Dr Simona Radaelli

RCVS and European Specialist in Veterinary Neurology

Nociception is the conscious perception and response to a noxious stimulus. It is sometimes also referred to as deep pain perception.

It should always be tested and in particular in spinal patients that show absent voluntary movements. The terms paralysis or plegia are used synonymously and refer to the absence of voluntary movements.

As a general rule, if the animal has voluntary movements, it should have nociception, especially if an extramedullary lesion is suspected.

When the spinal cord is damaged, loss of function develops in the following order: loss of proprioception, loss of movements and then loss of nociception.

Diagram showing a cross-section of the spinal cord with the distribution of the relevant pathways.

In non-ambulatory patients with a suspected spinal cord lesion, it is very important to assess the nociception, to ensure that the pathways (spinothalamic tract) carrying pain sensation are still intact within the spinal cord. These neurons lie deeply in the spinal cord white matter and they form a bilateral multisynaptic network. The nociceptive fibers are more resistant to damage than other pathways, therefore they are affected only by severe damage to the spinal cord. Nociception is a very important prognostic factor in these patients.

In the past, nociception has been divided in to superficial and deep. Superficial (fast) pain commonly originates from the skin and it is well localized and sharp. Deep pain (slow) originates often also from deeper structures and it is poorly localized and burning, aching. This distinction is no longer being used in clinical neurology, as the pathways responsible for the two types of pain perception are poorly divided anatomically.

How do I assess nociception?

When performing a neurological examination, it is very important to be aware of the key tests that are relevant to localize the lesion within the spinal cord and to help the clinician to determine prognosis for the patient.

A rubber hammer and a haemostat are useful tools to perform the key tests that will allow precise assessment of the spinal cord functions.

Tools commonly used for the neurological examination (light source, hammer, haemostat) (© Simona Radaelli).

In spinal patients, spinal reflexes and nociception (deep pain) are assessed in lateral recumbency. It is often useful to start with the hind limbs as most animals are sensitive to the limbs being touched, especially the front limbs. It is important to have the patients’ full cooperation for these tests, as they could voluntarily interfere with the response to the tests. Cats might prefer to be held on their back on the examiner’s lap, in a slightly seated position, for these tests.

It is recommended to test nociception at the end of the neurological examination, to avoid losing the animal’s cooperation for further tests.

The nociception is tested by pinching the animal’s digits or tail with the fingers first and then if there is no initial response, with a haemostat. Progressively increasing pressure should be applied if no response is elicited. When assessing nociception, all the digits and all the legs should be tested for comparison.

Assessment of nociception: note that this patient is turning to look towards the leg being pinched and had to be restrained from snapping at the clinician.
This means that nociception is present in the right hind limb (© Simona Radaelli).

It is important to test both medial and lateral digits, especially in the pelvic limbs, as the femoral nerve innervates the medial digits and the sciatic nerve the lateral digits. Nociception of the tail and perineal region should also be routinely tested, especially in animals where a L7-S3 lesion is suspected.

An animal with intact nociception should react by showing awareness of the pain: turning the head, crying, vocalizing, trying to bite, trying to move away with the whole body are signs of intact nociception.

Nociception is a behavioural response of the patient to a painful stimulus. Withdrawal of the limb IS NOT a sign of intact nociception, but only of an intact segmental reflex (peripheral nerve and spinal segments).

Animals with severe UMN (upper motor neuron) lesions might have intact withdrawal reflex and absent nociception, therefore it is useful to look at the animal’s head when testing the nociception, not the legs, as well as testing non affected limbs.

What about the withdrawal of the leg?

Withdrawal reflex is a spinal reflex; spinal reflexes are segmental, therefore they assess the reflex arc made by the limb’s sensory nerve, segmental spinal cord and motor components. This test assesses the spinal cord segments C6-T2 (thoracic intumescence) and L6-S2 (lumbar intumescence) and the nerves originating from these segments. When testing the withdrawal (flexor) reflex the skin between the digits is pinched and the animal should quickly and strongly withdraw the paw towards the body, flexing all the joints.

Spinal reflexes do not assess communication to the brain: they are a local unconscious reactions. The presence or absence of withdrawal reflex should not be confused with the response to nociception (which should show a conscious response, like turning the head, growling etc.) and it should not be used for prognosis.

How do I use nociception for the prognosis?

When assessing a patient with a spinal cord injury, neurological signs (like the presence or absence of voluntary movements) and the response to key tests, like nociception, are very important prognostic indicators.

As a general rule, a non-ambulatory animal with a spinal lesion and absent nociception has a guarded prognosis for recovery.

Animals with a suspected extramedullary compression (like intervertebral disc disease), that have had absent nociception for more than 48 hours, have a poor prognosis, even if the cause of compression is surgically removed. It is therefore very important to assess nociception with accuracy.

Animals that have suffered from a spinal trauma (fracture, luxation) and have absent nociception, have a very poor prognosis for recovery of function of the spinal cord, even if the fracture is reduced and stabilised soon after the trauma.

In case of spinal trauma, the loss of nociception carries a very poor prognosis, regardless of the length of time from the trauma. Once the nociception is lost, the chances of recovery are immediately very low.

In animals with severe L6-S2 lower motor neuron spinal cord lesions, the anal tone could also be lost, the perineal reflex absent and the perineal area (and base of the tail) could have absent nociception when pinched with a haemostat. The absence of nociception at this level has a negative impact on the chances of recovery of urinary and faecal continence.

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