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Micropenis is a significantly small penis with normal internal male genitalia. Micropenis is usually diagnosed shortly after birth, and the cause should be established; in addition, it should be differentiated from other associated syndromes. The role of the pediatric nurse practitioner is to diagnose the micropenis, guide the parents through the options of management, and support all involved through the selected treatment, whether hormonal or surgical. Patients affected with micropenis will need long-term management from their pediatric nurse practitioners, as well as follow-up by endocrinologists, urologists, pediatric surgeons (if surgery is chosen as the treatment), psychologists, and social workers. The need of more long-term research on patients with micropenis also is discussed.
The human body can vary greatly in both shape and size, but the size of certain body parts may signal an underlying pathology, one example being an extremely small penis. Micropenis refers to an abnormally small, normally structured penis with a stretched penile length of less than 2.5 standard deviations (SD) below the mean for age or stage of sexual development (
). Micropenis can be considered a clinical sign of a larger syndrome or an independent anomaly. One epidemiological study showed that 1.5 per 10,000 infant males were born with micropenis between 1997 and 2000 in the United States (
). Another similar term is “microphallus,” which refers to ambiguous-looking genitalia, but for the purpose of this article we will solely discuss the subject of micropenis.
The purpose of this article is to familiarize the medical professional, more specifically the pediatric nurse practitioner (PNP), with the phenomenon of micropenis and to review normal penile development, as well as the diagnostic criteria, causes, and treatments for micropenis. Long-term effects in terms of treatment and gender role also are discussed, along with the important role of the PNP in caring for affected patients. This article aims to provide PNPs with insights on this condition and information on management, including necessary referrals and treatments.
Criteria for Review
The studies chosen for review were limited to recent articles and books from 1997 to 2008 and dealt with all aspects involving micropenis. The exception to these criteria was the article “Micropenis. I. Criteria, Etiologies and Classification” by
, which was published in 1970 and is considered one of the “gold standards” on micropenis. The search was completed using Ovid Medline 1950 to 2007 and PubMED, as well as the Augustus C. Long Health Sciences Library at Columbia University and D. Samuel Gottesman Library at Albert Einstein College of Medicine of Yeshiva University. Key words used in the search were “micropenis,” “5-alpha reductase deficiency,” hypogonadism,” “androgen insensitivity,” and “ambiguous genitalia.” The criteria focused on classifications, causes, management, treatments, and future outcomes of patients with micropenis.
Human male sexual differentiation begins with the sex determination (testis development) and is followed by sex differentiation (genital development) (
). Normal sexual differentiation begins the seventh week of gestation when the fetus contains both the male and female genital ducts. In normal male differentiation, the sex-determining region of the Y region (SRY) directs the development of testes, and in the absence of the SRY gene, ovaries will be formed.
Early in gestation, placental human chorionic gonadotropin stimulates the developing testes to produce androgens, and by the fifteenth week, the regulation of androgen secretion is taken over by gonadotropins (luteinizing hormones and follicle-stimulating hormones) from the fetal pituitary, which is regulated by the fetal hypothalamus to form the hypothalamic pituitary-gonadal axis. The androgens produced are Müllerian inhibitory substance, which cause the female internal genital structures to regress, and 5α-dihydrotestosterone (DHT), which is required for the development of male internal genital structures and the differentiation of male external genitalia (
Activation of androgen receptors by testosterone or DHT also appears to contribute to the masculinization of brain structure and function in some species. Fetal androgen levels in males are elevated between weeks 8 to 24 of gestation, with peak levels occurring between weeks 14 to 16, while postnatal male testosterone levels increase from birth to a peak at 1 to 3 months and then decrease to prepubertal levels by 4 to 6 months (
). It has been found that the highest penile growth velocity is associated with the increased levels of testosterone during the 0- to 3-month period after birth. Thus the postnatal surge in reproductive hormones appears to be important for genital growth (
). Without satisfactory hypothalamic or pituitary function, inadequate penile growth will occur despite a normally shaped penis. Similarly, a primary testicular disorder that causes insufficient testosterone production near the end of gestation also can result in minimal penile growth (
The early diagnosis of “true micropenis” is essential and can give practitioners and parents an early opportunity for various treatment options. The first step to diagnosing micropenis is a physical examination of the external genitalia of the patient. Micropenis is by definition a condition that solely affects XY males, characterized by a small penis with a median raphe, foreskin, and glans in which the urethral meatus is normally placed, that is, no hypospadias (Figure 1). It may be either retracted or pendulous depending upon the length of the shaft and may or may not be erectile depending upon the presence or absence of the corpora cavernosa and corpus spongiosum. The scrotum is present and normally fused but may be underdeveloped, and the testes typically are descended and may or may not be normal in function (
). In general, there should not be any signs of feminization.
