Behavioral and anatomical consequences of repetitive mild thoracic spinal cord contusion injury in the rat

Highlights

• We present a novel model of repeated mild spinal contusion.

• Functional recovery is reduced after repeated mild contusion.

• Repeated contusion expands the injury and increases inflammation/apoptosis.

• Thus, mild contusion predisposes the spinal cord to increased vulnerability.

Abstract

Moderate and severe spinal cord contusion injuries have been extensively studied, yet much less is known about mild injuries. Mild contusions result in transient functional deficits, proceeding to near-complete recovery, but they may render the spinal cord vulnerable to future injuries. However, to date there have been no appropriate models to study the behavioral consequences, anatomical changes, and susceptibility of a mild contusion to repeated injuries, which may occur in children as well as adults during competitive sport activities. We have developed a novel mild spinal cord contusion injury model characterized by a sequence of transient functional deficits after the first injury and restoration to near-complete motor and sensory function, which is then followed up by a second injury. This model can serve not only to study the effects of repeated injuries on behavioral and anatomical changes, but also to examine the relationship between successive tissue damage and recovery of function. In the present study, we confirmed that mild thoracic spinal cord contusion, utilizing the NYU impactor device, resulted in localized tissue damage, characterized by a cystic cavity and peripheral rim of spared white matter at the injury epicenter, and rapid functional recovery to near-normal levels utilizing several behavioral tests. Repeated injury after 3 weeks, when functional recovery has been completed, resulted in worsening of both motor and sensory function, which did not recover to prior levels. Anatomical analyses showed no differences in the volumes of spared white matter, lesion, or cyst, but revealed modest extension of lesion area rostral to the injury epicenter as well as an increase in inflammation and apoptosis. These studies demonstrate that a mild injury model can be used to test efficacy of treatments for repeated injuries and may serve to assist in the formulation of policies and clinical practice regarding mild SCI injury and spinal concussion.

Introduction

Injuries to the spinal cord result in functional deficits, which depend on the severity and level of injury. Various animal models for spinal cord injury (SCI) including contusion, compression, transection, and hemisection have been developed and used depending upon the experimental goals of the study. Among these injuries, contusion injuries are the most common in human SCI. Contusion lesions created by a computer controlled impactor can be designed to be mild, moderate, or severe (Basso et al., 1996). While moderate and severe contusion injuries have been extensively studied, much less is known about the consequences of mild injuries. A mild injury, clinically defined as a spinal cord concussion, results in transient neurologic disturbances with deficits in sensory and motor function. A complete neurological recovery usually occurs within two to three weeks in animal mild contusion models (Basso et al., 1995, Basso et al., 1996, Scheff et al., 2003, Zhang et al., 2008), but the long term consequences of the injury are not known. Importantly, there is a long clinical history of spinal cord concussion, though it has assumed a variety of clinical terms, including transient paraplegia/quadriplegia/paresis, transient traumatic paraplegia/quadriplegia, neurapraxia, and spinal cord concussion (Brigham and Capo, 2013, Cantu and Cantu, 2005, Maroon et al., 2007, Torg et al., 1997, Torreman et al., 1996, Winder et al., 2011, Zwimpfer and Bernstein, 1990). Both pediatric and adult patients experience transient motor and sensory dysfunction following an acute blow to the cervical or thoracic spinal cord that gradually resolves over a period of 10 min to 48 h, without any radiological abnormalities observed within the spinal cord itself. Most patients sustain injuries following participation in a diverse array of contact sport activity, including American football, rugby, hockey, and wrestling, with the highest incidence rate occurring in American football — approximately 1.3 cases per 10,000, though this likely represents a significant underrepresentation due to failed reporting (Torg et al., 1986). Repeated spinal cord concussions are well known to occur, and numerous reports have documented increased recovery times, progressive neurological damage, and complete paralysis upon secondary concussion.

Unlike spinal cord concussion, brain concussions (mild traumatic brain injuries, mTBIs) have been studied extensively, as hundreds of thousands of sport- and combat-related injuries occur each year (Dietrich et al., 1994, Johnson et al., 2010, Povlishock et al., 1983, Prins et al., 2010). Although mTBI is usually not life threatening, their effects can have serious consequences. People who have had one concussion are more susceptible to another, especially if the new injury occurs before symptoms from the previous concussion have been completely resolved (Gronwall and Wrightson, 1974). In addition, there is also a negative progressive process in which smaller impacts cause the same symptom severity. Repeated concussions may also increase the risk in later life for dementia, Parkinson's disease, and/or depression (Mannix et al., 2013, Plassman et al., 2000). Animal studies indicate a complex pathology that includes the disruption of neuronal cell membrane accompanied by release of glutamate and a lower metabolic state which may persist for weeks after injury (Giza and Hovda, 2001). The efforts to increase awareness about symptoms of mTBI and how to manage them have culminated in the Zurich Consensus Statement on Concussion in Sport, which recommends persons to be symptom free before restarting activity, and then not all at once, but rather through a series of graded steps (McCrory et al., 2009).

Thus far, there have been no comparable studies on the effects and risks of repeated mild SCI in animal models. For example, it is not clear whether repeated mild contusion SCI results in cumulative or synergistic effects, with detrimental consequences on the potential for functional recovery, and whether the increased vulnerability is associated with specific biochemical, immunological, or histological changes. In this study, we addressed these issues in an established animal model, mild spinal cord contusion using the NYU-MASCIS (New York University — Multicenter Animal Spinal Cord Injury Study) impactor. Our findings indicate that even when the second injury occurs following the completion of recovery, the resulting deficits in both motor and sensory functions worsen, and there is no longer recovery to the same levels as the first injury.