temporary measure for animals that have contraindications for surgery, for example, with deep paresis and too at a young age individuals. This treatment is aimed at stabilizing the animal before surgical intervention and allows young individuals to achieve relatively safe age for surgery.

According to D.P. Beaver and others, the prognosis for dogs with congenital AO instability is in most cases favorable if the animal survives the operation and tolerates the postoperative period well. Operative mortality reaches about 10% of cases, and about 5% of animals require reoperation.

Among congenital anomalies of the spinal column, the most common in small dogs is the abnormal formation of the first two cervical vertebrae. In dwarf breeds, such as the Pekingese, Japanese Chin, Toy Terrier, Chihuahua, Yorkshire Terrier and some others, this can cause not only rotational, but also non-physiological angular displacement of the second cervical vertebra relative to the first, that is, subluxation. As a result, compression of the spinal cord occurs, leading to very serious consequences.

Among congenital anomalies of the spinal column, the most common in small dogs is malformation of the first two cervical vertebrae. Anatomically, the first cervical vertebra, the atlas, is a ring with wings extending to the sides, mounted, like an axis, on the protruding odontoid process of the second cervical vertebra - the epistrophea. From above, the structure is additionally strengthened by ligaments that attach a special crest of the second cervical vertebra to the occipital bone and atlas (Fig. 1). This connection allows the animal to make rotational movements with its head (for example, shaking its ears), while the spinal cord passing through these vertebrae is not deformed or compressed.

In dwarf breeds, such as the Pekingese, Japanese Chin, Toy Terrier, Chihuahua, Yorkshire Terrier and some others, due to insufficient development of the processes and fixing ligaments, not only rotational, but also non-physiological angular displacement of the second cervical vertebra relative to the first is possible, that is subluxation (Fig. 2). As a result, compression of the spinal cord occurs, leading to very serious consequences.

Puppies born with an anomaly of the first cervical vertebrae do not show any signs in the first months of life. They develop normally, are active and mobile. Usually, no earlier than 6 months, owners notice a decrease in the dog’s mobility. Sometimes the appearance of the first signs is preceded by an unsuccessful jump, a fall, or a head injury while running. Unfortunately, as a rule, only obvious movement disorders make you see a doctor.

A typical sign is weakness of the forelimbs. At first, the dog periodically cannot place its front paws correctly on the pillows and relies on a bent hand. Then he cannot rise on his forelimbs above the floor and crawls on his stomach. Motor disorders of the hind limbs appear later and are not as pronounced. No neck deformities are detected during external examination. In most cases there are no pain symptoms.

The described signs are clearly visible in toy terriers and Chihuahuas, less pronounced in chins and at first difficult to distinguish in Pekingese due to the large amount of hair and breed-specific deformation of the paws in this breed. Accordingly, dogs of the same breeds should be consulted at initial stage diseases, and with others they come when the animal cannot walk at all.

Rice. 2 Since the displacement of the second cervical vertebra is not noticeable externally, the only possible way reliable recognition of this disease is an x-ray examination. Two photographs are taken in lateral projection. In the first, the animal’s head should be extended along the length of the spine; in the other, the head is bent towards the handle of the sternum. In restless animals, short-term sedation should be used, since forceful flexion of the neck poses a danger to them.

In healthy animals, flexion of the neck does not lead to a change in the relative position of the atlas and epistrophe. The process of the second cervical vertebra is located above the arch of the atlas in any position of the head. In the case of subluxation, there is a noticeable departure of the process from the arch and the presence of an angle between the first and second cervical vertebrae. Special X-ray techniques for subluxation, epistrophy is usually not required and the risk of their use is unreasonably high.

Since the displacement of the vertebrae, leading to dysfunction of the spinal cord, is due to anatomical reasons, the treatment of epistrophic subluxation must be surgical. Fixing the animal’s head and neck with a wide collar and prescribing various medications only gives temporary effect and often only aggravates the situation, since restoring the mobility of a sick animal leads to further destabilization of the vertebrae. Sometimes it can be used to prove to animal owners that the problem is not in the paws and the effect of conservative treatment will only be temporary.