Occasionally older boys are evaluated for small genitalia due to parental concern. However, these boys usually are prepubertal and obese, and the minimal size of their penis is a result of the penis being hidden in the prepubic fat. The size of their penis is warranted as normal unless the size falls within the micropenis criteria. An accurate penile length measurement and thorough physical assessment can distinguish micropenis from other conditions (
). Misdiagnosing this condition may delay treatment and cause more frustration for the parents.
Getting the correct penile measurement is very important because a diagnosis of micropenis really hinges on the size. An accurate measurement that correlates with the requirement of 2.5 SD below the norm for age plus external and internal organs of an XY karyotype can be sufficient evidence to support a diagnosis of micropenis.
The conventional method for measuring the penis is using a ruler or calipers. The penile length should be the fully stretched rather than flaccid and can be achieved by grasping the glans between the thumb and forefinger and measuring from the pubic ramus, along the dorsum, to the tip of the glans penis. The suprapubic fat pad should be compressed in as completely as possible and the foreskin, if present, is not included in the measurement (
). Another option is using a 10-mL disposable syringe in a modified construction. The needle-bearing end is removed, and the piston is inserted in to the cut end (Figure 2). The flanged end is then placed over the penis, firmly pressing in to the fat pad while the piston is withdrawn, causing the penis to be suctioned into the injector. Once the penis is stretched out within the syringe during optimal suction, the penis length is read from the attached scale. Using this method eliminates measurement variability caused by the suprapubic fat pad by gently pressing the syringe on to the symphysis pubis (
After obtaining a correct measurement of the penis, the value is compared with the normal size range for chronological age. The penis must measure less than 2.5 SD below the mean for age to qualify as micropenis. Table 1 contains the average stretched penile length (in centimeters) of normal male subjects based on age and the length 2.5 SD below. For premature newborns, the following formula describes the expected penile length for infants born between 24 and 36 weeks gestation:
Table 1Stretched penile length in normal male subjects (cm)
Mean –2.5 SD
Newborn: 30 wk
2.5 ± 0.4
Newborn: 34 wk
3.0 ± 0.4
3.2 ± 0.4
3.9 ± 0.8
4.3 ± 0.8
4.7 ± 0.8
5.1 ± 0.9
5.5 ± 0.9
5.7 ± 0.9
6.0 ± 0.9
6.1 ± 0.9
6.2 ± 1.0
6.3 ± 1.0
6.3 ± 1.0
6.4 ± 1.1
13.3 ± 1.6
This table was published in Endocrinology, Custer, J., & Rau, R., The Harriet Lane handbook, 18th ed., 269-300, Copyright Elsevier (2009).
). A penile length that falls into the micropenis category warrants further evaluation of cause and treatment (Table 1).
