What is the initial workup for a patient presenting with micropenis?

Initial Workup for Micropenis

The initial workup for micropenis should include measurement of stretched penile length, hormonal evaluation, karyotype assessment, and evaluation for associated anomalies to determine the underlying etiology. 1

Definition and Diagnostic Criteria

• Micropenis is defined as a structurally normal but abnormally small penis with stretched penile length (SPL) 2.5 standard deviations below the mean for age and sexual stage 1
• Appropriate diagnostic cutoffs:
  • Below 2 cm at birth (term infant)
  • Below 4 cm after 5 years of age 1
• Important: Differentiate true micropenis from buried penis, webbed penis, or apparent micropenis due to obesity

Physical Examination Components

1. Penile measurements:

   • Measure stretched penile length (SPL) from pubic ramus to tip of glans
   • Compare with age-appropriate normative data
   • Document width of penis

2. Genital examination:

   • Assess position of urethral meatus (rule out hypospadias)
   • Evaluate scrotal development and fusion
   • Check for presence and position of testes (cryptorchidism)
   • Look for incomplete scrotal fusion or other genital anomalies that may suggest disorders of sex development (DSD) 1

3. General physical examination:

   • Evaluate for dysmorphic features suggesting syndromic causes
   • Assess for midline defects
   • Check for other congenital anomalies

Laboratory Evaluation

1. Hormonal assessment:

   • Basal hormone levels:
     • Luteinizing hormone (LH)
     • Follicle-stimulating hormone (FSH)
     • Testosterone
     • Dihydrotestosterone (DHT)
     • Androstenedione

2. Stimulation testing:

   • Human chorionic gonadotropin (hCG) stimulation test to assess testicular function
   • Measure testosterone and DHT response to hCG stimulation 1, 2

3. Genetic testing:

   • Karyotype analysis (essential in all cases) 1
   • Consider specific genetic testing based on clinical presentation

Imaging Studies

1. Endocrine-related imaging:

   • Brain MRI if hypothalamic-pituitary abnormalities are suspected
   • Pelvic ultrasound to evaluate internal genital structures if DSD is suspected

2. Additional imaging:

   • Renal ultrasound to rule out associated urinary tract anomalies
   • Other imaging based on associated findings

Differential Diagnosis Evaluation

The workup should aim to identify the underlying etiology, which typically falls into these categories:

1. Hypothalamic-pituitary disorders (hypogonadotropic hypogonadism):

   • Growth hormone deficiency
   • Multiple pituitary hormone deficiencies
   • Isolated gonadotropin deficiency

2. Primary testicular disorders (hypergonadotropic hypogonadism):

   • Partial gonadal dysgenesis
   • Testicular regression syndrome
   • Disorders of testosterone biosynthesis

3. Androgen insensitivity:

   • Partial androgen insensitivity syndrome
   • 5α-reductase deficiency

4. Genetic syndromes:

   • Prader-Willi syndrome
   • Noonan syndrome
   • Others

5. Iatrogenic causes:

   • Maternal medication during pregnancy 2

Management Considerations

After diagnosis, management typically involves:

• Hormonal therapy with testosterone in infancy (typically 25 mg testosterone enanthate per month for 3 months) 2, 3
• Long-term follow-up with pediatric endocrinology, urology, and psychology
• Assessment of response to hormone therapy 4

Important Clinical Pearls

• Early diagnosis and treatment are critical for optimal outcomes
• Testosterone treatment in infancy and childhood can significantly improve adult penile size 4
• Studies show that with appropriate hormonal treatment, most patients can achieve adult penile length within 2 standard deviations of the mean 4
• Long-term psychological support and follow-up are essential components of care 5
• A multidisciplinary team approach involving pediatric endocrinology, urology, genetics, and psychology provides optimal care

Remember that micropenis develops after the 14th week of pregnancy, which helps distinguish it from other disorders of sexual differentiation that occur earlier in fetal development 2.