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Microphallus

  • Author: Karen S Vogt, MD; Chief Editor: Stephen Kemp, MD, PhD  more...
 
Updated: Jun 23, 2016
 

Background

Microphallus, or micropenis, is defined as a stretched penile length of less than 2.5 standard deviations (SDs) below the mean for age. Traditionally, the term micropenis refers to a penis that is otherwise normally formed, and the term microphallus has been used when associated hypospadias is present.

The mean stretched penile length in a full-term newborn male is 3.5 cm. Measurements of less than 2-2.5 cm (2.5 SDs below the mean) in a full-term newborn male meet the definition of micropenis and warrant evaluation.[1, 2, 3, 4, 5] Penile growth is essentially linear during mid-to-late gestation. Tuladhar et al (1998) reported the following formula to describe the relationship between penile length and gestational age for infants born at 24-36 weeks gestation:[6]

Penile length in centimeters = -2.27 + 0.16 X (gestational age in weeks)

Although micropenis can be considered a form of ambiguous genitalia, the presence of a normal scrotum and palpable testes indicates a high probability of a normal male karyotype. If the testes are not palpable and/or the penile urethra is absent, the examination is better described as ambiguous, and an evaluation and counseling for disorders of sex development should be performed.

After the first few years of life, the penis grows very little until puberty when testosterone levels begin to rise. Mean stretched penile lengths and 2.5 SDs below the mean for various age groups can be found in the popular Harriet Lane Handbook (Table 10-15, 19th ed.).[7]

Occasionally, older boys are brought for evaluation because of concerns of small genitalia. These boys are usually prepubertal and obese. Most often, these individuals have normal penis size based on stretched penile length, and the apparent smallness is secondary to the penis being concealed in the suprapubic fat pad ("buried penis"). However, if the penis does measure less than 2.5 SDs below the mean (approximately 4 cm) or other abnormalities are present, such as cryptorchidism or hypospadias, further evaluation is indicated.

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Pathophysiology

Fetal production of testosterone and its peripheral conversion to dihydrotestosterone (DHT) is necessary for normal male development. Early in gestation, placental human chorionic gonadotropin (hCG) stimulates the developing testes to produce testosterone by binding to the luteinizing hormone (LH) receptor. By approximately 14 weeks' gestation, the fetal hypothalamic-pituitary-gonadal axis is active, and testosterone production falls under the control of fetal LH. Therefore, penile growth after the first trimester depends on fetal testosterone production. Testosterone is peripherally converted by the enzyme 5-alpha reductase to the more potent androgen DHT, which is responsible for virilization of the male external genitalia. Finally, intact peripheral androgen receptors are necessary for normal male development.[8, 9]

After an initial surge of LH and testosterone at birth, lasting about 12 hours, gonadotropin (LH and follicle stimulating hormone [FSH]) and testosterone levels are low during the first few days of life. At about 1 week of age, gonadotropin and testosterone levels begin to rise to pubertal levels, peaking at age 1-3 months, and then decreasing to prepubertal levels by age 6 months.[10, 11] After age 6 months, the little subsequent penile growth that occurs parallels general somatic growth. With the onset of puberty penis growth resumes because of increased testosterone production. Growth hormone also plays a role in penis growth as micropenis has been observed in children with isolated growth hormone deficiency.

Micropenis may be caused by a defect anywhere along the hypothalamic-pituitary-gonadal axis, a defect in peripheral androgen action, isolated growth hormone deficiency, a primary structural anomaly, or may be part of a genetic syndrome. The most common cause of micropenis is abnormal hypothalamic or pituitary function. In the absence of normal hypothalamic or pituitary function, a normally shaped penis may develop due to maternal hCG effect on fetal testosterone production, but adequate penile growth does not occur after 14 weeks' gestation when testosterone production depends on intact fetal pituitary LH secretion. Failure of adequate testosterone production toward the end of gestation due to a primary testicular disorder can also result in inadequate penis growth.