Other syndromes can also be mistaken for micropenis, such as concealed penises (Figure 3) which are further categorized as buried penis, trapped penis, and webbed penis. A buried penis is a typical-sized penis enclosed in the suprapubic fat pad because of a lack of skin attachment to the shaft, while the trapped penis results from excessive circumcision where adhesions form between the scrotal and penile skin. A webbed penis is characterized by the skin of the shaft being tethered to the ventral surface of the scrotum (
Micropenis can occur as an isolated incident or in association with other conditions, especially those involving hormone deficiencies. Some etiological factors of micropenis due to hormone deficiencies are hypogonadotropic hypogonadism resulting from an endocrine anomaly of the hypothalamic-pituitary-gonadal axis and androgen insensitivity syndrome with inadequate penile growth despite normal or excessive androgen secretion (
). Other possible causes of gonadotropin deficiency are caused by hypothalamic dysfunction, such as Kallmann's syndrome or Prader-Willi syndrome. There also can be decreased synthesis of testosterone or conversion of testosterone to DHT, as is the case of 5-alpha reductase deficiency (5αRD), decreased testosterone sensitivity, and growth hormone deficiency. Table 2 describes in more detail the pathophysiology, clinical findings, and diagnostic work-up results of hypogonadotropic hypogonadism, androgen insensitivity, and 5αRD, while the Box lists other conditions associated with micropenis.
Table 2Hypogonadotropic hypogonadism, androgen insensitivity, and 5α reductase deficiency
Hypogonadotropic hypogonadism (idiopathic or Kallman's syndrome)
Failure of the hypothalamus to secrete gonadotropin releasing hormones
Idiopathic can present with micropenis; Kallman's syndrome: micropenis or cryptorchidism, anosomia, hyposomia, sensorineural deafness, visual abnormalities, cleft lip and palate, congenital heart disease, renal agenesis, cerebellar ataxia, arm span greater than height, short metatarsals
Demonstration of prepubertal serum testosterone concentrations of less than 100 ng/dL in males
Low or normal serum LH and FSH concentrations (usually less than 4 to 5 IU/L)
Otherwise normal anterior pituitary function
Normal appearance of the hypothalamus and pituitary region on MRI
Androgen insensitivity (complete or partial)
Defective mechanism of action of the androgen on target cell or defective androgen receptor gene (loss-of-function); caused by defects in gene coding, environmental chemicals that impair DHT synthesis, and AR mediation
Micropenis or ambiguous genitalia; CAIS: female external genitalia with normal labia, clitoris, and vaginal canal; PAIS: wide range, female with only clitoromegaly or male with micropenis and/or hypospadias
Karyotyping, chromosomally male or female
Normal or abnormal serum testosterone levels
Mutations on the coding region for AR seen upon sequencing
5α reductase deficiency
Mutations in the steroid 5α-reductase type 2 gene causing a lack of enzyme production or defects in androgen receptor site
Isolated micropenis or hypospadias, severe ambiguity of external genitalia; internal male genitalia are normally developed
Increased ratio of serum testosterone to DHT
DHT level is low to undetectable
Testosterone level is normal to moderately elevated
Molecular analysis of 5αR type 2 gene (not commercially available)
). Hypogonadotropic hypogonadism is the failure of the hypothalamus to secrete gonadotropin-releasing hormones (GnRH) that normally stimulate the pituitary gland to secrete the gonadotropins luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which stimulate male testes to secrete hormones that are responsible for normal pubertal maturation and reproductive function (
). The hypothalamic-pituitary function is otherwise normal in most patients, and hypothalamic-pituitary imaging reveals no space-occupying lesions. This condition can be idiopathic or a result of Kallman's syndrome (
). In Kallman's syndrome, the failure of the hypothalamus to secrete GnRH leaves a deficiency in sex hormones that presents with micropenis or cryptorchidism and anosmia (a lack of smell) and hyposomia (inadequate development of the body). Kallman's syndrome is an autosomal dominant, autosomal recessive or X-linked recessive disease and can be distinguished from idiopathic hypogonadotropic hypogonadism with an objective smell test to document anosmia (Fechner et al.). The syndrome is diagnosed on the basis of clinical presentation, while the GnRH deficiency is diagnosed biochemically.
