Introduction

Microchipping pets in veterinary medicine is a common practice.^1–3 Adverse reactions to microchips are rare but include migration, inflammatory reaction, and tumor formation.^2–5 However, improper placement is another potential complication, which can lead to severe neurologic deficits.^2–6 Microchip placement affecting the spinal cord of both dogs and cats has been reported in a small number of cases.^2–6 In veterinary medicine, the treatment and management of these cases vary, and a preferred method has not been identified. The prognosis for recovery is unknown and thus guarded dependent upon the amount of damage to the spinal cord. In the paper, we report on the treatment and outcome of a 5 mo old male Japanese chin with neurologic deficits resulting from improper microchip placement.

Case Report

A 1.1 kg 5 mo old male Japanese chin presented for pelvic limb weakness and ataxia of 1 mo duration. The patient had no other health concerns. The patient history included being microchipped with immediate pain and ataxia appreciated after the procedure. On presentation at the referral center 3 mo later, his physical examination findings included a mildly dome-shaped cranium with the remainder of the physical examination falling within normal limits. On neurologic examination, the patient was bright, alert, and responsive. Posture was normal with no obvious head tilt or turn. Cranial nerves were all intact and appropriate. The patient was ambulatory in all four limbs with moderate paraparesis and ataxia, which was worse on the right side. Conscious proprioception was decreased in the right pelvic limb and normal in the remaining limbs. His withdrawal reflex in his right thoracic limb was decreased, indicating weakness in his right forelimb. Spinal reflexes were normal in the remaining limbs. No focal spinal pain was noted on palpation of the neck or back, and cutaneous trunci reflex was normal with no apparent cut-off. The neurologic assessment was consistent with ambulatory paraparesis (worse on the right) and weakness in the right forelimb localized to a caudal cervical lesion (C6–T2).

Complete blood count, serum biochemistry, and electrolytes were evaluated. The only value outside of the normal limits was an increased alkaline phosphatase, which is consistent with patient signalment. A cervical spine radiograph revealed a microchip located at C7–T1 spinal vertebrae (Figure 1A). On lateral views, the microchip appeared to be invading the intervertebral space of C7–T1. The patient was anesthetized and positioned in dorsal recumbency for cervical spine computed tomography (CT). Scans were performed using a 64-multislice CT scanner^a with a slice thickness of 0.5 mm for bone. On CT, the microchip was identified as a hyperattenuating, linear structure positioned within the vertebral canal at C7–T1 surrounded by streak artifacts (Figure 1C).

View Figure

• Download as Powerpoint
• Download Figure

The ataxia and pelvic limb weakness developed immediately after microchip placement, which is supportive of the microchip being the cause of the neurologic deficits. The owner elected for surgical intervention to remove the microchip. A dorsal approach to the caudal cervical spine was performed, centered over the seventh cervical vertebrae and the first thoracic vertebrae (C7 and T1, respectively) as described in a surgical textbook.⁷ During dissection of the soft tissues, the nuchal ligament was identified and left intact. Dissection along the right lateral border of the vertebrae was performed based on advanced imaging findings. The microchip was visualized protruding on the right lateral aspect of the intervertebral space C7–T1 (Figure 2A). The microchip was removed, and a culture swab of the area was sent for culture and sensitivity. A laminectomy was not performed. The site was copiously lavaged. The muscular layers were closed with a simple continuous pattern with 3-0 PDS.^b The subcutaneous and intradermal layers were closed in a simple continuous pattern with 4-0 Monocryl.^c The culture results returned no aerobic or anaerobic growth. There were no complications during surgery.

View Figure

• Download as Powerpoint
• Download Figure

The patient recovered uneventfully. During recovery, <12 hr after surgery, the patient was independently ambulatory, with moderate paraparesis and ataxia. Before being discharged from the hospital, his neurologic examination showed mild pelvic limb ataxia, with a mild delay in conscious proprioception of the right pelvic limb. The patient was discharged with a tapering dose of corticosteroids (starting at 0.5 mg/kg daily), methocarbamol, and gabapentin. The patient was also to be kept activity restricted for at least 2 wk following surgery. The patient continued to maintain a mild degree of ataxia in the pelvic limbs during follow-up via videos sent by the owner 3 mo after surgery but was overall comfortable.

Discussion

Sudden acute neurologic deficits caused by improper placement of a microchip have been reported in a small number of cases in animals—three dogs and two cats.^2–6 All these cases, except one, underwent surgical removal of the microchip with positive outcomes.^2–5 All animals, regardless of management selection, maintained neurologic deficits.^2–6 Of those cases reported, a kitten with sudden onset nonambulatory tetraparesis following microchip placement underwent solely medical management with corticosteroids and physiotherapy.⁶ The kitten recovered to ambulatory status but had persistent neurologic deficits.⁶ Of the cases that were managed surgically, many did not require a laminectomy, and the microchips were removed with soft-tissue dissection and manual extraction.^2,4,5 This is similar to our case in which a laminectomy was not required.

In the majority of cases reported, residual and lasting neurologic deficits remained.^2–6 This is consistent in our case with continued pelvic limb ataxia present several months following surgery. One limitation of this case report is that the cause of the persistent neurologic deficits cannot be definitively proven. Possible sources are the initial compressive injury, iatrogenic damage during surgery, or postoperative complications. Hicks and Bagley reported in their case that magnetic resonance imaging after surgical removal of an improperly placed microchip showed a linear hyperintensity from the muscle to the spinal cord highlighting the track of the microchip placement.⁴ This linear hyperintensity was consistent with edema and/or hemorrhage.⁴ This suggests that the microchip itself can cause damage. The possibility of damage from surgical removal should be a topic of future investigation.

The decision to select medical or surgical management of improper microchip placement should be case dependent; however, currently, surgical management seems to be preferred.^2–5 Recovery after surgery resulted in a positive outcome in all cases reported.^2–6 This is consistent in our case in which the patient recovered uneventfully. In the case report in which solely medical management was performed, the patient also recovered well.⁶ However, follow-up CT in the patient showed migration of the microchip located at the dens axis to the foramen magnum.⁶ Although the patient showed no neurologic changes due to the migration of the microchip, delayed onset of neurologic signs could occur.^5,6 A delayed onset of spinal cord injury caused by a microchip has been reported.⁵ In this report, they suggest that the close proximity of the microchip to bone resulted in a slow erosion of the lamina, leading to osteolysis, which eventually led to the microchip’s contact with the spinal cord.⁵ The knowledge of delayed-onset spinal cord injury could put surgical removal of misplaced microchips even in animals in which no neurologic signs are yet seen as the preferred treatment. Further investigation on medical versus surgical management is needed.

In four of the five cases reported, the animals were either small (<2 kg) or the microchip was placed by nontrained individuals.^2–6 To avoid complications, microchips should only be placed by licensed veterinarians or under the supervision of a licensed veterinarian.⁸ Proper placement of microchips in dogs and cats is subcutaneously midline between the shoulder blades, which will limit migration.^1–3 If complications occur immediately after microchip placement or a microchip is found to be an issue later in life, consultation with a veterinary surgeon or neurologist could be pursued.

Conclusion

Our case report supports that surgical management of improper microchip implantation can result in a favorable outcome. Clinicians and owners should be aware that surgical management of improper microchip placement does carry a risk of further damage to the spinal cord due to the close proximity of the surgery. In our case and all other cases reported, there were persistent neurologic deficits after medical or surgical management. Clinicians should inform owners that complete neurologic recovery may be unlikely.