10. What causes the lack of sense of smell (anosmia) ?

The one feature that separates Kallmann syndrome from other forms of congenital hypogonadotropic hypogonadism is the lack of sense of smell, other wise known as anosmia. The sense of smell is produced via olfactory system.

The normal olfactory system:

Figure 9 shows the main components go the olfactory system.

The olfactory hairs are normally found at the top of the nasal cavity, ready to detect a smell coming through the nostril. The detected smell is converted into a tiny electrical signal by the olfactory cells anchored to the olfactory bulb and sent through the nerve cells or neurones along the olfactory tract to those regions of the brain where it can be identified and recognised as a particular smell. There is one olfactory bulb and tract for each nostril.

The absent olfactory system and Kallmann syndrome:

In patients with Kallmann syndrome the olfactory tract or the olfactory bulbs are either partially formed or missing totally. There may be olfactory cells present at the top of the nasal cavity but if there is no connection between them and the brain it will be impossible for any smell to be recognised by the brain. 

Some patients with Kallmann syndrome have partially formed olfactory tracts and olfactory bulbs so retain a slight sense of smell.

Patients with congenital hypogonadotropic hypogonadism have a normal sense of smell as their olfactory tracts and olfactory bulbs are either fully formed or formed well enough to enable the sense of smell to appear unaffected.

Even patients with total anosmia can still detect certain volatile or strong aromatic compounds known as irritants. This is due to an alternative detection system, totally separate from the olfactory system mentioned above. At the back of the throat there are sensory cells linked to a cranial nerve called the trigeminal nerve which has a direct connection to the sensory centre of the brain. This system is totally unaffected in Kallmann syndrome.