Why do some dogs recover from injury and resume their pre-injury performance while others take months of rehabilitation and never really return to full performance?

Why does some dogs’ pain persist after the tissue has healed and how can it be treated?

Pain, while unpleasant, protects the body from further injury by limiting our movement and allowing the tissue to heal. However, when pain persists after the injured tissue has healed, it is described as maladaptive and no longer serves a biologic function. When pain persists after muscle healing, the condition is considered to have transitioned to a chronic state. This transition from acute to chronic may be linked to neuroplastic changes observed in the nervous systems of human and animals with chronic, painful musculoskeletal conditions.

What is neuroplasticity?

Neuroplasticity is defined as changes in structure, function and organisation of the nervous system throughout an animal’s life. Neuroplasticity is the method by which the animal’s brain encodes new experiences, learns and develops new behaviours.

Changes to the central nervous system (CNS) can occur over a short training period and can occur in response to:

• Cognitive processes

• Internal changes in afferent sensory transmission

• External stressors such as motor learning / training

• Peripheral sensory stimulation

For neuroplastic changes to occur, the stimuli need to be repetitive, salient, involve learning, and require the animal’s attention. This has implications for the original changes occurring in the CNS but also for developing effective rehabilitation approaches.

Link between changes in the nervous system and musculoskeletal conditions

Human studies of people with chronic musculoskeletal disorders have found structural and functional changes in the peripheral and central nervous system. Conditions in humans where changes in sensory transmission and processing changes are observed are osteoarthritis, tendinitis, and lumbar and cervical injuries. Similar effects are also observed in dogs with these conditions.

Chronic musculoskeletal disorders result from the interaction between the initial injury to the anatomical structure and changes in the following:

• the afferent information being conveyed from the peripheral receptors toward the spinal code, brain stem and cortical areas and

• neuronal processing of noxious stimuli.

It is thought that associative learning from the initial trauma reinforces the pairing of pain and movement. There is growing evidence that the pain associated with musculoskeletal disorders result in plasticity of the sensory representation of the body and disturbance in perception. This aversive association between movement and pain is reflected and maintained by neuroplastic changes even without continued insult to the musculoskeletal structures. This is the transition of the pain state from adaptive to maladaptive.

Neuroplastic changes are seen throughout the CNS of humans and animals with chronic musculoskeletal conditions including:

• Peripheral nervous system and spinal cord

• Brain stem

• Sensorimotor areas

• Mesolimbic and prefrontal areas of the cortex

1. Changes in the spinal cord

Human and some animal studies have found evidence of adaptations in the peripheral receptors and dorsal horn of the spinal cord in subjects with musculoskeletal conditions.

This research showed increases in the receptor field size of some dorsal horn neurons which results in the amplification of noxious and innocuous stimuli from the injury site. Sensory amplification of painful stimuli leads to central sensitization to pain and secondary hyperalgesia and allodynia.

Sensitization to pain following a soft tissue injury serves a protective function. The animal associates movement with pain to protect the structure from re-injury. Sensitization, however should be transient and normalise when the tissue repairs.

2. Changes in the brain stem

Changes in the brain stem associated with chronic musculoskeletal disorders relate specifically to areas involving the descending pain modulatory pathways namely

• PAG (Periaqueductal grey pathway) and

• RVM (Rostral Ventro Medial pathway).

These pathways are influenced by the meso-limbic and opioid systems and normally inhibit transmission of noxious stimuli. With chronic musculoskeletal conditions these pathways can be disrupted and in fact amplify transmission of noxious stimuli.

3. Changes in the sensorimotor areas of the brain

Musculoskeletal conditions have been shown to alter neuronal properties and organisation within the primary and secondary somatosensory cortices (S1 and S2) and insula. These areas of the brain discriminate sensory inputs specifically the transmission and processing of noxious stimuli.

These neurological changes also affect the animal’s motor control. They alter patterns of muscle loading, recruitment and timing. Specifically, they affect the animal’s ability to selectively recruit muscle.

It is hypothesized that changes in the sensorimotor areas of the brain have a protective function to reduce the load on the injured muscles and tendons and therefore reduce pain. This is referred to as the pain adaptation model. This model states that when a muscle is injured the body adapts by reducing agonist muscle activity and increasing antagonist muscle activity.

4. Neuroplastic changes in meso-limbic and pre-frontal structures

There is a correlation between changes in the meso-limbic and pre-frontal structures of the brain and chronic musculoskeletal conditions. It appears that changes in these areas of the brain may be predictive of the transition from acute to a chronic condition.

The brains of subjects with chronic musculoskeletal pain differ from those of healthy subjects and these differences are principally observed in the meso-limbic and pre-frontal structures. Affected subjects experience spontaneous fluctuations in pain not necessarily associated with noxious stimuli. The abnormal activity in these parts of the brain results in increased vigilance and decreased ability to disengage from the pain.

Meso-limbic and pre-frontal structures also have connections with sensorimotor areas and influence descending pain modulating pathways (PAG and RVM) which leads to ongoing, abnormal augmented pain transmission.

What are the effects of changes in the nervous system?

The effects of neuroplastic changes can manifest as:

Altered sensory transmission including:

• Sensory amplification and persistence of pain perception often at sites remote to the original injury site – referred to a central sensitization.

Changes in motor control including:

• Altered muscle recruitment patterns which may lead to reduced variability in movement. In a study of humans with patellar tendinopathy, subjects demonstrated different jumping and landing patterns and generally less variability in their movements.

Changes in perceptual processes including:

• Alterations in body image perception (proprioception) e.g. distorted perception of size or position of a limb or body part resulting in increased error in repositioning the limb and decreased ability to detect joint motion.
• Changes in perception threshold to noxious stimuli
• Decreased accuracy in recognising tactile stimulation
• Symptoms being observed bilaterally when the original injury was unilateral

Psychological and behavioural changes including:

• Avoidance and decreased engagement with family and other pets, decreased attention and ability to learn, and guarding behaviours.

The implications for rehabilitating a dog with maladaptive pain responses due to a chronic musculoskeletal condition are significant. Treatment approaches need to not only address tissue healing but also the neural changes to restore the dog to their pre-injury state.

The next blog will review treatment approaches and explore current research into motor learning to potentially prevent neural changes occurring when a dog is injured. Please see https://www.fullstride.com.au/blog/rehabilitation-approaches-for-chronic-muscle-conditions for more information on treatment approaches.

Full Strides provides canine remedial massage treatments to support your dogs’ rehabilitation plan. For qualified canine and equine massage therapists in your local area please see the practitioner listing at www.saena.com.au .

Until next time, enjoy your dogs.

Sources:

Millis, D.L, Levine, D & Taylor R.A (ed) (2004) Canine Rehabilitation and Physical Therapy, Elsevier Inc, Missouri USA

Pelletier, R, Higgins, J, Bourbonnais, D (2015) “Addressing neuroplastic changes in distributed areas of the nervous system associated with chronic musculoskeletal disorders.” Physical Therapy, Vol. 95, No. 11, Nov 2015

Pelletier, R, Higgins, J, Bourbonnais, D (2015) “Is neuroplasticity in the central nervous system the missing link to our understanding of chronic musculoskeletal disorders?” BMC Musculoskeletal Disorders 16:25.

Rio, E, Kidgell, D, Lorimer Moseley, G, Gaida, J, Docking, S, Purdam, G and Cook, J (2016) “Tendon neuroplastic training: changing the way we think about tendon rehabilitation: a narrative review.” British Journal of Sports Medicine, Feb 2016, 50 (4): 209 – 215