Anosmia in an Adolescent

Anosmia, loss of smell, or hyposmia, decreased sense of smell, may present to the otolaryngologist as a sole complaint or as part of a variety of rhinological symptoms. It can be acute or chronic and intermittent or continuous. Common causes of anosmia or hyposmia include nasal obstruction associated with an upper respiratory infection, allergic rhinitis, nasal polyposis, septal deviation, and acute or recurrent rhinosinusitis. Kallmann syndrome (KS) is a rare disease that presents with anosmia or hyposomia and hypogonadotropic hypogonadism. ¹ Kallmann syndrome is present since birth but is often identified in adolescents who complain of reduced or lack of smell in addition to lack of secondary sexual characteristics and concerns for delayed puberty. Kallmann syndrome can lead to infertility if not treated with hormone replacement therapy. ² Magnetic resonance imaging (MRI) shows absence of the first cranial nerve. ^{3 -7}

A 14-year-old girl presented to the pediatric otolaryngologist with anosmia. She was never able to smell even perfumes but occasionally could smell malodorous scents. There was no history of sinus disease or other symptoms related to the head and neck. Her weight-to-age percentile dropped from 86 to 39 over 4 years and height-to-age was on the sixth percentile. Examination of the head and neck including rigid nasal endoscopy was unremarkable. Magnetic resonance imaging showed bilateral absence of olfactory bulbs and tracts (Figure 1). All other neural anatomy, including corpus callosum, fourth ventricle size, neurohypophysis, pituitary gland, and olfactory sulci, were normal. A preliminary diagnosis of Kallmann syndrome (KS) was made, and the patient was referred for endocrinology and genetic consults.

Figure 1.

A coronal T2-weighted MR image through the anterior cranial fossa. The olfactory bulbs and olfactory tracts are absent (orange arrows). MR indicates magnetic resonance.

Kallmann syndrome has an incidence of 1 in 8000 to 30 000 in males and 1 in 40 000 to 120 000 in females. ^8,9 Only 30% of cases have an identifiable genetic cause. ⁶ Genetic testing is recommended to identify for known associations.

The pathogenesis of anosmia with KS is well described. Neural development of the olfactory system and gonadotropin-releasing hormone (GnRH) neurons are closely associated during the ninth week of embryogenesis. ^3,6 Originating from the extracranial olfactory placode, GnRH neurons migrate along the olfactory neurons through the cribriform plate to the hypothalamus. ³ Disruption of olfactory neuronal migration in KS leads to failure of both olfactory and GnRH neurons in reaching their intracranial destination. This error leads to permanent inability to smell and an absent gonadotropin release. ⁶

Magnetic resonance imaging of the olfactory system with high-resolution coronal fast spin echo T2-W and T1-W images is recommended. ⁵ In healthy controls, the olfactory bulbs can be visualized near the cribriform plate as isointense relative to cerebral gray matter (Figure 2). ^3,5 The MRI from this patient has absent olfactory bulbs and tracts (Figure 1).

Figure 2.

A coronal T2-weighted MR image through the anterior cranial fossa. The olfactory bulbs and olfactory tracts are present in a normal patient (blue arrows). MR indicates magnetic resonance.

Kallmann syndrome is an uncommon cause of anosmia or hyposmia in children. Far more commonly, it is associated with nasal congestion or obstructive polyps or other lesions that prevent airflow to the superior nasal cavity. Such pathology should always be excluded. The ability to smell malodorous scents, such as ammonia, should not exclude the diagnosis due to an intact the trigeminal chemosensory system. In a child with anosmia and small stature, delayed puberty or evidence of hypogonadism, an MRI is the investigation of choice either following a computed tomography scan or as the first-line investigation. ^5,6 Blood tests revealing low levels of serum gonadotropin, testosterone, and estrogen levels aid in the diagnosis.