Myofascial pain syndrome (MPS) has been studied in humans and animals. It is defined as a regional pain syndrome caused by a dysfunction of muscle and the surrounding fascia. MPS is characterised by muscle pain caused by myofascial trigger points (MTrPs) or tender points. Secondary dysfunctions may include weakness, stiffness, restricted range of motion, and sleep disturbances.

What is a trigger point?

A tender point in a muscle is characterised as a trigger point when the following are present:

• The tender point has developed into a taut band or nodule within the muscle belly where the motor end plate (juncture of between motor neuron and muscle fibre) is located.

• There is referred pain in the area surrounding the tender point when palpated or with movement.

• There is a local twitch response (LTR) when any pressure is applied to the point.

These abnormalities in the region surrounding a MTrP have been identified in human and animal studies and can be categorised as sensory and motor.

Sensory abnormalities

Pressure is applied to the area elicits a pain response in the area and potentially, referred pain (in another area of the body) and a local twitch response. (LTR). The cause of these sensory responses is understood to be sensitized nociceptors at the site of the local twitch response.

Motor abnormalities

The second aspect of a myofascial trigger point area is the motor responses. Studies have shown evidence of dysfunctional motor endplates (where the motor nerve fibres meet the skeletal muscle fibres at the neuromuscular junction). This dysfunction is evidenced by spontaneous electrical activity including end plate “noise” and spikes in electrical activity within the region of the trigger point.

Pathology of MPS

It is thought that motor abnormalities and the development of a taut band is due to excessive leakage of acetylcholine in the end plate region. Acetylcholine (ACh) is a neurotransmitter released from motor neurons to activate muscles. Excessive release of ACh causes regional shortening of sacromeres of muscle fibres involved in the taut band leading to a contracture knot in the region of the motor end plate. Sacromeres in the contracture knot remain hyper-contracted while the remaining muscle fibres are maximally stretched as compensation for the missing length of the shortened sacromeres.

This mechanism for the development of a taut band is thought to occur due to an “energy crisis”. The “energy crisis” theory posits that the increased release of acetylcholine in the neuromuscular junction cause muscle fibre tension resulting in the taut band which in turn increases metabolism and inadequate blood supply and oxygen to the muscles. This condition leads to increased secretion of sensitising substances leading to pain and an increase of acetylcholine.

Studies investigating the biochemicals found in trigger points support the energy crisis theory. These studies found that substances associated with pain and inflammation (Substance P and Calcitonin gene-related peptide (CGRP)) were present in active MTrP (presence of pain and trigger point) but not latent trigger points (no pain and trigger point) or normal skeletal muscle (no pain or trigger points).

Human and animal studies of myofascial pain indicate that the referred pain and local twitch response seen in MTrPs is a function of the spinal cord mechanism. These studies suggest that central sensitisation in the spinal cord can diffuse substance P and Calcitonin gene-related peptide (CGRP) to other dorsal horn neurons. These chemicals are released when noxious stimuli from the MTrP region send impulses to the corresponding dorsal horn neurons hence pain responses in other areas of the body. Animal studies found a local twitch response could be elicited when pressure is applied to the trigger point only when a nerve that innervated the muscle was intact and had a correct connection with the spinal cord. The conclusion from these studies is that the local twitch response is mediated by the spinal cord reflex.

Trigger points can be classified as latent and active. Latent trigger points are those where a taut band can be felt in the muscle but there is no pain response. Active trigger points on the other hand elicit a pain response, potentially referred pain and a local twitch response. Human studies showed that referred pain could be elicited from active and latent trigger points. These studies showed that more pressure was required to elicit a response from latent trigger points than active ones. The amount of pressure to elicit a response directly correlates with the degree of irritability of the trigger point.

A characteristic of MPS is that latent trigger points can be activated. The mechanism by which latent trigger points are activated through MTrP circuits. A MTrP circuit is defined as the neural network with connections among dorsal horn neurons related to a MTrP. The MTrP circuit send nerve branches to connect with other MTrP circuits. In this way, latent MTrPs can become active if stimuli from other circuits are strong enough.

How to identify myofascial pain?

In veterinary patients, myofascial pain may be undiagnosed and therefore untreated. Animals with undiagnosed myofascial pain can be labelled as aggressive, lazy or stubborn.

Diagnosis of MTrP is mainly through manual palpation to identify the following:

• spot tenderness,

• taut bands in muscles,

• restricted range of motion, and

• regional pain

Where these are signs are present, referred pain and local twitch response can confirm the presence of myofascial pain.

What causes myofascial pain?

MPS can be caused by acute and chronic muscle overload.

In the acute state, MPS may be caused by acute muscle overload such as a trauma (car accident) or incidence of poor co-ordination when fatigued (slip, trip, fall).

Chronic MPS can be caused by sustained muscle contraction from repetitive movement, minor muscle strain, poor posture, systemic disease or neuromuscular lesions such as strain, sprain, bursitis, arthritis or vertebra disc lesion. These lesions can be caused by muscle overactivity.
The chronic state of MPS is characterised by an increase in the number of MTrPs and increase in localised and referred pain in other muscles.

While myofascial pain syndrome is still not well documented for veterinary patients, animal studies suggest that dogs can suffer from this syndrome. In the next article, conservative methods for treating myofascial trigger points will be discussed.

Full Stride provides canine myofunctional therapy treatments to keep dogs active and exercising regularly without pain. For certified animal myofunctional therapists working in your area please see www.saena.com.au

Until next time, enjoy your dogs.

Sources:

Kuan, T. S. (2009). Current studies on myofascial pain syndrome. Current pain and headache reports, 13(5), 365-369.

Qureshi, N. A., Alsubaie, H. A., & Ali, G. I. (2019). Myofascial Pain Syndrome: A Concise Update on Clinical, Diagnostic and Integrative and Alternative Therapeutic Perspectives. International Neuropsychiatric Disease Journal, 1-14.

Simons, D. G. (2002). Understanding effective treatments of myofascial trigger points. Journal of Bodywork and movement therapies, 6(2), 81-88.

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