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Also known as
KS, Hypogonadotropic hypogonadism with anosmia, Olfactogenital dysplasia, Olfactory-gonadal dysplasia, de Morsier syndrome
Definition
Kallmann syndrome (KS) is a congenital genetic disorder characterized by the dual presentation of hypogonadotropic hypogonadism (HH), leading to absent or delayed puberty and infertility, and anosmia or hyposmia (absent or reduced sense of smell).1 This condition arises from a developmental failure in the embryonic migration of gonadotropin-releasing hormone (GnRH) neurons from the olfactory placode to the hypothalamus.9, 10 Normally, these neurons, along with olfactory nerves, traverse the cribriform plate to reach the hypothalamus, where they regulate reproductive function.9, 10 Defects in this migratory pathway disrupt both olfactory function and GnRH secretion, resulting in the characteristic features of KS.9, 10 If GnRH deficiency occurs without olfactory defects, the condition is termed normosmic idiopathic hypogonadotropic hypogonadism (nIHH).7, 11
The genetic basis of Kallmann syndrome is heterogeneous, with mutations in over 40 genes implicated in its etiology, often leading to varied secondary characteristics.12 The most frequently identified genetic mutations involve ANOS1 (formerly KAL1) and FGFR1. However, a significant portion of cases (35-45%) remain without a currently identifiable genetic cause.6, 13, 14, 15 Other genes associated with KS include PROK2, PROKR2, CHD7, FGF8, and SOX10, among others.6, 16, 17 Inheritance patterns can be X-linked recessive (e.g., ANOS1 mutations), autosomal dominant, or autosomal recessive, though many cases arise from sporadic mutations.6
Clinical Context
Kallmann syndrome is typically suspected in adolescents presenting with delayed or absent puberty, often accompanied by a reported lack of smell (anosmia or hyposmia).6 Clinical evaluation may reveal other associated features such as cryptorchidism (undescended testes), micropenis in males, cleft lip or palate, unilateral renal agenesis, and synkinesia (mirror movements of the hands).6, 15, 20, 21, 22 A family history of delayed puberty or diagnosed KS can also be indicative.6
Diagnostic confirmation involves hormonal assessments, imaging, and potentially genetic testing. Laboratory findings characteristically show low levels of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and sex steroids (testosterone in males, estradiol in females), despite normal or low GnRH (due to hypothalamic dysfunction).6, 48 Pituitary function beyond gonadotropin secretion is generally normal.5 MRI of the brain may reveal olfactory bulb aplasia or hypoplasia, a key indicator of KS.5, 6, 48, 50
Management of Kallmann syndrome focuses on hormone replacement therapy (HRT) to induce and maintain secondary sexual characteristics, support bone health, and improve quality of life. For fertility induction, pulsatile GnRH therapy or gonadotropin (hCG and hMG/FSH) treatments can be effective.6 (Source: PMC9477064, Introduction) Surgical correction for cryptorchidism may be necessary. Due to the risk of osteoporosis from prolonged hypogonadism, bone density monitoring and vitamin D/calcium supplementation are often recommended. (Source: StatPearls NBK538210, Introduction)