There are several ways to stabilize the excessively mobile connection of the atlas and epistrophe. Foreign literature describes methods aimed at obtaining a fixed fusion between the lower surfaces of the vertebrae. Probably these methods have their advantages, but the lack of special plates and screws, as well as high risk injuries to the spinal cord when they are incorrectly located on the tiny vertebrae of small dogs make these methods inapplicable in practice.

In addition to these methods, it is proposed to attach the process of the second cervical vertebra to the arch of the atlas with wire or non-absorbable cords. Moreover, the second approach is considered not reliable enough due to the possibility of secondary displacement of the vertebrae.

Over the past few years, our clinic has been using vertebral fixation using Mylar cords. original technique. To gain access to the problem area of ​​the spine, the skin is incised from the occipital crest to the third cervical vertebra. The muscles along the midline, focusing on the well-defined crest of the epistrophy, partly sharply, partly bluntly, move apart to the vertebrae. The crest of the second cervical vertebra is carefully removed from soft tissue throughout its entire length. Then, very carefully, the muscles are separated from the arch of the first cervical vertebra. Due to the insufficient development of the first and second cervical vertebrae and their displacement, the spaces between them gape widely, which makes possible damage to the spinal cord at this moment.

By spreading the muscles wide, they cut through the hard meninges along the anterior and posterior edges of the arch of the atlas. This moment of the operation is also very dangerous. Since the use of one loop around the arch of the atlas is, in general opinion, not reliable enough, we use two cords, passed independently of each other. The result is a more reliable system that allows movement between the vertebrae within physiological limits, but prevents the resumption of pressure on the spinal cord.

The insertion of the threads should be as careful as possible; the angular displacement of the vertebrae, inevitable at this moment, should be minimized. Since all manipulations are performed in the area where vital centers are located and breathing is quite possible, intubation and artificial ventilation lungs throughout the intervention.

Careful preoperative preparation, maintenance of vital functions during surgery, careful manipulation of the wound, anti-shock measures upon recovery from anesthesia help reduce the risk surgical treatment subluxation of the epistrophy is minimal, but it still remains, and dog owners should be warned about this. Since they ultimately make the decision to carry out the operation, the decision must be balanced and deliberate. Animal owners must understand that there is no other way out, and part of the responsibility for the fate of the dog lies with them.

With rare exceptions, the results of surgical treatment are good or excellent. This is facilitated not only by the surgical technique, but also by properly performed postoperative rehabilitation of the animal. Happening full recovery motor ability, we observed relapses only when we used the traditional technique with a wire loop. We consider external neck fixators unnecessary.

Thus, timely recognition of this congenital anomaly, which should be facilitated by the neurological alertness of the doctor performing the initial examination of dogs of breeds susceptible to this problem, allows correct treatment and get a quick recovery of the injured animal.

Portugeis A. A., veterinary clinic "Exvet", Odessa.

List of abbreviations: C1–C2 – atlantoaxial joint; AAN – atlantoaxial instability; C1 – atlas (first cervical vertebra); C2 – epistrophy (second cervical vertebra); NSAIDs – non-steroidal anti-inflammatory drugs; GCS – glucocorticosteroids.

AAN in dogs was first described in 1967. This pathology mainly occurs in young dogs of dwarf breeds (Chihuahua, Yorkie, Toy Terrier, Spitz), but can also occur in larger breeds and even in cats 1. The usual age interval for the onset of this disease is from 4 months to 2 years. This pathology is most often the result birth defect development of the C1, C2 vertebrae and the ligaments connecting them.
In the ontogeny of the epistropheus, there are seven centers of ossification, while its tooth consists of two such centers. The cranial center arises in the atlas, and the caudal center in the epistrophe. The fusion of ossification centers occurs at 4 months of age. The main causes of AAN are dysplasia, hypoplasia or aplasia of the epistrophic tooth (32%), as well as underdevelopment of the internal ligaments C1–C2 (mainly the transverse ligament of the atlas) (Fig. 1) 2. Trauma can also be the cause of this pathology.

