INTRODUCTION

Lactation is one of the most important physiologic processes necessary for the survival of mammalian offspring in the absence of human intervention. Breast milk contains species-specific nutrients and immune factors that allow for the appropriate growth and development of newborns. For breastfeeding to be successful, the mammary glands must develop and transform into milk-producing organs by a hormone-driven process referred to as lactogenesis. An understanding of this process has taken on increasing importance as the tremendous benefits of breastfeeding have become widely known in the past two decades.

Galactorrhea, in its broadest sense, refers to the spontaneous flow of milk from the nipple at any time other than during breastfeeding. Derived from the Greek, galactorrhea literally means “milk flow.” Physiologic galactorrhea is experienced by many pregnant women late in gestation, as well as during the first few weeks or months after delivery when not breastfeeding or after cessation of breastfeeding.

Clinically, the term galactorrhea is used to refer to abnormal or inappropriate secretion of breast milk or a milklike fluid. This term often applies to milk secretion that occurs more than 6 months after either cessation of breastfeeding or delivery when not breastfeeding or in a woman who has never been pregnant. However, it is important to be aware that nonphysiologic galactorrhea has no universally agreed-upon definition.

Nonphysiologic galactorrhea can span the continuum from an incidental finding on examination to a significant social problem for the patient. The discharge may be unilateral or bilateral and it may occur spontaneously or only with nipple stimulation. Galactorrhea must be differentiated from a nipple discharge other than milk. The etiology can be idiopathic or related to any of a number of potentially serious underlying or iatrogenic causes.

This chapter begins with a review of the normal physiology of the breast and lactation. The various etiologies of pathologic galactorrhea are discussed, followed by a discussion of evaluation, treatment, and long-term prognosis for women with this problem.

PROLACTIN

A thorough comprehension of prolactin is required to understand both lactation and galactorrhea. Prolactin is responsible for the primary endocrine control of breast secretions, although multiple other hormones also influence breast milk production.

Prolactin is a 198-amino acid polypeptide hormone secreted in pulsatile fashion from the anterior pituitary (adenohypophysis) by specific cells referred to as lactotrophs.¹ These cells have a common origin with the growth hormone-secreting cells (i.e., somatotrophs). Both prolactin and growth hormone are classified as somatomammotropic hormones based on their structural similarities. Due to its structural similarity to growth hormone, prolactin was the last pituitary hormone to be isolated.

Prolactin has a short half-life of only 20 minutes and exists in heterogeneous forms, including big prolactin, a dimeric glycosylated form, and big big prolactin.^2,³ Secretion has a circadian rhythm, with a higher average level of serum prolactin during sleep, especially during the rapid eye movement phase.

The prolactin receptor is a member of the cytokine receptor family. It is a single transmembrane polypeptide found in multiple tissues in addition to the mammary glands, including the liver, adrenal glands, lungs, testes, and ovaries.⁴ The effects of prolactin in many of these tissues remains unknown.⁵

Neuroendocrinology of Prolactin Secretion

The control of prolactin secretion by the anterior pituitary lies within the hypothalamus and is predominantly inhibitory. However, the hypothalamus releases both prolactin-inhibiting factors (PIF) and prolactin-releasing factors (PRF) that modulate the secretion of prolactin.⁶

Prolactin-inhibiting factors are released by the hypothalamus into the hypothalamo-hypophyseal portal system. Although other compounds have been shown to have inhibitory activity, dopamine appears to be the most important PIF.⁷ Interruption of the tuberoinfundibular tract and blocking dopamine receptors with “gene knockout” techniques both result in high prolactin levels.⁸ Gonadotropin-associated peptide and γ-aminobutyric acid are also PIFs.⁹

Prolactin secretion is regulated by a negative feedback loop wherein prolactin, acting via prolactin receptors in the median eminence, stimulates dopamine secretion.¹⁰ Dopamine acts via the adenylyl cyclase pathway to reduce prolactin secretion from the pituitary lactotrophs. The predominant dopamine receptor in the adenohypophysis is D₂ (DRD2). Binding of dopamine to this receptor decreases cellular adenylyl cyclase activity and cyclic adenosine monophosphate.

Although the major control mechanism for prolactin is inhibitory, the hypothalamus also releases PRFs, which stimulate lactotrophs to secrete prolactin (Table 17-1). The primary physiologic PRF appears to be vasoactive intestinal peptide. However, there appear to be two more clinically relevant PRFs.^11,¹² The ability of thyrotropin-releasing hormone (TRH) to act as a PRF is believed to be the basis of the association between hypothyroidism and hyperprolactinemia, a condition that resolves with thyroid hormone replacement. Serotonin appears to be another PRF; use of any of the various selective serotonin reuptake inhibitors (SSRIs) is commonly associated with hyperprolactinemia.

Table 17-1 Prolactin Releasing Factors

Thyrotropin-releasing hormone

Serotonin

Vasoactive intestinal peptide

Opioids

Growth hormone-releasing hormone

Gonadotropin-releasing hormone

LACTATION

The process of lactation requires normal breast development followed by appropriate hormonal and mechanical stimulation of the breast.

Breast Development

Prenatal

Normal development in utero requires fetal exposure to many hormones, including estrogens, progesterone, prolactin, insulin, cortisol, thyroxine, and growth hormone (Table 17-2). The endocrine system is essential for the proper development and function of the mammary glands.^13,¹⁴ Gene knockout studies have demonstrated the importance of each of these hormones in fetal mammary gland development.¹⁵. These hormones work through normal hormone–receptor interactions using local growth factors in many cases.^16,¹⁷

Table 17-2 Hormones Required for Normal Mammary Development

Puberty

At puberty, estradiol is the major influence on breast development, and the breast contains both α and β estrogen receptors. However, multiple hormones are necessary for normal development, including all the hormones necessary for normal in utero development (see Table 17-2).

Under the influence of rising estradiol levels at puberty, the breast increases in size and the areola gains pigmentation.¹⁸ Most breast development at puberty is adipocyte differentiation under the influence of estrogens. Estrogens stimulate the lactiferous ducts to branch extensively. The terminal alveoli unit, however, is rudimentary.

The Mature Breast

Anatomy

The breasts consist of glands, fat, and connective tissue. The basic glandular unit consists of ducts and secretory lobules (Fig. 17-1). Twenty or more lactiferous ducts converge on the nipple. Each large duct is connected to smaller branching ducts and eventually lobules. The ducts are lined by a cuboidal or columnar epithelium, at the base of which are myoepithelial cells. Each lobule contains alveoli, milk glands lined by epithelium.^19,²⁰ In a nonactive state, the ductules often have only rudimentary terminal alveoli that secrete milk.

Figure 17-1 The breast and nipple–areolar complex. Distal ducts are lined by epithelial and myoephithelial cells. The breast lobules empty into lactiferous ducts, which terminate at the nipple. The glands of Montgomery are associated with sebaceous glands and terminate on the areola.

(From Powell DE: The normal breast: Structure, function and epidemiology. In Powell DB, Stelling CB. The Diagnosis and Detection of Breast Disease. Mosby, 1995,)