A reduction in the quantity of sperm may be due to a deficit in their production or a partial obstruction of the seminal duct, with the former being the more common. The causes may be the same as those mentioned for secretory azoospermia, but of less severity.
When the quantity of sperm is below 5 million/ml it is advisable to undertake genetic studies (karyotype and study of the Y chromosome). The majority of oligozoospermias are accompanied by disorders or irregularities in the quality of the seminal fluid (asthenozoospermia, teratozoospermia). A testicular biopsy in the most severe cases may differentiate between a secretory or an obstructive problem, however, the presence of sperm even if scarce does permit the application of the ICSI technique, and therefore the testicular biopsy would only be indicated in certain cases. When the oligozoospermia is moderate (10-20 mill sperm/ml) medical treatment may be recommended despite there being none that would consistently ensure an improvement in sperm concentration. In these cases it is advisable to rule out a series of factors referred to in the following section.