Ectopic neurohypophysis is a rare congenital condition characterized by the abnormal positioning of the posterior lobe of the pituitary gland. This anatomical variation is present from birth, differing from its typical location within the sella turcica, a bony cavity in the skull. It represents a developmental anomaly and often correlates with other structural variations in the brain.
The Pituitary Gland and Ectopic Neurohypophysis
The pituitary gland, also known as the hypophysis, is a small, pea-sized endocrine gland situated at the base of the brain, nestled within the sella turcica. It is often referred to as the “master gland” because it produces hormones that regulate the function of many other endocrine glands, including the thyroid, adrenal, and reproductive glands. The pituitary gland is divided into two main parts: the anterior lobe (adenohypophysis) and the posterior lobe (neurohypophysis).
The anterior pituitary is composed of hormone-secreting epithelial cells, while the posterior pituitary consists of unmyelinated secretory neurons. These two lobes are connected to the hypothalamus by a stalk, known as the pituitary stalk or infundibulum. In ectopic neurohypophysis, the posterior pituitary lobe is found in an unusual location, most commonly at the median eminence, instead of its normal position within the sella turcica. This abnormal placement often coincides with an absent or underdeveloped pituitary stalk, disrupting the normal anatomical connection between the hypothalamus and the pituitary gland.
Developmental Origins
Ectopic neurohypophysis is a congenital anomaly. The pituitary gland has a dual embryonic origin: the anterior lobe develops from oral ectoderm, and the posterior lobe (neurohypophysis) arises from neural ectoderm. During normal development, the neurohypophysis forms as a downward extension of the diencephalon, a part of the brain, and extends into the sellar fossa.
Disruptions in these developmental signals, such as mutations in certain genes like HESX1, LHX4, or SOX3, can lead to developmental disorders like ectopic neurohypophysis. The incomplete downward extension of the diencephalon (infundibulum) during embryogenesis is the underlying reason for the posterior pituitary’s misplaced location. This developmental error results in the posterior pituitary remaining in a higher position than it typically would, often near the hypothalamus.
Clinical Manifestations
The clinical signs and symptoms associated with ectopic neurohypophysis primarily stem from deficiencies in pituitary hormones, particularly those from the anterior pituitary. Growth hormone deficiency is the most commonly observed hormonal problem, leading to short stature or pituitary dwarfism. Children with this condition may present with growth failure and developmental motor delay.
Other anterior pituitary hormone deficiencies can occur, including deficiencies in thyroid-stimulating hormone (TSH) and adrenocorticotropic hormone (ACTH). These can result in central hypothyroidism or adrenal insufficiency. Deficiencies in gonadotropins (LH and FSH) may lead to delayed puberty or infertility. While posterior pituitary function is typically preserved, antidiuretic hormone (ADH) deficiency, leading to diabetes insipidus, can occur in some cases. The severity of these hormonal deficits can vary, ranging from isolated growth hormone deficiency to panhypopituitarism, where multiple anterior pituitary hormones are affected.
Ectopic neurohypophysis can also be associated with other brain malformations, such as septo-optic dysplasia, which involves underdeveloped optic nerves and midline brain anomalies. Patients may also experience neurological findings like seizures, developmental delay, or cerebral palsy. Neonatal hypoglycemia is another common finding in infants with this condition.
Detection and Treatment Approaches
Diagnosing ectopic neurohypophysis involves a combination of clinical evaluation, specialized imaging, and blood tests. Magnetic resonance imaging (MRI) of the brain and pituitary gland is the primary diagnostic tool. MRI allows visualization of the misplaced posterior pituitary as a bright spot, typically at the median eminence or near the floor of the third ventricle. It can also reveal an absent or thin pituitary stalk and assess the size and development of the anterior pituitary gland.
Blood tests evaluate the levels of various pituitary hormones and those regulated by the pituitary gland. These tests identify specific hormone deficiencies, such as low growth hormone, thyroid hormone, or cortisol levels. Dynamic or stimulation tests may also be conducted, where hormone levels are measured before and after administering medications that stimulate hormone production.
The primary treatment approach for ectopic neurohypophysis focuses on hormone replacement therapy to address any identified deficiencies. For growth hormone deficiency, daily growth hormone injections are administered. If thyroid hormone is deficient, levothyroxine is prescribed, and for cortisol deficiency, hydrocortisone or prednisone is used. Sex hormone replacement therapy may be initiated for delayed puberty.
Long-term monitoring and management by an endocrinologist are important to adjust hormone dosages as needed. Consistent hormone replacement can significantly improve a patient’s quality of life and address associated health issues.