Clinical Case: Degenerative Sacrolisthesis

Objectives

• What is meant by the pars interarticularis? How it relates to spondylolisthesis.
• How muscle strength testing is done and measured; how tendon reflex testing is done and measured. 
• Why bladder control was affected in this patient.
• What is meant by the anal wink response; and what that meant for this patient.

Pars interarticularis and spondylotisthesis relation

The pars interarticularis is the part of vertebra located between the inferior and superior articular processes of the facet joint (Figures 9&10). In the transverse plane, it lies between the lamina and pedicle. It is abnormal in spondylolysis, either due to fracture or a congenital defect.

On an anterior oblique radiograph of the lumbar spine, the pars is the neck of the imaginary Scottie dog; the Scottie dog's eye is the pedicle, its hindlegs the spinous process, its nose the transverse process, its ear the superior articular facet and its forelegs the inferior articular facet.

In typical spondylolytic spondylolisthesis (congenital) a bilateral break in the pars interarticularis allows the L5 vertebral to slip forward on the S1 vertebral body. As noted in this case the forward slippage is called spondylolisthesis; this case was unusual in that the slippage occurred between S1and S2.

Muscle strength and tendon reflex tests

The strength of specific muscle groups in the body is tested against resistance, and one side is compared with the other. However, a caveat is that pain may preclude the patient from revealing their full muscle strength potential. The most common muscle strength scale was originally developed by The Medical Research Council of the United Kingdom:

• 0: No visible muscle contraction
• 1: Visible muscle contraction with no or trace movement
• 2: Limb movement, but not against gravity
• 3: Movement against gravity but not resistance
• 4: Movement against at least some resistance supplied by the examiner
• 5: Full strength

The difficulty with this and similar scales is its subjectivity and the especially large range in potential strength between grades 4 and 5. Deep tendon reflexes are assessed using a reflex hammer to stretch the tendon and elicit a quick stretch reflex response. The most widely known reflex is that done by striking the patellar tendon to elicit active extension of the knee by the quadriceps femoris muscle. The extent of the reflex is evaluated both visually and by palpation of the tendon or muscle in question. The most common scale for tendon reflexes is based on the number of plus signs assigned to the response: 

• 0+ No response or absent reflex 
• 1+ Trace or Decreased response 
• 2+ Normal response
• 3+ Exaggerated or brisk response 
• 4+ Sustained response (clonus)

Bladder control in spondylotisthesis

Urine formed in the kidneys flows through the ureters and collects in the bladder. The muscular layer of the bladder is composed of a complex network of smooth muscle, known as the detrusor muscle (Figure 11.), which upon contraction reduces all dimensions of the bladder.

In the male, a distinct collar of circularly oriented non-striated muscle occurs in the bladder neck. This smooth muscle sphincter is supplied by sympathetic fibers that cause the sphincter to contract, thereby preventing retrograde flow of semen into the bladder during ejaculation. In the female, no such muscle sphincter is anatomically recognizable; thus, the role of this circular muscle in urinary continence is probably negligible.

To attain urinary continence the bladder acts as a passive reservoir that retains fluid because the intra-urethra pressure is greater than intravesical pressure. Intra-urethra pressure is generated by smooth muscle within the urethra, striated muscle in the external sphincter urethra (which is morphologically adapted to maintain tone over long periods), and levator ani (especially in women and especially when there is an increase in intra-abdominal pressure as in coughing, sneezing, etc.).

The detrusor muscle is innervated by parasympathetic nerves. These fibers originate as pelvic splanchnic nerves, pass through the inferior hypogastric plexus, reach the bladder wall and synapse within the wall with postganglionic fibers, which, in turn, stimulate muscle contraction. Although sympathetic fibers also reach the bladder wall (from L1 and L2 segments via hypogastric plexus), these nerves probably are mainly vasomotor (in males however they supply the distinct muscle at the bladder neck as described above). In the patient, in this case, the compression of the sacral nerves due to the spondylolisthesis (Figure 3) affected the parasympathetic fibers in spinal nerves S2-4 resulting in a flaccid bladder that retained urine.

Anal wink response

The anal wink, anal reflex, perineal reflex, or anocutaneous reflex is the reflexive contraction (puckering) of the external anal sphincter upon stroking of the skin around the anus. The stimulus is detected by the nociceptors in the skin that are supplied by the inferior rectal nerves (branches of the pudendal nerve); their reflex response is mediated via spinal cord sacral segments S2-S4. The absence of this reflex indicates that there is an interruption of the reflex arc, or damage to the spinal cord. In the case of this patient, the S2-S4 spinal nerves were being compressed as evidenced by the absence of this reflex as well as the urine retention (Figure 3).