Clinical signs

Basic clinical sign AAN—variable intensity neck pain—occurs in 55–73% of cases (Cerda-Gonzalez & Dewey, 2010; Parent, 2010). The pain can be either periodic, mild, manifested during periods of any specific movements, or high intensity, accompanied by obvious vocalization, lowering of the head, and careful and minimal body movements. Neurological deficits can also vary in severity, from mild ataxia of movement, which can manifest as weakness in the fore and hind limbs, to moderate and, in rare cases, severe tetraparesis. In exceptional cases, precomatose and coma(Fig. 3). Asymmetric symptoms of spinal cord damage may occur (displacement of the epistrophy can occur not only in the dorsoventral, but also in the lateral direction). The development of symptoms can be either acute or chronically progressive. In dwarf dog breeds with defects in the development of the C1–C2 junction acute symptoms diseases can occur due to minor injuries (jumping from a sofa, sudden jumping out of the owner’s hands, etc.) Most owners of mini-breeds with this pathology go to the clinic before their pets are one and a half years old.

Visual diagnostics

AAN should be suspected in all toy dog ​​breeds with pain, cervical stiffness, and ataxia even after 2 years of age. Differential diagnoses these patients may have Chiari-like malformation, atlanto-occipital overlap, dorsal compression of C1–C2 (Dewey's cavity), syringomyelia, arachnoid cyst, trauma, intervertebral hernia(up to 1.5 years unlikely 3)
Plain lateral radiographs may show the presence of C1–C2 instability (Figure 4). Sometimes it is necessary to gently bend the patient's head during an x-ray. The sensitivity of the radiographic method is 56% (Plessas & Volk, 2014). You should not neglect this simple and accessible research, especially if you have already initial examination an assumption arises about the presence of AAN; in addition, this will help to avoid accidental deterioration of the patient’s condition as a result of careless handling in the future. Sedation before Rg-imaging should be carried out with great caution. Due to the relaxation of the neck muscles, compression of the spinal cord may worsen, however, if this is necessary, it is better to use more precise methods diagnostics such as CT or MRI. CT has high sensitivity detection of various bone pathologies. Also, this method is good at identifying changes in the location of bone structures/implants (atlanto-occipital overlap, AAN, malformation and incomplete ossification of the vertebrae). The sensitivity of the method is 94%. (Rylander & Robles, 2007; Cerda-Gonzalez & Dewey, 2010; Parry, Upjohn et al., 2010) (Figure 5).
Preference is given to the MRI method, which is the gold standard for studying the nervous system (Fig. 6). It can show not only the location of compression, but also secondary changes in neural tissue (Westworth & Sturges, 2010; Middleton, Hillmann et al., 2012).

Treatment

The goal of treatment for AAN is to stabilize the C1–C2 vertebrae. There are conservative and surgical treatments. The latter is preferred. There was a direct relationship between the speed and completeness of restoration of neurological functions and the speed of contacting the clinic with the development of AAN 4.

