DigiGait for Spinal Cord Injury

The most devastating effect of spinal cord injury [SCI] is the loss of the ability to walk. Animal models of SCI are critical to the development of therapies to restore locomotive capacity. The DigiGait™ Imaging System is in use worldwide to better understand and develop therapies for numerous human conditions, including amyotrophic lateral sclerosis, muscular dystrophy, and arthritis.

The ability of DigiGait™ to describe the posture and kinematics of animals with nerve injury under a wide range of conditions with high repeatability is making the instrumentation the gold standard for SCI research.

Based on various animal models, it is generally accepted that central pattern generators (CPG) exists for the rhythmic generation of stepping movements. Computerized gait analysis is becoming increasingly important to the SCI research community, to provide quantitative metrics broadcast by the CPG. DigiGait™ uses ventral plane videography to three-dimensional gait analysis in rodents, reporting metrics that reflect the numerous postural and kinematic aspects of rhythmic stepping. See how DigiGait™ works! Inking of paws or paradigms that hope the animal might cross a walkway, do not provide data regarding the rhythmicity of truly ambulating animals. Watch how DigiGait™ “digitally paints” the paws:

The most important confounder in the interpretation of gait data are differences in walking speeds between healthy sham control animals and animals with SCI.  Overground gait analysis, therefore, often provides inconclusive data because of the usual slower walking speed in injured animals. Injured animals will often prefer not to walk, or may walk slowly, and therefore many of the gait metrics, such as reduced stride length, are secondary to a slower walking speed. DigiGait™ empowers the investigator to have all animals walk the same speed, or a range of speeds [0.1 cm/s to 99.9 cm/s]. Quantifiable differences, such as more open paw placement angle or reduced duty cycle, can then be attributed to a gait disturbance rather than a slower walking speed.

For subtle gait disturbances, challenging the animals to walk faster and uphill or downhill can provide early subtle evidence of motor dysfunction. DigiGait™ provides researchers with much more accurate measurements, helping to improve their SCI research. Hoping an animal might cross a catwalk, a researcher might determine several gait indices from an animal having walked a few strides at often indeterminate speeds. Via DigiGait™, however, the investigator can expect good copious data with low standard errors based on multiple strides [10-30 strides typical, vs. ~5 strides with overground methodology]. The figure below depicts 20 gait signals for this subject walking at a known speed:

Data is from animals walking at the same speed, so there is no worry about the effects of speed on the metrics. This results in a faster cleaner study with a higher degree of repeatability.

Typical treadmill walking speeds for rat models are in the range of 20-40 cm/s, while for mouse models the range is slightly larger at 15-60 cm/s. The speeds achievable are often dependent on strain, age, and disease model. Protocols are model dependent, and can include challenging the animals with uphill and downhill walking and running. Even animals with complete transection of the spinal cord are able to perform the walking test on the DigiGait™ treadmill. Videos 1 and 2 illustrate a rat with transection of the spinal cord walking 20 cm/s, the same speed as its sham controls.

Video 1:SCI rat

Video 2: Normal rat

Despite the loss of hind limb function, the forelimbs operated more frequently to maintain the animal’s speed. Forepaw and forelimb postural adjustments were also apparent.

Figure 1 and 2 display the dynamic gait signals generated by the DigiGait™ system, indicating stepping characteristics of forelimbs (upper) and hind limbs (lower) of a rat recovering from SCI. Note the qualitative differences in a forelimb gait signal compared to a hind limb gait signal [e.g., sharp upstroke during the braking phase in the hind limbs].

Figure 1: SCI rat

Figure 2: Normal rat

As spinal injured animals recover, hind limb stepping is gradually restored, but with a marked asymmetry in the forelimb and hind limb stepping frequency. Note in Figure 1, for example, the increased stepping frequency of the forelimbs vs. the hind limbs in a rat recovering from SCI. The Basso-Beattie-Bresnahan (BBB) score for this rat was 14. Gait symmetry, which is the ratio of forelimb-to-hind limb stepping, was ~1.4 for this rat. [Note: in normal animals, and in animals with a BBB score of 21, gait symmetry is ~1.0.] Phase dispersion, a metric introduced by Michelle Basso’s laboratory [1] provides a quantitative metric of coordination. DigiGait™ adopts the published mathematical algorithm, which was based on the study of several rats walking overground at indeterminate speeds, and applies it to any animal walking at any speed. Below is a summary of coordination values for a rat with SCI and a sham control rat.

Correct assessment of forelimb and hind limb coordination during walking is one limitation within the BBB locomotor rating scale to quantify locomotor recovery following spinal cord injury [2]. Analysis of overground walking, moreover, is significantly confounded by differences in walking speeds among and between treatment groups of animals [3]. Ventral plane videography using the patented DigiGait™ imaging treadmill provides accurate assessment of coordination and robust gait kinematics at comparable walking speeds [4].

DigiGait™ reports over 30 metrics of posture and locomotion, many of them developed specifically for spinal cord injury research. Such metrics include: gait symmetry, phase dispersion, paw area variability, and weight support, which provide quantitative indices reflective of loss of coordination, dorsal stepping, and weight bearing. DigiGait™ also reports external and internal rotations of the paws, step sequence pattern, and nerve functional indices, including the sciatic, peroneal, and tibial functional indices (SFI, PFI, & TFI). Quantitative and controlled assessment of gait indices should facilitate testing of new drugs to restore locomotor function following spinal cord injury. To date, DigiGait has investigated transgenic inhibition of Nogo-66 receptor function [3], chondroitinase [5], and NMDA receptor-mediated excitotoxicity plus oxidative stress, as potential therapies for spinal injury [6].

By using DigiGait™’s specifically designed metrics, researchers of SCI can best investigate rodent gait, allowing for simpler, quicker, and more accurate results.

References

1. Stepwise motor and all-or-none sensory recovery is associated with nonlinear sparing after incremental spinal cord injury in rats. Exp Neurol. 2005;191:251-65.
2. The assessment of locomotor function in spinal cord injured rats: the importance of objective analysis of coordination. J Neurotrauma. 2005; 22:214-225.
3. Transgenic inhibition of Nogo-66 receptor function allows axonal sprouting and improved locomotion after spinal injury. Mol Cell Neurosci. 2005;29:26-39.
4. Quantification of locomotor recovery following spinal cord injury contusion in adult rats. J Neurotrauma. 2006; 23:1632-1653.
5. Effects of acute chondroitinase treatment and training on functional recovery following moderate spinal cord injury in rats. Program No. 405.6/TT10. Neuroscience Meeting Planner. San Diego, CA: Society for Neuroscience, 2007; online.
6. The functional and neuroprotective actions of Neu2000, a dual-acting pharmacological agent, in the treatment of acute spinal cord injury. J Neurotrauma. 2010;27:139-49.