Spinal Injuries in Birds

Key Points

  • Traumatic injuries are common in free-ranging avian species.
  • Clinical signs associated with spinal trauma are variable, largely due to the degree of spinal cord involvement but may include hind limb paraplegia or paraparesis, decreased pain perception, and the presence of upper motor neuron signs.
  • The unique nature of the avian spine renders plain radiography less sensitive in birds than in mammals, particularly for acute lesions.
  • Alternate imaging techniques such as computed tomography and magnetic resonance imaging are much more sensitive diagnostic tests.
  • The mid or caudal thoracic spine, cranial to the synsacrum and caudal to the notarium, is particularly vulnerable to blunt impact trauma leading to vertebral fracture and spinal cord trauma. Focus imaging modalities on this region if neurologic assessment fails to offer clues regarding lesion localization.

Introduction

Traumatic injury is the most common reason for admission of wild birds to rehabilitation centers. Spinal injuries in birds are particularly problematic, as they are incredibly difficult to diagnose, localize, and manage.

 

Unique anatomic features of the avian spine

The avian spine is segmentally fused to provide the stability needed for flight while also protecting the spinal cord.

  • In many species, the cranial thoracic vertebrae are fused to form the notarium. This area of the spine is further stabilized by the ribs, which articulate with the sternum.
  • Fusion of caudal thoracic, lumbar, sacral, and caudal vertebrae forms the synsacrum (Fig 1, Fig 2).
  • The pygostyle represents a fusion of the last few caudal vertebrae.
skeleton diagram

Figure 1. Bird skeleton diagram illustrating the synsacrum (green) and pygostyle (arrow). Photo credit: Brechet modified by J. Rockley via Wikimedia Commons. Click image to enlarge.

synsacrum renegade lisp

Figure 2. Close-up of the avian skeleton. Photo credit: Renegade Lisp via Wikimedia Commons. Click image to enlarge.

When injury occurs, it is usually within the segment of spinal cord between the notarium and synsacrum.

bird skeleton diagram

Figure 3. Bird skeleton diagram: pygostyle (17), synsacrum (18) and notarium (20). Photo credit: Svtiste via Wikimedia Commons. Click image to enlarge.

Neurologic exam

Despite recent strides in the understanding of avian neuroscience and a variety of advanced diagnostic modalities, it remains frustratingly difficult to definitively classify spinal trauma lesions in birds. Optimal neurological assessment is achieved by examination of a calm, conscious patient in lateral recumbency with minimal restraint, a state that is rarely possible in the avian patient.

Neurologic examination requires careful observation to evaluate mentation, gait, posture, and activity level.

  • Use cranial nerve evaluation to rule out the possibility of brain or brain stem injury.
  • Palpate limbs to assess symmetry and relative limb strength in an effort to differentiate between central and peripheral lesions.
  • Test postural reactions assess the integrity of the sensory and motor nerve pathways throughout the body as well as integration by the brain. Postural reactions include wing flapping, and wing repositioning for the forelimb and extensor postural thrust, conscious proprioception, and tactile placing for the hind limb.
  • Use spinal reflexes to differentiate upper motor neuron (UMN) versus lower motor neuron lesions. Reflexes assessed include cloacal sphincter tone, leg withdrawal, and wing withdrawal.
  • Assess nociception by means of a toe pinch. Determine response to pain by noting if the bird looks toward the inciting stimulus, vocalizes, or attempts to bite in response to the stimulus.

Birds suffering from spinal trauma generally show varying degrees of paresis or paralysis in one or more limbs, decreased pain perception, and the presence of UMN signs.

 

Imaging the avian spine

Survey radiographs

Plain radiography is an excellent tool for diagnosing most spinal fractures in virtually every species with a bony skeleton, except in birds. In a clinical trial performed at the University of Illinois, less than half of spinal fractures in birds presented were visualized by survey radiography of the spine. This may be due to the fact that most of these fractures were non-displaced on gross evaluation during postmortem examination. Bony remodeling and periosteal reaction seen with chronic and displaced fractures increase the likelihood of visualizing spinal fractures through radiographic evaluation.

Contrast radiography

Myelography has been described in birds but the procedure is difficult to perform. The avian subarachnoid space is comparatively small, and the presence of the synsacrum means that thoracolumbar rather than lumbar puncture is necessary. Even thoracolumbar injection may not be possible in ducks, geese, and swans, because these species have overlapping thoracolumbar vertebrae that may prevent needle positioning in the subarachnoid space.

A myelographic technique has also been described in which contrast material is injected into the fourth ventricle. When this procedure was performed, it yielded excellent visualization of all but the distal 10–15% of the spinal cord most likely due to the glycogen body, which acts as a space-occupying structure blocking the caudal flow of contrast medium.

Alternate imaging

Computed tomography and magnetic resonance imaging offer a significant increase in sensitivity in the diagnosis of avian spinal fractures when compared to plain films. Remember spinal cord between fused segments is most commonly affected, so focus imaging modalities between the notarium and synsacrum if neurologic assessment fails to offer clues regarding lesion localization.

 

Management of spinal lesions

Because of the high degree of stability in the avian spine, injury sufficient to cause neurologic deficits is generally severe. Humane euthanasia is frequently selected in wildlife patients, however management may be attempted in the rare pet bird.

The National Acute Spinal Cord Injury Studies (NASCIS) have verified significant improvement in motor function and sensation in humans with spinal cord injury treated with high dose methylprednisolone sodium succinate (Solu-Medrol, Pharmacia & Upjohn) within 8 hours of injury. Of course most wildlife are not presented for care in a timely manner, and these patients do not tolerate the frequent handling and intensive management required. Remember that corticosteroid use is also associated with an increased risk of aspergillosis in the avian patient.

Maintain lightweight birds on thick padding, while larger birds should be housed in makeshift slings to prevent the development of pressure sores. Taking into account the nature of the spinal cord injury, perform careful, gentle physical therapy. I am unaware of any reports describing surgical management of spinal injury in birds.

 

Conclusion

It is essential to accurately assess spinal injury in order to determine treatment and prognosis. The unique nature of the avian spine renders plain radiography less sensitive in birds than in mammals, particularly for acute lesions. Computed tomography and magnetic resonance imaging are much more sensitive in the diagnosis of avian spinal fractures. Most spinal fractures in birds are located in the mid or caudal thoracic region, cranial to the synsacrum and caudal to the notarium. Unfortunately spinal trauma that results in permanent loss of function is catastrophic for the patient’s chance of release and euthanasia is almost always recommended.

References