Conservative treatment is acceptable in cases of very early age patient (up to 4 months) when the owner refuses surgery, this treatment option can also be considered in cases of mild and intermittent pain symptoms. Conservative treatment is aimed at strictly limiting head mobility (applying a corset, which should start from the middle of the head and end in the caudal third thoracic) for 1.5–2 months" (Fig. 7). NSAIDs/steroids are also necessary.
The point of this method is that within 1.5–2 months, scar tissue develops in the unstable C1–C2 joint, which can further support this connection and prevent compression of the spinal cord. In a study of 19 dogs (observation period - 12 months), this method showed 62% positive results. Dogs that did not respond to therapy died or were euthanized. Thus, the mortality rate was 38% 5. Possible complications when using this technique: corneal ulcer, bedsores at the points of contact of the corset with the skin, wet dermatitis under the corset (poor ventilation, food getting behind the corset), otitis externa, aspiration pneumonia (associated with difficulty swallowing in a position of permanent fixation of the head and neck, and weakness of the larynx and pharynx may also be present). In a study by Havig and Cornell, the complication rate was 44% (Havig, Cornell et al., 2005). The disadvantage of this technique is the high relapse rate.
Surgery indicated for relapse after conservative treatment and for moderate to severe symptoms of the disease.
There are two types of C1–C2 fixation: dorsal and ventral methods.
The dorsal method consists of dorsal access to C1–C2 and reduction and fixation using an orthopedic wire/polypropylene suture over the C1 arch and the C2 ridge (Fig. 8). After this, the same corset is applied as for conservative treatment for 1–1.5 months. The method was described in 1967 by Dr. Geary (Geary, Oliver et al., 1967).

The advantage of this technique is the relative simplicity of its implementation, however, the implants are often much denser than the arch of the atlas bone, resulting in numerous relapses. Also, due to the specific position of the patient on the surgical table (sternal position with a bolster under the ventral part of the neck and flexion of the head), iatrogenic compression of the spinal cord is created, which can significantly aggravate the patient’s vital functions until his death. This technique does not eliminate rotational movements and shear forces that continue to operate at the C1–C2 junction 8. Complications associated with migration/fracture of implants or bone when using the dorsal technique are 35–57% 6, 7. The success rate of the method ranges between 29 and 75%. The mortality rate can average 25%. (Beaver, Ellison et al., 2000).
The ventral method has two modifications. The first technique is the installation of transarticular implants (wires/screws) with or without cement (it is better to use cement with an antibiotic). The method was described by Drs Sorjonen and Shires (Sorjonen & Shires, 1981). Positive results were recorded in 71% of cases (44–90%) (Beaver, Ellison et al., 2000) (Fig. 9).
The second technique is the placement of multiple implants (wires/screws) in C1–C2, including transarticular placement and placement of bone cement (Schulz, Waldron et al., 1997). Positive results were achieved on average in 87–90% of patients (Fig. 10). At the same time, mortality was up to 10% of cases (Aikawa, Shibata et al., 2014).

A required element of any of ventral techniques is the removal of cartilage from the articular surfaces of C1–C2 and transfer of cancellous bone to create arthrodesis at this level. Cartilage is removed with a scalpel, curette or bur. When using a bur, care must be taken not to remove too much bone. Cancellous bone is most often harvested from the proximal humerus because this area can easily be included in the surgical site. Dental acrylic can be used as cement, but you need to be sure that the operation is highly sterile (Fig. 11).

The stages of ventral stabilization of C1–C2 using the multiple fixation technique are shown in Fig. 13-17.

Advantages of the method: high stability and functional fixation, complete neutralization of all forces acting in the C1–C2 joint, no additional fixation of the cervical spine with a corset (except for patients of medium and large breeds). The probability of a positive outcome is 60–92% 9. The success rate is related to the surgeon’s experience in performing this operation.
Disadvantages of the method: the surgical technique is much more complicated compared to the dorsal method, there is a possibility of damage to the spinal cord if implants are placed incorrectly, the most common postoperative complications are laryngeal paralysis (damage to the recurrent laryngeal nerve during access), swallowing disorders (may occur due to too much amount of cement), aspiration pneumonia, infection. The rate of postoperative complications can be about 30% 9.

Conclusion

The method of choice for the treatment of pathology such as AAN today is anterior fixation using multiple implants and bone cement. With a certain level of training in the technical performance of this operation, very good statistical indicators can be achieved. It provides a large margin of safety C1–C2. Thanks to arthrodesis, the load on the implants will last a short time(2–4 months). There is no need for additional actions (corset). Due to a certain positioning of the patient, good reposition of C1–C2 is achieved, which is not always possible to achieve when using the dorsal method.

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