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The Hands-On Evaluation - Spinal Reflexes

Spinal Reflexes

Spinal reflex evaluation should be considered as a continuation of the evaluation of the gait and postural reactions testing and not as a sole entity. 

It is extremely unlikely that you will have a reflex abnormality and not have a gait abnormality and or a proprioceptive abnormality as well. If you do, ask yourself whether you really believe your reflex findings.

Functionally the spinal cord can be divided into four regions:  cranial cervical [C1 to C5] – cervicothoracic [C6 to T2] – thoracolumbar [T3 to L3] and lumbosacral [L4 to S3] (see our section on Spinal Cord Segments).  Lower motor neuron (LMN) cell bodies are located within the grey matter of the cervicothoracic intumescence (segments C6 to T2) for the thoracic limbs and lumbosacral intumescence (segments L4 to S3) for the pelvic limbs.  Following gait and postural reactions testing, the clinician should be in a position to narrow down the lesion localization as being cranial to T3 spinal cord segments, caudal to T3 spinal cord segments, or within the peripheral nervous system (peripheral nerve, neuromuscular junction or muscles). 

Spinal reflex evaluation helps to further narrow down the lesion localization by testing the integrity of the C6 – T2 and L4 – S3 intumescences as well as respective segmental sensory and motor nerves (LMNs) that form the peripheral nerves and the muscles innervated. 

Note the contribution of multiple nerve roots and spinal nerves (originating from the cervical intumescence) to the peripheral nerves of the thoracic limb in the picture below. The radial nerve is highlighted.

Note the contribution of multiple nerve roots and spinal nerves (originating from the lumboasacral intumescence) to the peripheral nerves of the pelvic limb in the pictures below. The femoral nerve is highlighted in the top picture and the sciatic nerve is highlighted in the bottom picture.

Spinal reflexes are SEGMENTAL. They only evaluate the spinal segment(s) within the intumescences corresponding to the stimulated nerve.  They do NOT require consciousness.  Lesions at the level of these intumescences or affecting the peripheral nervous system result in loss of segmental spinal reflexes as well as reduced muscle tone and size.  Lesions cranial to the intumescence (upper motor neuron (UMN) dysfunction) will result in normal to increased segmental spinal reflexes (release of the inhibitory modulatory effect of the UMN on the LMN). 

One exception to that rule is worth mentioning in the context of the emergency patient.  Animals with severe, peracute transverse thoracolumbar spinal cord lesions usually show severe pelvic limb hypotonia and depressed spinal reflexes for a few hours to days after the onset.  Although it has been compared to a similar condition in man called spinal shock, the reasons that a UMN pathway interruption causes LMN-like pelvic limb signs are poorly understood. 

Despite many spinal reflexes being described, the most reliable are the withdrawal reflex in the thoracic limb and the patellar and withdrawal reflexes in the pelvic limbs.  Other spinal reflexes (triceps, biceps, extensor carpal radialis and gastrocnemius) are more difficult to perform and to interpret - they can be much more dependent on the experience of the examiner.  

It's important to note that reflexes are not affected by sedation. However, propofol and isoflurane eliminate all spinal reflex responses and are not suitable for neurological examinations.

For a more complete overview of the functional neuroanatomy responsible for spinal reflexes and how to test, visit our neuroanatomy pages.

Withdrawal (flexor) reflex

How to perform - A noxious stimulus is applied to the tested limb by pinching the nail bed or digit with the fingers or a haemostat.  This stimulus causes a reflex contraction of the flexor muscles and withdrawal of the tested limb.  If this withdrawal reflex is absent, individual toes can be tested to detect if specific nerve deficits are present.  In the thoracic limb, compression of the digits stimulates nociceptors in the radial nerve dorsally (and ulnar nerve in digit five) and in the median or ulnar nerve on the palmar surface.  In the pelvic limb, compression of the digit three to five stimulates nociceptors of the sciatic nerve (peroneal branch dorsally and tibial branch on the plantar surface).  

