On the 500year anniversary of his death, it is time to consider how Leonardo da Vinci’s iconic view of man in a square and a circle has shaped our traditional view of the human body. For patients and physicians alike, we are taught to think of the body from “head to toe”, but this standard view fails to emphasize the true relation of the pelvic floor to the rest of the body. 
The neural axis and neural crest cells have a key role in guiding tissue growth and form. (1) These nerves create templates of cranial to caudal segmental anatomy that guide tissue growth and development of the embryo. Let us consider a new view from the perspective of the nervous system, and explore how Leonardo might help to reveal some of the clinically important patterns of human form and function associated with pelvic floor problems.
The spinal nerves of the neck are ordered so that proximal nerves move proximal joints and more distal segments innervate more distal joints to move the muscles of the upper limbs. Thus C5 moves the shoulder, C6 elbow, C7 wrist, C8 long flexors and extensors of the fingers, and T1 moves the intrinsic muscles of the hand. In the same way, nerves of the lower limb are ordered such that the spinal segments that innervate the hip L2 and L3 are proximal to those for the knee L4 and L5, which in turn are proximal to the ankle S1 and toes S2 and S3. The spinal nerve segments responsible for pelvic floor muscle function include S2, S3 and S4, and thus overlap with those to the feet and extend more caudally in the conus of the cord. 
We propose a re-configured da Vinci figure set in a triangle. Simply the figure must assume a new position: sitting, with legs out in a split, arms folded across the chest. In this posture, we view the human figure as the nervous system does and reveal a new truth about our human condition. (2)
No longer the familiar figure standing strong and impervious, but pressed down into the triangle we present the body in a way that more accurately reflects our anatomy and development, not only vulnerable to congenital anomalies, but also subject to a range of clinical variation. Our figure in a triangle retains the safe space around the head, but the bottom sits directly on the vulnerable perineum, with no wiggle room below. Toes still reach to the margin of the figure, but now the tail end is “on the line”, making human newborns vulnerable to developmental shortfall such as imperforate or ectopic anus. Thus, instead of “head to toe”, the figure encapsulated in a triangle reaches from head to tail, and like the spinal cord from the base of the skull down to the coccyx and anus reaching not only to, but also beyond the spinal segments that extend nerves into the feet and toes. Experience tells us, it is easier to buy gloves that fit well than to buy shoes, because our hands are more similar than our feet are to one another. 
We have recognized clinical patterns of asymmetry in our patients with severe voiding symptoms and wondered what the significance might be. For example, those requiring surgical therapy such as electrical stimulation demand a choice of lead placement either on the right or on the left, and in this setting we have an opportunity to explore the clinical relevance of asymmetry. We have assessed the association between laterality of lead placement and patient asymmetry and hypothesize a better outcome with lead placement on the less well-formed and less versatile side.

We performed a ten-year chart review of all electrical stimulation placements (Medtronic, Interstim). A total of 57 patients were identified, who had undergone first-stage lead placement for non-obstructive urinary retention or OAB. Physical examination findings were recorded prospectively using a standard clinical protocol. Patient symmetry was assessed by preoperative physical examination findings including bilateral sacral sensory appreciation (two point discrimination of 4cm light touch in sacral dermatomes), gluteal muscle mass (standing), intrinsic muscles of the feet (abduction of the toes), and pelvic floor muscle strength (Oxford score). Asymmetry was defined as a significant, consistent lateralized difference in findings based on a single observer. Among asymmetric patients, the less well-formed side was deemed less versatile. Patients with only subtle findings were considered to be symmetrical. Successful lead placement was defined as a 50% or greater improvement in urinary symptoms and progression to surgical placement of the internal program generator.

Of the 57 patients reviewed, 47 (82%) progressed to successful second stage placement while 10 (18%) failed. A majority, 39/57 (67%) of patients had clinical features of significant asymmetry and of these 33 (84%) progressed to second stage. Of the 18 symmetrical patients, success was achieved in 14 (78%). In a subgroup analysis of 19 asymmetrical patients who underwent initial trial with bilateral lead placements, 13 reported greater benefit with the lead placed on the less versatile side, 2 were equivocal, and only 4 favored the more versatile side (p=0.003 Student’s T test).

Human evolution has prioritized heads over tails; human heads and brains are larger than ever, yielding great progress for the intellect, reason, and imagination. But, those same forces of selection have created problems for the caudal segments of our bodies – some of which we can manage better if we understand how to spot the signs, that something may be amiss. If you have funny-looking feet, for instance, you should be prepared for bladder and bowel problems, because the nerves that shape the form and function of the feet also shape the perineum and pelvic floor, so one will mirror the other in predictable ways.

Clinical features of asymmetry are very common, and in our practice account for the majority of patients with severe pelvic floor dysfunction. For those who are candidates for surgical neuromodulation, lead placement on the less versatile side correlated with significantly improved clinical outcomes. We suggest that clinical examination features of asymmetry in the sacral segments should be considered more carefully during preoperative evaluation of neuromodulation candidates to guide the choice of side for lead placement. 

Leonardo’s re-envisioned man in a triangle helps students and patients alike to understand the close relationships that exist between the form and function of the feet and pelvic floor. Furthermore appreciation of physical examination findings in the feet that distinguish the patient from an asymptomatic friend or relative might serve to provide an organic factor that could be contributing to the clinical pattern of problems. Recognizing these physical exam findings not only helps to set more realistic expectations for treatment outcomes, but in pointing to a possible organic cause may help to reduce some of the stigma associated with bowel and bladder problems.

The Neural Crest and Neural Crest Cells – Vertebrate Development and Evolution  ISBN 978-0-387-09845-6
Seeking Symmetry: Finding Patterns in Human Health  ISBN 978-1-912085-11-8