For several months, a 57-year-old woman has noticed that food seems to have less taste. She also has had intermittent midfrontal headaches. On examination, there is normal taste sensation to sugar, salt, vinegar, and quinine, but smell is absent to peppermint, lemon, and coffee. Except for mildly decreased corrected visual acuity on the left, the neurological examination is normal. Magnetic resonance imaging reveals a 3-cm sharply demarcated mass occupying the olfactory grooves beneath the frontal lobe and strongly enhancing with gadolinium. A diagnosis of meningioma is confirmed at surgery; following total removal of the tumor, her visual acuity on the left returns to normal, but she remains anosmic.
Meningiomas arise from arachnoidal cell clusters either within arachnoidal villi or where cranial nerves or blood vessels penetrate the dura. Sporadic meningiomas are frequently associated with chromosomal copy number alterations, especially involving the tumor suppressor gene responsible for neurofibromatosis type 2. Comprising 20% of intracranial tumors, they are usually encapsulated and benign, and they cause symptoms by compressing adjacent structures. Approximately 10% of meningiomas are located at the olfactory cribriform and ethmoid regions, where they cause unilateral or bilateral anosmia by damaging the olfactory bulbs. As these slow-growing tumors enlarge, they push the optic nerves or chiasm posteriorly, producing impaired vision or even complete blindness. Some patients present with headache in the absence of olfactory or visual impairment. Very large olfactory meningiomas cause mental symptoms—particularly indifference or abulia—by compressing the frontal lobes or the anterior cerebral arteries (see Case 77).
Because olfactory areas of the frontal and temporal lobes are so widely distributed, lesions of frontal or temporal cortex seldom cause selective anosmia (Figure 11–1). (Subtle disturbances in olfactory discrimination, however, have been described in patients with inferomedial temporal lobe lesions; the impairment is often unilateral on the same side as the temporal lobe pathology.) Neocortical orbitofrontal olfactory areas receive projections from the piriform cortices of the temporal lobes (both directly and through the dorsomedial nucleus of the thalamus), and surgical removal of these orbitofrontal areas in animals results in impaired olfactory discrimination. These neocortical olfactory areas are too laterally placed, however, to be affected by most medially located olfactory meningiomas. More vulnerable are the olfactory bulbs and the primary olfactory fibers projecting to them from the nasal olfactory epithelium (which contains, among its several million olfactory neurons, many different G protein-coupled odorant receptors). On the other hand, anosmia following head injury—a common occurrence—might result from shearing of olfactory fibers passing through the cribriform plate, bilateral contusion of the orbitofrontal cortex, or both.
Afferent pathways to olfactory cortex. The axons of mitral and tufted relay neurons of the olfactory bulb project through the lateral olfactory tract to the olfactory cortex. The olfactory cortex consists of a number of distinct areas, the largest of which is the piriform cortex. From these areas, olfactory ...