In complete androgen insensitivity syndrome (CAIS) and partial androgen insensitivity syndrome (PAIS), the mechanism of the androgen action on the target cell is defective, or in other words, there is a loss-of-function mutation in the androgen receptor gene (
). Individuals with CAIS and PAIS are chromosomally and gonadally male; however, those with CAIS have female external genitalia with a normal labia, clitoris, and vaginal canal, while PAIS external genitalia can range from slightly virilized female to slightly undervirilized male. For the purpose of this article, we will be focusing on PAIS, although diagnostic screening would be the same for both. Male subjects affected with PAIS have normal testes, most likely undistended, with normal production of testosterone and normal conversion to DHT, and because they produce normal amounts of Müllerian inhibitory substance, they do not have fallopian tubes, a uterus, or a vagina. They also may or may not have normal serum testosterone levels because they only have partial enzyme defects (
). 5αRD is primarily due to mutations leading to lack of enzyme production, but it also can be caused by defects in the androgen receptor site. Patients affected with 5αRD are genetically male, but some patients are mistakenly assigned as female because of under-virilization and are raised as such. However, during puberty a variable degree of virilization will occur because of the increased surge of testosterone, and a change in gender may be needed (
). It has been questioned as to how much a role 5αRD plays in a male subject with congenital micropenis. In the study by Gad and colleagues, it was found that 5αRD played a minimal role in isolated micropenis and was more prevalent in cases of ambiguous genitalia with micropenis; however, the study also found that 5αRD did correlate with penile length in intersex cases. If 5αRD is not the main cause of micropenis, it does seem to play a part in the growth of a penis.
The initial step of treatment of micropenis is to have a complete work-up ordered by the primary care provider or endocrinologist, including chromosomal analysis or karyotyping with Y fluorescence to determine the genetic sex and to rule out other syndromes (
). Levels of serum gonadotropin (LH and FSH), testosterone, DHT, and androstenedione (a precursor sex hormone) also are measured (Table 3). The gonadotropin levels will be able to narrow down the differential diagnoses, while testosterone and DHT levels can determine the testes' responsiveness to gonadotropin stimulation. It also can be used to diagnose 5αRD. A GnRH stimulation test will evaluate the pituitary gland's ability to respond and produce LH and FSH. Levels of the growth hormone cortisol, as well as total and free thyroxine levels, will determine hypopituitarism, and a human chorionic gonadotropin stimulation test will measure the body's ability to perform testosterone biosynthesis (
). Imaging studies such as pelvic ultrasonography can be helpful in cases of ambiguous genitalia to visualize the internal reproductive organs, or a magnetic resonance imaging scan of the head can be used to assess the pituitary and hypothalamic area (
Initial treatment is a short course of testosterone to assess the ability of the penis to respond to the hormone. Treatment can be given via intramuscular injections or applied topically. Testosterone cypionate or enanthate in oil, 25 mg, is given intramuscularly every 3 weeks for 4 months for the initial course. The adverse effects are minimal and include temporary accelerated growth velocity and bone age (
administered 5% testosterone ointment daily to 50 boys aged 5 months to 8 years for 30 days, which resulted in increased penile length. Transdermally absorbed testosterone also has been shown to stimulate the secretion of growth hormone from the pituitary gland, which will increase the production of insulin-like growth factor-I, a factor that promotes bone growth. Long-term administration has the potential to enhance growth in penile length as well as skeletal growth (Arisaka et al.).
