Cranial Nerve I

Published on 03/03/2015 by admin

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Last modified 03/03/2015

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3 Cranial Nerve I

Olfactory

Michal Vytopil, H. Royden Jones, Jr.

Clinical Vignette

This 58-year-old lady, a culinary expert, as well as a neurologist, was hiking when she unexpectedly and forcefully struck her forehead on a low-hanging tree limb that she had not seen because of a very low visor on her hat. She immediately fell backwards powerfully striking her occiput, seeing stars but not losing consciousness. She acutely developed terrible vertigo. Profound spontaneous nystagmus was the only abnormality on a neurologic examination shortly after her fall. Within just a few hours, she noted that potato salad had no smell or taste. Her husband, also a neurologic physician, soon tested her olfactory sensation with perfume; he demonstrated that she had total loss of the sense of smell. Brain and skull computed tomographic (CT) scanning failed to demonstrate a skull fracture or hematoma. This lady maintained a total loss of smell for a few years. Gradually, fleeting inappropriate dysosmias occurred, along with some functional return. Eventually, some appropriate olfactory sense returned, particularly for smells of citrus and cucumber. The normally pleasant smell of raspberries is negatively altered. In contrast, car exhaust now has a paradoxically pleasant perfume-like smell. Currently, she notes that the best thing is a return of her smell and taste for garlic. Despite such effects, this epicurean still has significant difficulties cooking. Most interestingly, she perceives “smells” in her dreams and she still misses her spouse’s aroma whenever they are physically close. As a physician, she can no longer appreciate patient odors, such as too much ethanol.

Comment: The olfactory nerves are particularly liable to shearing trauma such as occurs with a closed head injury. Their very thin axons are relatively easily severed. In this instance, the patient experienced two rapidly sequential severe head injuries, first frontal and than occipital. This trauma presumably led to a shearing force totally interrupting the tiny olfactory nerves as they crossed the cribriform plate at the base of the skull prior to entering the olfactory bulb at the base of the frontal lobe. Although there were no overt fractures demonstrated with CT scanning, one can easily suggest that the olfactory nerves were severed at their intracranial point of entry.

The olfactory nerve (CN-I) provides for the sense of smell. This sensory modality provides an important warning system by enabling the identification of spoiled and potentially toxic foods or noxious chemicals. Smell function also contributes to various life qualities as this sensory modality provides awareness of many pleasurable sensations, including appreciation of certain foods and beverages as well as subtle attractions eventually leading to sexual desire and reproduction.

Dysfunction of the olfactory nerve is quite rare, occurring in certain very select circumstances. Examination of olfactory function is not routinely pursued during the average neurologic evaluation (Chapter 1). However, in clinical settings such as in the above vignette, it is essential to routinely evaluate olfactory function by asking the patient to identify a few familiar odors such as coffee, perfumes, tobacco, etc. On occasion, the patient may not be aware of or assign much importance to the loss of his or her smell sensation. This may be particularly true when there is a concomitant neurologic deficit as seen in occasional patients with potentially treatable olfactory groove meningiomas that also compromise frontal lobe function.

Anatomy

When identifying odors, humans rely on volatile substances entering their nasal cavity to excite receptors. Olfactory receptor cells are bipolar sensory neurons whose dendrites form a delicate sensory carpet on the superior aspect of the nasal cavity (Fig. 3-1). The thin, unmyelinated axons of the bipolar sensory cells collectively form the olfactory nerve. These travel through the cribriform plate into the olfactory bulb at the base of the fronto-orbital lobe. Within the bulb, CN-I fibers synapse with the dendrites of the large mitral cells, whose axons constitute the olfactory tract passing along the base of the frontal lobe and projecting directly into the primary olfactory cortex within the temporal lobe. In contrast to all other sensory modalities, olfactory sensation does not have a central processing site such as within the thalamic nuclei (Fig. 3-2). This direct pathway to the cerebral limbic structures may have had an important evolutionary function in lower animals and, later, primates.

Figure 3-1 Olfactory Receptors.

Figure 3-2 Olfactory Pathways.

The human primary olfactory cortex includes the uncus, hippocampal gyrus, amygdaloid complex, and entorhinal cortex (Fig. 3-2). Cortical representation of smell is bilateral. Although most of the olfactory tract fibers supply the ipsilateral olfactory cortex, some fibers decussate in the anterior commissure and terminate in the opposite hemisphere. Consequently, a unilateral lesion distal to the decussation rarely produces any olfactory dysfunction.

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