AHH is an acquired form of the disease often occurring after sexual maturation and is not related to genetic defects. Pathogenesis
CHH is a type of HH resulting from the abnormal migration of GnRH neurons during embryonic development. GnRH neurons are derived from the olfactory placode and migrate into the central nervous system (CNS) during embryonic development. Embryonic migration can be affected by several gene mutations including but not limited to, KAL1, fibroblast growth factor (FGF8), sex determining region Y-Box 10 (SOX10), GNRHR, GNRH1 and KISS1R. Kallmann syndrome results in a loss of smell (anosmia) and is associated with KAL1 mutations. The KAL1 gene encodes anosmin-1, an extracellular adhesion molecule that plays a role in GnRH neuronal migration and adhesion. Mutated KAL1 genes leads to ill GnRH neuronal migration as well as olfactory neuron disorder causing anosmia and non-functional GnRH releasing neurons. Mutations of KAL1 are mostly nucleotide insertion or deletion causing frame shifts in the translation of anosmin-1 resulting in a faulty protein. Inactivating mutations in the genes encoding GNRH1 or its receptor will result in the failure of the HPG axis and give rise to normosmic CHH. Inactivating mutations of KISS1 or KISS1R causes normosmic CHH in humans. This is because KISS1 is the mediator for the feedback loop in the HPG axis allowing low levels of sex steroid to stimulate GnRH secretion from the hypothalamus.CHH is a genetically heterogenous disorder with cases reported as being X-linked, recessive and autosomally inherited.
Another clinical sign of CHH, more specifically Kallmann syndrome, is a lack of a sense of smell due to the altered migration of GnRH neurons on the olfactory placode. Kallmann syndrome can also be shown through MRI imaging with irregular morphology or aplasia of the olfactory bulb and olfactory sulci. Anterior pituitary function must be normal for all other axes in CHH as it is an isolated disorder. Testing anterior pituitary function is helpful to identify if the HH is due to hyperprolactinemia. Management
The goal for HH therapy is to induce pubertal development, sexual function, fertility, bone health, and psychological wellbeing. Testosterone therapy for males and estradiol therapy for females is used to improve genital development, develop secondary sexual characteristics, allow for the growth and closure of the epiphyseal plate, as well as improving sexual function. This therapy does not restore fertility as gonadotropins are required for spermatogenesis and folliculogenesis. If fertility is desired, pulsatile GnRH therapy or gonadotropin therapy is necessary.Gonadotropin therapy involves the use of human chorionic gonadotropin (hCG) and FSH. In the male, hCG stimulates Leydig cells to produce testosterone so that plasma and testicular levels increase. With the increased levels of testosterone, sexual activity, libido and overall wellbeing should improve. Administration of FSH is required to induce spermatogenesis by acting on Sertoli cells. FSH is required for maintaining the production of high numbers of good quality sperm.