Azoospermia may occur because of reproductive tract obstruction (obstructive azoospermia) or inadequate production of spermatozoa, such that spermatozoa do not appear in the ejaculate (non-obstructive azoospermia). Azoospermia is diagnosed based on the absence of spermatozoa after centrifugation of complete semen specimens using microscopic analysis.
Azoospermia can be classified into three major types as listed. Many conditions listed may also cause various degrees of oligospermia rather than azoospermia.
Pretesticular azoospermia is characterized by inadequate stimulation of otherwise normal testicles and genital tract. Typically, follicle-stimulating hormone (FSH) levels are low (Hypogonadotrophic) commensurate with inadequate stimulation of the testes to produce sperm. Examples include hypopituitarism (for various causes), hyperprolactinemia, and exogenous FSH suppression by testosterone. Chemotherapy may suppress spermatogenesis. Pretesticular azoospermia is seen in about 2% of azoospermia
In this situation the testes are abnormal, atrophic, or absent, and sperm production severely disturbed to absent. FSH levels tend to be elevated (hypergonadotropic) as the feedback loop is interrupted (lack of feedback inhibition on FSH). The condition is seen in 49-93% of men with azoospermia. Testicular failure includes absence of failure production as well as low production and maturation arrest during the process of spermatogenesis.
Causes for testicular failure include congenital issues such as in certain genetic conditions (e.g. Klinefelter syndrome), some cases of cryptorchidism or Sertoli cell-only syndrome as well as acquired conditions by infection (orchitis), surgery (trauma, cancer), radiation, or other causes. Mast cells releasing inflammatory mediators appear to directly suppress sperm motility in a potentially reversible manner, and may be a common path physiological mechanism for many causes leading to inflammation. Generally, men with unexplained hypergonadotropic azoospermia need to undergo a chromosomal evaluation.
Post testicular azoospermia post testicular azoospermia sperm are produced but not ejaculated, a condition that affects 7-51% of azoospermic men. The main cause is a physical obstruction (obstructive azoospermia) of the post testicular genital tracts. The most common reason is a vasectomy done to induce contraceptive sterility. Other obstructions can be congenital (example agenesis of the vas deferens as seen in certain cases of cystic fibrosis) or acquired, such as ejaculatory duct obstruction for instance by infection.
Ejaculatory disorders include retrograde ejaculation and an ejaculation; in these conditions sperm are produced but not expelled.
Idiopathic azoospermia is where there is no known cause of the condition. It may be a result of multiple risk factors, such as age and weight. For example, a review in 2013 came to the result that oligospermia and azoospermia are significantly associated with being overweight (odds ratio 1.1), obese (odds ratio 1.3) and morbidly obese (odds ratio 2.0), but the cause of this is unknown. The review found no significant relation between oligospermia and being underweight.