Introduction

Congenital penile abnormalities reported in the dog include diphallia (duplication of the penis), penile frenulum, hypospadias, penile hypoplasia, and penile immaturity.^1–4 Diphallia has also been documented in other species, including cats, sheep, donkeys, cattle, and humans.^5–12 In humans, diphallia occurs in one of every 5–6 million live births.^10–12 Only a handful of cases have been reported in veterinary medicine, so the true incidence in animals is not known.^2–9

Diphallia is often accompanied by other congenital anomalies, including duplication of caudal midline structures (ureters, urinary bladder, prostate, colon), polymelia, renal hypoplasia, cryptorchidism, accessory scrotum, and atresia ani, among others.^2–12 The dog described herein is unusual because diphallia occurred in isolation; no other congenital abnormalities were identified. Given that the patient was otherwise healthy, the nonfunctional, accessory penis was a source of discomfort, and the prognosis for surgery was excellent, surgical correction was elected.

Case Report

An 8 mo old 9.4 kg intact male mixed-breed dog was presented for paraphimosis of a nonfunctional, accessory penis. The patient had been acquired as a stray by a rescue organization. The dog’s prior medical history and lineage were unknown. The caretaker’s presenting complaint was that the accessory penis was extruded constantly and was painful to the patient. Additionally, the accessory penis did not produce a urine stream when the patient postured to urinate, but a full urine stream was produced from the larger penis. Hematuria, stranguria, and incontinence were not observed.

On physical examination, the patient was bright, alert, and responsive. He was in good body condition with normal hydration status and normal vital parameters. Thoracic auscultation was unremarkable. Gross examination of the external urogenital structures revealed a single prepuce with two penises, with each exiting from a separate preputial orifice. The right penis was smaller and protruding from its own respective preputial orifice, while the left was larger and contained within the prepuce (Figure 1A). Given discomfort on examination, the patient was sedated with dexmedetomidine^a (375 mcg/m² IV) to facilitate a more thorough evaluation. After sedation, it was noted that the mucosa of the accessory penis was erythematous and edematous with areas of ulceration, which were thought to be secondary to exposure. No urethral opening could be identified grossly. The functional, left penis was extruded and was grossly normal (Figure 1B). Both testicles were present in the scrotum and palpated normally. Bloodwork revealed a mild anemia (hematocrit 34%, reference range 36–60%, hemoglobin 11.9 g/dL, reference range 12.1–20.3 g/dL), likely attributable to the patient’s age. Serum chemistry was within normal limits.

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An abdominal ultrasound showed no ultrasonographic abnormalities within the abdominal viscera. A positive-contrast retrograde urethrogram was performed using diatrizoate meglumine and diatrizoate sodium^b (1 mL/kg via urinary catheter), an iodinated contrast media, to evaluate the lower urinary tract prior to surgery. A 10-French red rubber catheter was passed retrograde into the left urethra with the catheter tip just proximal to the urethral meatus, and serial radiographs were taken as the contrast media was administered. One patent urethra was identified within the larger left penis, but it had no communication with the accessory penis, and there was no evidence of a urethra within the accessory penis (Figure 2). Each penis was noted to have an os penis on survey radiographs. There were no other abnormalities noted.

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Given the lack of concurrent congenital anomalies and the perceived level of patient discomfort, surgical excision of the accessory penis was elected. The patient was premedicated with hydromorphone hydrochloride^c (0.1 mg/kg IV) and midazolam hydrochloride^d (0.1 mg/kg IV) and then induced with propofol^e (4 mg/kg IV). General anesthesia was maintained with gas inhalant, isoflurane^f, in oxygen, administered via endotracheal tube. The patient was placed in dorsal recumbency; routine preparation and draping were performed. On initial exam during surgery, both the accessory penis and functional penis could be extruded through separate preputial orifices. The prepuce of the right (accessory) penis was explored, and no communication between the right preputial cavity and the preputial cavity of the left (functional) penis was identified. Saline was flushed into the right prepuce via a catheter tip 6-mL syringe as digital pressure sealed the orifice around the syringe. Pressure was maintained in the accessory prepuce, thus confirming that there was no communication between the accessory penis and the left penis. A 10-French red rubber catheter was passed into the urinary bladder via the left penis.

A ventral midline incision was made into the prepuce, over the accessory penis, into the cavity between the lamina interna and the penis. The incision was extended from the preputial orifice to the fornix. Dissection was continued caudal to accessory fornix into the subcutaneous, and the shared connective tissue became the lateral wall of the primary prepuce. The shared connective tissue between the noncommunicating penises extended 5 cm cranial toward the preputial orifice and was very thin. The mucosal portion of the prepuce was isolated and dissected circumferentially with a combination of sharp and blunt dissection with Metzenbaum scissors (Figure 1C). Vessels were ligated with 4-0 poliglecaprone 25 suture^g, and hemorrhage was controlled with electrocautery. Upon dissection, a 4-mm rent into the internal layer (lamina interna) developed at the fornix of left prepuce. The rent was sutured closed using 4-0 poliglecaprone 25 suture in a simple continuous pattern. The subcutis was closed with 4-0 poliglecaprone 25 suture in a simple continuous pattern, and the skin was closed with 4-0 nylon^h using cruciate sutures (Figure 1D). Following amputation of the accessory penis, a standard prescrotal castration was performed.

