Kallmann Syndrome

Definition and Pathogenesis

Kallmann syndrome is a genetic disorder classically characterized by the combination of isolated hypogonadotropic hypogonadism and an impaired or completely absent sense of smell (hyposmia or anosmia).
The primary pathophysiological defect is the disrupted embryonic migration of gonadotropin-releasing hormone (GnRH) neurons from the nasal placode to their final destination in the hypothalamus.

The syndrome is genetically heterogeneous and can be transmitted in X-linked, autosomal dominant, and autosomal recessive patterns.
In over 60% of patients, a specific gene defect may not be identified.
The X-linked form accounts for approximately 15% of cases and is caused by mutations in the KAL1 gene, which encodes the cell adhesion protein anosmin-1.
The autosomal dominant form (sometimes referred to as KAL2) occurs in up to 10% of patients and is frequently caused by loss-of-function variants in the Fibroblast Growth Factor Receptor 1 (FGFR1) gene.
Other implicated genes include FGF8, PROK2, PROKR2, WDR11, and CHD7.

Endocrine Manifestations: Due to GnRH deficiency, affected neonates (especially males) often present with cryptorchidism and a microphallus. Later in life, patients present with delayed or completely absent pubertal development.
Olfactory Deficits: Patients exhibit anosmia or hyposmia, though they may not independently report this symptom without targeted questioning or testing.
Associated Systemic Anomalies: Phenotypic expression is highly variable even within families. Features can include unilateral renal agenesis, bimanual synkinesia (mirror movements), sensorineural hearing loss, color blindness, and midline facial defects such as cleft lip and palate (which is particularly associated with FGFR1 variants).

Biochemical Evaluation: Hormonal profiling reveals prepubertal levels of sex steroids (testosterone or estradiol) accompanied by inappropriately low or normal gonadotropins (LH and FSH).
Neuroimaging: Magnetic Resonance Imaging (MRI) of the brain is indicated and characteristically demonstrates the absence or hypoplasia of the olfactory bulbs and/or sulci.
Olfactory Testing: Objective assessment of the sense of smell using standardized microencapsulated odorant booklets, such as the University of Pennsylvania Smell Identification Test (UPSIT), is used to confirm anosmia or hyposmia.