Cutaneus trunci reflex
Sensory: Segmental spinal nerves
Motor: Lateral thoracic nerve (originating at C8-T1)
Pathway: The sensory arm of this reflex involves segmental spinal nerves originating at each level of the vertebral column from L6 to the cranial thoracic region. The segmental spinal nerves exit the spinal cord and proceed caudolaterally into the periphery. As a result, the skin sensory receptors will be caudal to the location where the sensory fibers enter the spinal cord. These sensory fibers immediately partially cross over the spinal cord and synapse on long interneurons that ascend bilaterally and synapse on motor neurons in the C8-T1 region giving rise to the lateral thoracic nerves.
Technique: Pinch the skin with a hemostat approximately 2 cm lateral to the spinous processes starting at L6. If the response is present bilaterally at L6, you do not need to proceed cranially since this indicates the pathway is intact bilaterally. If there is no response, continue cranially until there is a positive response, which is called the “cutoff” level.
Normal response: Since the sensory arm of the reflex ascends the spinal cord bilaterally, you should see the skin twitch on both sides.
- Historically, it has been taught that the cutaneus trunci cutoff occurs approximately 2 vertebrae caudal to lesion location, but a prospective study analyzing the cutaneus trunci reflex in 41 dogs with spinal cord disease showed that the cutoff occurred from 0 to 4 vertebrae caudal to the lesion (Gutierrez-Quintana et al, J Sm Anim Pract, Aug 2012). In this study, 48.5% of dogs had a cutoff 2-3 vertebrae caudal to maximal spinal cord lesion location.
- In patients with evidence of spinal cord disease, a unilateral cutaneus trunci cutoff can help determine the side of the spinal cord lesion, which would be ipsilateral to the cutoff.
- Absent cutaneus reflex on one side only in patients with thoracic limb lameness or weakness suggests the lesion is located in the ipsilateral spinal cord at the C8-T1 spinal segments or in the ipsilateral lateral thoracic nerve. This helps identify patients with brachial plexus injuries or neoplasms.
Muscle tone & atrophy
Muscle tone can be assessed in several ways. While the patient is standing, gently press down on the vertebral column over the limbs to determine how much strength the patient has in the limbs. Muscle tone is also analyzed during withdrawal reflex testing. Push the limbs toward the body to determine how much resistance is present. Also, determine muscle strength by gently resisting patient limb flexion during withdrawal testing.
- Decreased tone is suggestive of dysfunction of the lower motor neuron in the spinal cord or peripherally in the peripheral nerve, neuromuscular junction or muscles.
- Increased tone is suggestive of an upper motor neuron lesion cranial to the intumesence of the limb being tested (cranial to C6 for the thoracic limbs and L4 for the pelvic limbs).
Palpate the muscles of the head, trunk and limbs to check muscle atrophy. There are two types of muscle atrophy.
- Denervation atrophy occurs secondary to dysfunction of the nerve innervating the muscle that is atrophied. Denervation atrophy is seen early in the course of disease and is often moderate to severe.
- Disuse atrophy occurs secondary to decreased use of the muscles involved. It occurs later in the course of disease and is often mild to moderate unless there is chronic disuse.
Anal tone (perineal reflex)
The perineal reflex tests the S1-S3 and caudal aspects of the spinal cord and the pudendal nerve (perineal and caudal rectal branches). Lightly stroke or pinch the perineal skin with a hemostat. The normal response is contraction of the anal sphincter and possibly flexion of the tail. Anal tone also can be determined during digital rectal exam.
It is very important to determine whether the patient has intact nociception (“pain perception”) in the limbs. Nociceptive testing should be performed in all patients that are paralyzed, but need not be performed in ambulatory patients or in nonambulatory paretic patients. Patients with voluntary movement of the limbs should have intact nociception. Use the least amount of pressure required to elicit a response. Start with your finger tips on the interdigital skin before moving on to the toes. Use a hemostat only if you are unable to elicit a response with your finger tips.
NOTE: Positive nociception requires a behavioral response to the noxious stimulus, such as crying, whining, whimpering, trying to bite, or more subtle signs (e.g., dilation of the pupils, increased respiratory rate). Withdrawal of the limb does not equate to intact nociception; withdrawal involves only a local spinal reflex. A behavioral response is needed to show that the patient consciously recognizes the pain sensation.
NOTE: Many texts and other sources describe “superficial pain” vs. “deep pain” perception and attempt to distinguish between them by the location and amount of force applied to the toes. From a practical and clinical point of view, the most important thing is to determine whether nociception is present or not. The patient’s prognosis following a spinal cord injury is determined by the presence or absence of ANY nociception in the affected limbs. For example, patients with an acute Type I intervertebral disc extrusion that are paraplegic with intact nociception have an 85-95% chance for recovery following surgery compared to only 50% chance for recovery in paraplegic dogs with absent nociception if surgery is performed within 24-48 hours.
Palpation of the head and vertebral column
This should be saved for last. Palpating the vertebral column earlier in the exam may lead to an uncooperative patient and compromise the neurological exam if the patient is painful. Palpate the entire length of the vertebral column to check for any swelling, obvious misalignment, or pain.
- Start with a light touch at the end of the body opposite to where you suspect the pet is painful. For example, if you suspect the patient has cervical discomfort, start palpating at the sacrum and move cranially.
- Palpate the abdomen to get an idea of how much the patient tenses up before palpating the thoracic and lumbar vertebral column with one hand on the abdomen. Caudal thoracic & lumbar discomfort may be manifested only by abdominal muscle tensing.
- Put the head and neck through a full range of motion. Normal patients should be able to touch their nose the the side of the thorax. Do not flex the neck in young to middle-aged toy & small breed dogs that have neck pain until you rule out an atlantoaxial subluxation. This could lead to potentially catastrophic worsening of neurological signs.
- It can be difficult at times to determine whether a patient has discomfort from the caudal lumbar / LS region or hips. Dorsal palpation of the lumbar vertebral column in standing animals also transfers pressure down onto the hips and can elicit discomfort if there’s coxofemoral osteoarthritis. Extension of the limbs to check for hip discomfort also extends the caudal lumbar pain so LS discomfort may be elicited on extension of the hip. LS pain should be strongly suspected if there is pain on elevation of the tail or during palpation of the ventral LS region during digital rectal exam as neither of these tests manipulates the hips.
- Palpate the head just above the zygomatic arches to check for discomfort in the head.