Anovulation is a chronic condition in which the normal monthly release of one egg from an ovary is disrupted for any reason. It is important because it is the cause of difficulty in conceiving a child in at least a third of cases. It often presents as irregular menstruation, with fewer periods per year than normal, or even the total cessation of periods.
The causes of anovulation are classified as those suitable for treatment by inducing egg release (or ovulation), and those which cannot be so treated.
What causes anovulation?
Hypothalamic and/or Pituitary Causes: Hypogonadotropic Hypogonadism
Functional Anovulation
In this condition, there is a failure of the pituitary gland to produce the gonadotropins, or the hormones that stimulate the normal ovarian cycle. These are follicle stimulating hormone (FSH) and luteinizing hormone (LH). In some cases, the defect is attributed to hypothalamic suppression by profound stress, over-rigorous exercise or being severely undernourished.
In hypothalamic anovulation, the hypothalamic secretion of gonadotropic releasing hormones is affected, with a secondary effect on the pituitary gland by failure to stimulate the release of gonadotropins. In this condition, basal levels of gonadotropins and estrogens are normal or only slightly reduced, but the pulsatile secretion of gonadotropins which is necessary for physiological action is not marked. Such women respond well to the administration of gonadotropin-releasing hormone.
Sheehan’s Syndrome
This is a condition which follows massive infarction of the anterior pituitary, such as after an episode of intense bleeding, usually following childbirth or trauma.
Kallman’s Syndrome
This is a congenital hypothalamic disorder in which the neurons producing gonadotropin releasing hormone are absent. It is often associated with anosmia, or absence of the sense of smell.
Cerebral Irradiation
Children who have received irradiation, for a cranial tumor or sometimes leukemia, may develop damage to the hypothalamic or pituitary area, causing failure of gonadotropin secretion.
Hyperprolactinemia
In this condition, there is a high level of the hormone prolactin, which inhibits the gonadotropins by exerting negative feedback on their secretion. It is usually produced by a pituitary tumor called a microadenoma (a small glandular tumor).
Idiopathic
In many cases it is not possible to identify any physical or physiological reason for hypothalamo-pituitary dysfunction, but the patient responds to gonadotropin administration by ovulating.
Ovarian Conditions
The most common ovarian cause of anovulation is the polycystic ovary syndrome (PCOS), which accounts for about 70% of all cases of infertility due to anovulation. This is almost always found in young adult women with hirsutism, acne or prolonged irregular menstrual cycles. Many are overweight and insulin resistance seems to play a primary role in causing the absence of ovulation.
Anovulation Not Responsive to Ovulation Induction
These conditions have a primary ovarian cause for not releasing eggs, and therefore cannot be corrected by ovulation induction.
Ovarian Failure
The ovary itself may not be properly developed (a ‘streak gonad’) and this causes failure of egg release. Streak gonads are mostly fibrous tissue and a few undeveloped follicles. This may occur with:
- Chromosomal anomalies such as Turner’s syndrome (only a single X chromosome instead of the usual pair) or androgen insensitivity (46 XY) syndrome
- Destruction of the ovary:
- Surgical removal or oophorectomy
- Autoimmune destruction of the ovaries
- Destruction of the ovary by chemotherapy or radiotherapy to the abdomen and pelvis area
- Idiopathic ovarian failure: Here there is no identifiable cause for the ovaries to degenerate, but they stop ovulating
Further Reading