Micropenis can also occur in children with LH-receptor defects and defects in testosterone biosynthesis (e.g. 17-beta hydroxysteroid dehydrogenase deficiency).[10] The genitalia of individuals with LH-receptor defects vary from normal female-appearing to male-appearing with micropenis. Individuals with 17-beta hydroxysteroid dehydrogenase deficiency most often have female-appearing genitalia and, less often, ambiguous genitalia.[12]

Defects in peripheral androgen action include 5-alpha reductase deficiency (failure of conversion of testosterone to DHT) and partial androgen insensitivity syndrome (PAIS) due to an androgen receptor defect. However, most children with these conditions have varying degrees of incomplete labioscrotal fusion, resulting in hypospadias and genital ambiguity.[12, 13]

Lastly, genetic syndromes in which micropenis may be a feature include Prader-Willi, Klinefelter, and Noonan syndromes, among others (see Causes).[12, 14]

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Epidemiology

Sex

By definition, microphallus is an exclusively male condition. However, distinguishing between a male with micropenis and bilateral cryptorchidism and a female with clitoromegaly is important and may be difficult.

Age

Micropenis is most often recognized and evaluated in the immediate newborn period, but delays in evaluation may also occur.

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Contributor Information and Disclosures
Author

Karen S Vogt, MD Pediatric Endocrinologist, Department of Pediatrics, Division of Endocrinology, Walter Reed National Military Medical Center

Karen S Vogt, MD is a member of the following medical societies: American Academy of Pediatrics, American Diabetes Association, Endocrine Society, Pediatric Endocrine Society

Disclosure: Nothing to disclose.

Coauthor(s)

Michael J Bourgeois, MD Director of Pediatric Undergraduate Medical Education, Associate Professor, Department of Pediatrics, Division of Pediatric Endocrinology and Metabolism, Texas Tech University School of Medicine

Michael J Bourgeois, MD is a member of the following medical societies: American Academy of Pediatrics, American Diabetes Association, Texas Medical Association

Disclosure: Nothing to disclose.

Specialty Editor Board

Mary L Windle, PharmD Adjunct Associate Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Nothing to disclose.

George P Chrousos, MD, FAAP, MACP, MACE, FRCP(London) Professor and Chair, First Department of Pediatrics, Athens University Medical School, Aghia Sophia Children's Hospital, Greece; UNESCO Chair on Adolescent Health Care, University of Athens, Greece

George P Chrousos, MD, FAAP, MACP, MACE, FRCP(London) is a member of the following medical societies: American Academy of Pediatrics, American College of Physicians, American Pediatric Society, American Society for Clinical Investigation, Association of American Physicians, Endocrine Society, Pediatric Endocrine Society, Society for Pediatric Research, American College of Endocrinology

Disclosure: Nothing to disclose.

Chief Editor

Stephen Kemp, MD, PhD Former Professor, Department of Pediatrics, Section of Pediatric Endocrinology, University of Arkansas for Medical Sciences College of Medicine, Arkansas Children's Hospital

Stephen Kemp, MD, PhD is a member of the following medical societies: American Academy of Pediatrics, American Association of Clinical Endocrinologists, American Pediatric Society, Endocrine Society, Phi Beta Kappa, Southern Medical Association, Southern Society for Pediatric Research

Disclosure: Nothing to disclose.

Additional Contributors

Arlan L Rosenbloom, MD Adjunct Distinguished Service Professor Emeritus of Pediatrics, University of Florida College of Medicine; Fellow of the American Academy of Pediatrics; Fellow of the American College of Epidemiology

Arlan L Rosenbloom, MD is a member of the following medical societies: American Academy of Pediatrics, American College of Epidemiology, American Pediatric Society, Endocrine Society, Pediatric Endocrine Society, Society for Pediatric Research, Florida Chapter of The American Academy of Pediatrics, Florida Pediatric Society, International Society for Pediatric and Adolescent Diabetes

Disclosure: Nothing to disclose.

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