Empirical evidence indicates that testosterone treatment has a positive impact on penile growth during infancy, yet it is unclear whether the growth will continue during adolescence and adulthood (
). A lack of response is likely to represent androgen resistance or androgen receptor deficiency, and therefore failure to virilize at puberty is possible. An important aspect of testosterone treatment is the recommendation to begin treatment early in infancy and childhood. Patients with hypogonadotropic hypogonadism show a decrease in penile androgen expression. There is a natural decrease in androgen receptors in early adulthood, and so the early administration of testosterone allows for increased penile androgen receptor concentration and duration during the period before this decline (
, the aforementioned treatment resulted in increased penile length and improved male genital development in a 46 X,Y infant. This treatment also has been shown to work with patients affected with 5αRD, although the large amount of testosterone produced at puberty may be enough for complete virilization. In a research study by Bertelloni and colleagues (2007), three Italian 46 X,Y newborns, two with 5αRD, were given a trial of DHT cream, which resulted in an increase of penile length of at least 120%. In a study by
, percutaneous 2.5% DHT gel was used on six children (ages 1.9 to 8.3 years) affected by micropenis of various etiologies. They found that administration of DHT, 0.2 to 0.3 mg/kg once daily for 3 to 4 months, was able to increase phallic growth. Adverse effects were minimal, with minor irritation to the skin, and similar to adverse effects of testosterone treatment (
). Topical DHT can be a good alternative for patients who do not respond to testosterone.
Luteinizing Hormone/Follicle-stimulating Hormone
In patients with hypogonadotropic hypogonadism, treatment with recombinant human LH and FSH during the first year of life has been successful in inducing testicular growth and a little increase in penile length. In a study by Main and colleagues (2002), a patient with micropenis was given recombinant human LH and FSH in doses of 20 and 21.3 IU as subcutaneous injections twice a week over a period of 6 months and was able to achieve sufficient phallic growth when testosterone was added to the treatment. Some adverse effects reported included increased body hair and pigmentation as well as intermittent nausea. In general, the treatment was well tolerated (Main et al.).
Although exogenous hormone treatments can result in increased penile growth in the patient with micropenis, penile length still may be below average in adulthood (
If the micropenis does not achieve adequate length through medical intervention, surgical options are available, but these options should only be considered after all other treatments have been exhausted. One option is surgical construction of the penis, or elongation plastic surgery. These procedures are highly complex and carry with them many risks, especially because the patient most likely will undergo multiple treatments and the results may not produce acceptable functional or cosmetic outcomes (
). Genitoplasty also can have other complications such as increased scarring, pain, and decreased sexual pleasure. These complex procedures require a skilled team of physicians, including plastic surgeons, microvascular surgeons, and urologists, and currently they are being performed only in highly specialized centers that can guarantee high standard outcomes (
). However, recent reports in long-term follow-up have shown that a majority of male patients with penile malformation who are raised as girls have demonstrated a marked male-typical shift in psychosocial and psychosexual development, with most declaring themselves to be male (
). This information provided by adults with micropenis who claim a male gender role, have a heterosexual orientation, and indicate a steadfast male gender identity has led to a change in attitude regarding gender reassignment. Furthermore, information concerning the impact of androgen on the fetal brain suggests that sex steroid exposure may have an organizing influence on gender, particularly in regard to gender role (
). More recent reports suggest that biological factors in the prenatal period also may have a significant role in gender identification, especially due to brain imprinting from testosterone. On the other hand, long-term psychosexual issues may occur in the presence of a small sex organ (
). Although gender reassignment is now discouraged, there are problems revolving around psychological concerns. Some concerns include patients expressing that their quality of life had been negatively affected by the presence of micropenis and seeking psychological counseling because of concerns of sexual inadequacy and/or depression. A high incidence of depression combined with the patients' perception that their micropenis has negatively affected their quality of life indicates that psychological counseling should be recommended routinely in the treatment of these patients (
The first health care professional to discover the anomaly is most likely the pediatrician or PNP during the newborn examination or a well-infant check-up during the first year of life. The first step in diagnosing suspected micropenis is to confirm that the length of the penis fits within the criteria of a diagnosis of micropenis. Measuring should be done on the stretched penile length using one of the two methods previously described, either the ruler or syringe. The PNP making the diagnosis should be familiar with the genetic and endocrinology principles that direct fetal sex differentiation as well as the influence of fetal sex hormones on gender identity (
). This knowledge enables the PNP to explain to parents how hormones affect penile growth and to provide basic information regarding their child's condition. Next, the PNP should refer the patient to a pediatric endocrinologist for consultation and more in-depth information on the condition. The laboratory studies needed to confirm the diagnosis can be ordered either by the PNP or the endocrinologist.
