Three theories have been postulated to explain the origin of varicocele, which are not mutually exclusive. The first describes the right-angled insertion of the left testicular vein into the left renal vein, with a consequent increase in the hydrostatic pressure that is ultimately transmitted to the pampiniform plexus [26, 34]. The second relies on congenital incompetent (or absent) venous valves, resulting in retrograde flux and dilatation [18, 34]. This theory has been supported by venographic and color Doppler studies. Based upon the level of these incompetent valves being at or below the communicating veins, which include the internal spermatic, cremasteric, vassal and external pudendal veins, two pathophysiologic subtypes have been described, namely shunt and stop types, as shown in Fig. 2.2a and b. When the incompetent valves are located only above the level of the communicating veins, a stop-type varicocele is present, which constitutes 14 % of all varicoceles. The stop-type varicocele is characterized by a brief retrograde flow from the internal spermatic vein towards and into the pampiniform plexus. No orthograde venous blood flow and reflux towards the communicating veins is seen because distal valves are present and are functionally competent. Surgical ligation of the stop-type varicocele shall cure the varicocele by offsetting the reflux-producing incompetent valve against valves from the remaining normal venous drainage system [35]. Conversely, when incompetent venous valves are present below the communicating veins, a shunt-type varicocele is present, which constitutes 86 % of all varicoceles [35, 36]. Shunt-type varicocele is characterized by retrograde blood both from the internal spermatic vein into the pampiniform plexus and orthograde reflux into the communicating veins (vasal and cremasteric veins) [37]. Surgical ligation of the shunt-type varicocele would be expected to be less effective because the incompetent valves are most numerous and widely distributed. Mohseni et al. [38] reported in a prospective controlled study involving 74 children and adolescents with varicocele that the shunt-type was associated with a greater risk of testicular hypotrophy compared to the stop-type varicocele. In addition, the authors noted that a higher recurrence rate occurred when the shunt-type varicocele had been repaired by the retroperitoneal approach compared to the inguinal approach.

The third theory involves the so-called nutcracker effect, in which compression of the left renal vein between the superior mesenteric artery and abdominal aorta would partially obstruct the blood flow through the left testicular vein and therefore increased the hydrostatic pressure inside the pampiniform plexus [39]. The nutcracker phenomenon builds up a steadily raised renocaval pressure gradient and reflux down the internal spermatic vein, resulting in the development of collateral venous pathways [40–43]. Evidence supporting this theory was provided by hemodynamic studies in adults and children with varicocele. In adults, Mali et al. [40] reported correlation between the renocaval pressure gradient and renospermatic reflux, thus showing that the severity of left renal vein compression in the upright position determines the velocity of retrograde flow in the left spermatic vein and varicocele size.

Doppler ultrasonography can be used as the first diagnostic test in patients with suspected nutcracker phenomenon [43, 46]. The B-mode sonographic measurement of the diameter of the LRV combined with Doppler sonographic measurement of the peak velocity (PV) of the LRV was used to diagnose LRV entrapment syndrome. It has been suggested that the distal-to-proximal diameter ratios and flow velocity ratios exceeding 5.0 represent nutcracker phenomenon cutoff levels [43, 46, 47]. In one study involving 67 men with varicocele, 55 % were demonstrated venographically to have an entrapment phenomenon [48].

The nutcracker phenomenon can be the chief cause of pediatric varicocele. In one report, children with the nutcracker phenomenon had higher grade varicocele and obvious collateral vein formation than did the patients with a lower renocaval pressure gradient [49]. However, the insufficiencies of the internal spermatic vein may be the main cause of renospermatic reflux in patients with a low renocaval pressure gradient [25, 40–42, 50, 51]. In children, the use of Doppler ultrasonography in the diagnosis of nutcracker phenomenon has been limited because the left renal vein sampling area is smaller and the Doppler angle is larger in children than in adults [47, 52].

Lastly, the etiology of varicocele may be a combination of all these mechanisms that are further aggravated by an upright posture. As shown in thin, tall athletic subjects, the incompetence of venous valves and lack of fat support around the left renal vein with narrowing of the aortomesenteric angle may lead to varicocele formation [14].

Approximately 80 % of men with varicocele are fertile and have normal fecundity [5, 6]. Although the pathophysiology of varicocele has been extensively studied, no conclusive mechanism fully explains why the remaining 15–20 % are infertile.

Scrotal hyperthermia, hormonal disturbances, testicular hypoperfusion and hypoxia as well as backflow of toxic metabolites are potential mediators of varicocele-mediated infertility [57]. Recently, oxidative stress has been implicated as an important mediator of varicocele-associated infertility [57]. Nonetheless, the reasons why some patients with varicocele are infertile, whereas the majority of patients are not, remain unclear. Such phenomenon may be partially explained as infertility being a combination of both male and female factors, in which a fully functional female reproductive system can compensate male factor deficiencies and therefore result in a successful conception. Different intrinsic susceptibility must exist among men with varicocele, which culminates in the various effects of varicocele on male fertility [34].