Kallmann Syndrome (KS) is a congenital form of hypogonadotropic hypogonadism (HH) that manifests with hypo- or anosmia. This decrease in gonadal function is due to a failure in the differentiation or migration of neurons that arise embryologically in the olfactory mucosa to take up residence in the hypothalamus serving as gonadotropin-releasing hormone (GnRH) neurons. A deficit in the GnRH hormone results in decreased levels of sex steroids leading to a lack of sexual maturity and the absence of secondary sexual characteristics. Typical diagnosis occurs when a child fails to begin puberty. The condition, first described in 1944, is a rare pediatric genetic disease that is estimated to affect 1 in 48,000 individuals [1].  Treatment involves life-long hormone replacement therapy. However, treatment for male infants may include early hormone treatment or surgery to correct undescended testicles [2].  Unfortunately, later in life, these patients have an increased risk for developing osteoporosis due to their decreased sex hormones production and are often prescribed Vitamin D supplementation and bisphosphonates [3]. 

Kallmann syndrome, like other HH conditions, is characterized by reproductive features centered around a lack of sexual maturation during the years of puberty. These signs can include a lack of testicular development as determined by testicular volume in men, and a failure to start menstruation (amenorrhoea) in women.  Poorly defined secondary sexual characteristics can include a lack of pubic hair and underdeveloped mammary glands.  Micropenis may also be present in a small portion of male cases, while cryptorchidism or undescended testicles may have been present at birth. All of these traits are related to low levels of luteinizing hormone (LH) and follicle stimulating hormone (FSH), which consequentially results in low testosterone in males and estrogen and progesterone in women [2].

In addition to the reproductive deficits of HH conditions, there will also be the presence of other non-reproductive characteristics which are often defects of embryological origin. KS is defined by its additional presentation of anosmia or hyposmia.  Approximately 60% of patients with GnRH deficiency present with an impaired sense of smell and could be identified as having KS, cleft palate and lip, hypodontia and cleft hand or foot are also frequently present along with unilateral renal agenesis [4]. Cerebral impairments may also be present, including central hearing impairment, mirror movements of the hands (synkinesis) and ataxia. Color-blindness and ocular window defects have also been observed [5][6].

Fundamentally, KS is the result of a defect in the GnRH neurons of the hypothalamus or their differentiation and migration to the hypothalamus during embryonic development. The cause of this condition is genetic but can be the result of a multitude of different genetic mutations. Mutations in approximately 40 different genes have been reported to be connected with HH conditions including KS and present with slight variations in secondary features. The most common defects related to KS are in the genes ANOS1 and FGFR1, but approximately 35 to 45% of cases are not explained by the currently identified genetic abnormalities [7].  Clinical genetic testing can be used to determine the specific genes involved in an individual patient. 

One gene of particular interest is the KISS1 gene encoding the kisspeptin signaling molecule. Kisspeptin is a potent initiator of the production of GnRH in the hypothalamus, and its production is known to be impacted by environmental factors [8].

One study of Kallmann Syndrome in Finland estimated the incidence of the condition within that country to be 1 in 48,000. The condition is genetic and often X-linked, resulting in increased prevalence among males.  Approximately 1 in 30,000 males has the condition compared to 1 in 125,000 females[1][9].

The KISS1/Kiss1 gene, which encodes for the kisspeptin hormone, is a well-known regulator of reproductive hormones, specifically acting upstream of GnRH [10][11]. Studies indicate that in some cases of HH, patients present with deletions and point mutations in the KISS1R [12][13]. The kisspeptin neuronal network” (KP) reside within the hypothalamus preoptic (POA) and the infundibular (INF) nuclei, respectively [14][12][13]. This elegant neuronal circuit regulates puberty and human reproductive functions by signaling GnRH secretion, subsequentially controlling FSH And LH. The cranial nerve zero (0) has been described in the literature as an innocuous neuroanatomical structure associated with GnRH and potentially involved in the regulation of human reproductive functions and behaviors[15][16]. Perhaps, the CN0's GnRH axons not only play a critical role in the development and differentiation of the HPA axis but may also trigger conceptually exhilarating endocrinologic responses independently or together with the KP neural circuit[15][16].