Thoracic limb flexor withdrawal

Pelvic limb flexor withdrawal

How to interpret - The withdrawal reflex is a segmental spinal cord reflex that only depends on the function of the local spinal cord segments.  On the thoracic limb, it evaluates the integrity of the spinal cord segments C6 to T2 (and associated nerve roots), brachial plexus, peripheral nerves (radial, axillary, musculocutaneous, median and ulnar nerves) and the muscles innervated.  On the pelvic limbs, it evaluates the integrity of the spinal cord segments L4 to S1 (and associated nerve roots) the femoral and sciatic nerves, and the muscle innervated.  It should be stressed that the withdrawal reflex in the thoracic or pelvic limbs does not depend on the animal’s nociception (perception of a noxious stimulus).  As we will discuss in a later chapter on disc disease, the withdrawal reflex in dogs with cervical disk herniation is not reliable for determining the affected site, as a decreased withdrawal reflex does not always indicate a lesion from C6 to T2 in these cases.

An incomplete thoracic limb withdrawal reflex

An incomplete pelvic limb withdrawal reflex - note that the dog feels the noxious stimulation and exhibits a behavioral response but does not exhibit a withdrawal of the limb with flexion of the joints.

Patellar reflex  

How to perform - The patellar reflex is elicited by hitting the patellar ligament and observing a reflex contraction of the quadriceps muscle and extension of the stifle joint.  This position allows the stifle to be slightly flexed and compared between the two sides.  Evaluation of the extensor tone on the pelvic limb can be used as a control in animals with an ambiguous patellar reflex as it involves the same neuro-anatomical components (L4 to L6 spinal cord segments, femoral nerve, and quadriceps muscle).

A normal patella reflex can be seen in this dog.

How to interpret - The patellar reflex evaluates the integrity of spinal cord segments L4 to L6 (and associated nerve roots) as well as the femoral nerve.  A weak or absent reflex indicates a lesion of the L4 to L6 spinal cord segments or the femoral nerve.  A similarly weak or absent reflex can on occasion be seen with stifle disease.  A lesion cranial to the L4 spinal cord segment can cause a normal or exaggerated patellar reflex.  In the absence of other neurological deficits, an exaggerated patellar reflex means little and can be observed in an excited or nervous animal.  Finally, the patellar reflex can appear hyperreflexic with a sciatic nerve or L6 to S1 spinal cord segment lesion.  This pseudo-hyperreflexia is a result of decreased tone in the muscles that flex the stifle and normally counteract stifle extension during the patellar reflex. Neurologically normal dogs may have an age-dependent decline in patellar reflex magnitude or a prolongation of total reflex time.

Perineal reflex 

How to perform - The perineal reflex is elicited by stimulation of the perineum with a hemostat resulting in contraction of the anal sphincter and flexion of the tail.  

A normal perineal reflex can be seen in this dog.

How to interpret - This reflex tests the integrity of the caudal nerves of the tail, the pudendal nerve, spinal cord segments S1 to Cd5, and associated nerve roots.

Cutaneous trunci (panniculus) reflex

How to perform - This reflex is elicited by pinching the dorsal skin of the trunk between the vertebral level T12 and L4 to L5 and observing a contraction of the cutaneous trunci muscles bilaterally producing a twitch of the overlying skin.  This reflex is present in the thoracolumbar region and is absent in the neck and sacral region.  Testing is started at the level of the ilial wings: if the reflex is present at this level the entire pathway is intact and further testing is not necessary.  

A normal cutaneous trunci reflex can be seen in this dog.

How to interpret - From the dermatome tested, the sensory nerve from the skin enters the spinal cord at the level of the segments corresponding to that dermatome (approximately two vertebrae cranial to the level tested).  Afferent sensory information ascends the spinal cord and synapses bilaterally at C8 to T1 spinal cord segments with the motoneurons of the lateral thoracic nerve, which courses through the brachial plexus and innervates the cutaneous trunci muscle.  The panniculus reflex can be decreased or lost with a lesion anywhere in this pathway (dorsal nerve roots, spinal cord, lateral thoracic nerve).   With spinal cord lesions, this reflex is lost caudal to the spinal cord segment affected, indicating the presence of a transverse myelopathy.  Pinching the skin cranial to the lesion results in normal reflex while stimulation of the skin caudal to the lesion does not elicit any reflex.  Such findings help to further localize lesions between T3 and L3.  This reflex can also be lost ipsilaterally (with normal reflex on the other side) with conditions affecting the brachial plexus regardless of the level that which the skin is stimulated.  In the absence of other neurological deficits, the absence of the cutaneous trunci reflex means very little.

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