Postoperative care consisted of standard incisional care, administration of tramadol hydrochlorideⁱ (2.7 mg/kg PO q 8 hr), and administration of cefpodoxime proxetil^j (10.6 mg/kg PO q 24 hr). At the time of suture removal, 14 days postoperatively, the incisions had healed completely, and the patient was urinating normally. The patient was still doing well when adopted 6 wk after the procedure.

The excised tissue was submitted for histopathology (Figure 1E). The excised tissue contained a bone in the center, and no urethra was present. The tip of the penis surface epithelium was eroded and infiltrated with neutrophils. The base of the excised tissue had cavernous vasculature spaces and scattered subepithelial lymphocytic aggregates. The excised tissue was consistent with a penis devoid of a urethra.

Discussion

Diphallia is an uncommonly reported congenital anomaly. Approximately 100 cases have been documented in the human literature, and a classification system has been developed: diphallia of the glans alone, complete diphallia, bifid diphallus, and pseudodiphallia. In patients with complete diphallia, each penis has two corpora cavernosa and a single corpus spongiosum. Bifid diphallus occurs when each penis has only one corpus cavernosum. The degree of bifid diphallus can be complete, in which case each penis terminates at the base of a shared shaft, or partial, in which case only the glans penis is duplicated. Pseudodiphallia is described as a true, partial diphallia in which the duplicated phallus is smaller or rudimentary.^10–12 Using this system, the anomaly present in the patient described herein would be classified as pseudodiphallia.

In veterinary medicine, diphallia has been rarely reported. In a case series of 10 male domestic shorthairs with reproductive anomalies, a 1 yr old Siamese cat was cryptorchid in addition to having a double glans penis.⁵ For dogs, there are three documented cases of diphallia in the English literature.^2–4 Johnston et al. described a 6 mo old poodle mixed-breed dog with diphallia, duplication of the urinary bladder, right renal hypoplasia, bifurcation of the descending colon, and bilateral cryptorchidism. In this patient, each penis had a patent urethra.² Zucker et al. reported a 5 mo old mixed-breed dog with diphallia and polymelia. This dog had the third, rudimentary limb amputated at a young age. He was then castrated 2 yr after adoption because of prostatitis but was lost to follow-up thereafter.³ Recently, a mixed-breed puppy with a supernumerary limb, anal atresia, an absent tail, absent scrotal sacs, and diphallia was described. This puppy died 72 hr after birth, presumably due to his congenital abnormalities.⁴

The cause for diphallia is not completely understood. Multiple theories have been proposed to explain why diphallia occurs, including embryologic duplication, inhibited fusion of embryonic structures or anlagen early in development, incomplete twinning or teratoid, and atavism.^2–12

During embryogenesis, mesodermal bands migrate around the cloacal membrane to form the genital tubercle. The genital tubercle goes on to form either the penis or the clitoris; this differentiation is dependent on either the presence or absence of dihydrotestosterone.¹³ Duplication of the cloacal membrane would lead to the presence of two genital tubercles and thus the development of two penises. Since other structures also arise from the cloacal membrane (urinary bladder, hindgut, neural tube), duplication of the membrane would explain some of the concurrent anomalies commonly seen in patients with diphallia.^{2–5,8,10–11}

Partial fission of a blastocyst prior to embryo formation would lead to incomplete twinning and duplication of only the caudal midline structures. The resulting duplicated tissues would be considered to be teratoid structures.^10–12 In the mixed-breed puppy described by Mazzullo et al., the duplication of multiple caudal structures (penis, supernumerary limb) was postulated to be secondary to incomplete twinning.⁴ Finally, it has been proposed that diphallia is actually a form of atavism, or reversion to ancestral type, since reptiles have a double penis or hemipenis.^10,12,14

Conclusion

While the etiopathogenesis of diphallia is not completely understood, the variety of presentations and concurrent anomalies suggest that more than one explanation may be correct.^2–12 The estimated incidence of diphallia in humans in the United States is one in every 5–6 million live births.^10–12 Data collected by the American Veterinary Medical Association on pet ownership in 2011 estimated that there were 69.9 million pet dogs in North America.¹⁵ With only three cases of diphallia in dogs in the English veterinary literature, it is likely that more cases have gone unreported.^2–4 This case report describes the first reported incidence of diphallia as an isolated anomaly in the dog and its successful surgical correction.