Once the work-up on the patient is complete and the endocrinologist involved in the child's care has determined the cause of the micropenis, treatment and management can begin. The person responsible for providing the parents with the diagnostic and prognostic information must present all the information gathered during the evaluation, as well as the pros and cons of each treatment option (
). The initial treatment is testosterone therapy to increase the size of the penis. This therapy can initiated by the endocrinologist and followed up with the PNP, with a re-evaluation with the endocrinologist at the end of the treatment. If the therapy is able to achieve adequate growth, treatment is considered a success, although it is uncertain whether growth is maintained into adulthood (
If the testosterone course fails in adding length, the other hormonal treatments described in the treatment section can be administered. If all the hormonal treatments fail, the PNP may provide the parents with information on surgical options, such as penile reconstruction or sex reassignment, and a referral to a pediatric surgeon or pediatric urology surgeon. The surgical options include the technically difficult phalloplasty or the easier vaginoplasty, and each procedure is associated with short- and long-term complications (
). If the parents decide to forgo surgery, the PNP must devise a management plan to help the patient adjust to life as a male with micropenis.
In all cases, psychological counseling and social services are helpful and most likely necessary to both the patient and the parents and should be started early because these patients seem to suffer emotionally (
). Psychiatric counseling may be needed as the child grows and deals with his condition. Common issues among male subjects with micropenis include fear of sexual rejection, sexual inadequacy, the size of their penis affecting their quality of life, and poor body image (Husmann). On another note, long-term follow-up in some patients has shown that there were no major alterations in male sexual activity, comfort, or identity (
The focus of management has remained fixed on determining which medical and/or surgical interventions will yield an optimal outcome for the condition. The most appropriate care has the medical team involving the parents in each step of decision making along the way. Each set of parents, with their unique sociocultural beliefs and backgrounds, would make different decisions based on what their conception of their child's gender is and how they see their child's future in the context of their family and society (
). Parents must feel convinced that no medical information has been kept from them and that the decision is done with full knowledge of alternatives with the support of the PNP and the rest of the medical team.
Management of these pediatric patients can be overseen by their PNP and will involve medical or surgical treatment, appropriate referrals to an endocrinologist and possibly a geneticist, psychological counseling, and follow-up throughout childhood.
Long-term follow-up is provided by the PNP within the context of the child's primary health care setting. The PNP continues to monitor penile length, maintains the hormone treatments or surgical outcomes, and screens for future complications. She or he refers the patient as needed to specialty providers and coordinates the care between the medical team and the patient and family. The PNP also provides holistic care, which includes emotional support for the parents during diagnosis and management and for the patient during long-term follow-up.
Research into the field of micropenis is needed, because there are not many long-term follow-up results. Long-term data are needed for patients with micropenis who underwent hormone therapy or surgical procedures for penile elongation or gender reassignment to see if the result remained permanent throughout childhood, adolescence, and adulthood. A comparative study also is needed to see which hormone treatment has yielded the most continuous results. Appropriate phalloplasty should be explored as an option for younger children because of the fact that some children experience significant psychological disturbance and the current surgical procedures are aimed at the older child or adult. Overall, more research is needed on which clinical practices are most likely to achieve the best quality of life for people affected by micropenis.
A boy with a penis measuring 2.5 SD below the norm for his age needs to be evaluated for micropenis. Laboratory studies are done to determine the possible cause and the best treatment to administer to get the best possible outcome. Management of these pediatric patients can be overseen by their PNP and will involve medical or surgical treatment, appropriate referrals to an endocrinologist and possibly a geneticist, psychological counseling, and follow-up throughout childhood. It is important to counsel the parents of these children every step of the way to ensure that the best quality of care is given.
I thank Dr. Rita Marie John, Director of the Pediatric Nurse Practitioner Program at the Columbia University School of Nursing, for her help and advice while I was writing this article.
Systemic effects of transdermal testosterone for the treatment of microphallus in children.