Chapter 3 - Olfaction and Vision
Vision and olfaction are two of the special senses. While patients usually easily recognize loss of vision, loss of olfaction may not be recognized without testing.
Unilateral depression or loss of olfaction (anosmia) is most commonly due to obstruction of the nasal passages. When it is due to damage to neural structures, it must affect the olfactory pathway at or rostral to the olfactory trigone (Fig. 3-1). Therefore, dysfunction must be in the olfactory tract, bulbs, nerve filaments, or olfactory mucosa in the roof of the nasal passage.
The olfactory pathway divides immediately anterior to the anterior perforated substance to travel via (1) the lateral olfactory stria to the primary olfactory cortex (prepiriform-piriform) in the ipsilateral mesial part of the temporal lobe, (2) the anterior limb or stria that dives into the anterior perforated substance to join the anterior commissure, which carries it to the contralateral olfactory cortex, and (3) the medial olfactory stria, which travels medially from the trigone to distribute to limbic cortex in the septal, subcallosal, and parasagittal frontal regions.
Damage to the olfactory epithelium, the olfactory filiments, the olfactory bulb or olfactory tract can cause unilateral anosmia. Destruction of olfactory cortex or olfactory pathways posterior to the trigone (where the tracts divide) must be bilateral to depress olfactory function. Potentially irritative lesions (tumor, post-traumatic or ischemic scarring, arteriovenous abnormalities, etc.) in the olfactory cortical regions may be the source of epileptic activity and cause olfactory symptoms; that is, the patient may complain of hallucinations of smell. Typically, these olfactory hallucinations are described as acrid and unpleasant and are not lateralized by the patient.
Olfaction is tested by having a patient with eyes closed, sniff a relatively familiar odor from a small vial, occluding the nares alternately to test each side separately. Substances such as acetic acid and ammonia should not be used for testing because they cause strong trigeminal stimulation and can therefore be sensed by an anosmic person. Coffee grounds are popular because they are recognized by approximately 80% of normal individuals and cause minimal trigeminal stimulation. Three distinct levels of function can be determined: (1) cannot smell, (2) can smell something, and (3) recognizes coffee. In addition, a person may recognize an asymmetry of sensitivity despite an inability to recognize the substance. Lack of recognition is not significant if the patient responds by saying that they smell a substance bilaterally; recognition on one side suggests contralateral hyposmia if the patient claims to smell but not recognize something on the opposite side.
Examples of disease processes that cause or are associated with decreased or otherwise abnormal smell are as follows:
- a. Common cold with occlusion of nasal passages (most common cause of hyposmia).
- b. Unilateral occlusion by deviated nasal septum.
- c. Occipital head trauma with shearing effect on olfactory nerve filaments passing through cribriform plate (Fig. 3-2).
- d. Frontal head trauma, if it results in a fracture line through the cribriform plate, causes anosmia by tearing the fine olfactory nerve filaments. (In the absence of fracture, frontal head trauma is less likely to cause hyposmia.)
- e. Tumors (most commonly meningioma) on the mid-portion of the sphenoid ridge or in the olfactory groove, both capable of pressing on the olfactory tract. If the tumor is on the sphenoid ridge so that, in addition to pushing up on the olfactory tract, it compresses down on the optic nerve, it causes the syndrome of ipsilateral anosmia, optic atrophy, and possibly also contralateral papilledema because of increased intracranial pressure caused by the tumor mass effect (Foster-Kennedy syndrome).
- a. Pernicious anemia (vitamin B12 deficiency) is frequently associated with bilateral decreased olfaction.
- b. Vitamin A deficiency is associated with hyposmia and dysosmia (odors are unpleasant), possibly the result of nasal mucosal abnormalities.
- c. Zinc deficiency has been associated with hyposmia and dysosmia.
- d. Diabetes mellitus. Presumably, this hyposmia is secondary to demyelination in the olfactory tracts or loss of the peripheral olfactory neuron.
- e. Multiple sclerosis is a rare cause of hyposmia, presumably on the basis of olfactory tract demyelination.
- f. Herpes simplex encephalitis, which tends to localize in the temporal lobes and cause severe, hemorrhagic, necrotic destruction, may cause anosmia secondary to bilateral olfactory cortex destruction or alternatively, because the virus may enter the nervous system via the olfactory mucosa, may destroy one or both olfactory nerves and bulbs in the process. The presentation of acute-onset anosmia and a severe memory-encoding deficit (the latter secondary to bilateral mesial temporal lobe destruction) in a person who is febrile suggests the possibility of herpes simplex encephalitis, a treatable disorder.
- g. Hepatic disease, particularly acute hepatitis, is frequently associated with an unpleasantness of odors (dysosmia).
Unilateral destructive or compressing (mass) lesions in the anterior temporal lobe may cause olfactory hallucinations, a focal epileptiform discharge that often spreads to involve other portions of the limbic system and neocortex (see Chap.22).
For a more detailed elaboration of visual system anatomy and function, refer to a neuroanatomy text. Figure 3-3 shows diagrammatic representations of the visual pathway including expected abnormalities of vision caused by lesions of its various parts.
Unilateral lesions of the retina and optic nerves cause monocular defects. Lesions from the chiasm back give rise to binocular field defects because of crossing of the nasal half of the retinal fibers from each eye. An exception to this rule occurs when there is involvement of the fibers representing the nasal retinal (temporal field) peripheral crescents. The fibers from this portion of the retina have no homonymous counterpart in the opposite peripheral temporal retina (nasal field). The peripheral crescents, therefore, remain monocular in representation from the retina to the visual cortex (Figs. 3-3, 3-4 and 3-5).
Formal testing of vision divides this function into two basic aspects: (1) central or cone vision, and (2) peripheral or rod vision. Peripheral vision is the greatest part of the visual field, whereas central vision represents a relatively small segment of the projected visual world. Nevertheless, it is mainly central vision that is responsible for visual acuity and color vision. However, cones require relatively bright light in order to function effectively. Peripheral vision also subserves a major function of directing central vision by visual-oculomotor responses toward the peripheral stimuli. The more peripheral the field, the less capable of form or figure perception it becomes, which is in keeping with the centrifugal thinning out of the population of peripheral field receptors (rods) in the retina. At the far periphery one is capable of perceiving only moving objects, although reflex movements of the eye (directing the eyes toward a moving stimulus) can be elicited from this region. Rods have a very low threshold for activation by light as compared with cones and are thus more suited for night vision. In daylight, pigment "bleaches" and the rods are insensitive. The cones, which have a high threshold for pigment bleaching by light, are relatively useless in the dark.
Visual acuity is first tested by having a person read a chart (Snellen chart) containing standard-sized figures (numbers, letters or other forms) as perceived at a standard distance. The notation 20/20 vision means that the patient can recognize objects at 20 ft. that a normal person can recognize at that distance. The designation 20/70 means that the patient, at best, can only recognize at 20 feet what a normal individual can recognize at 70 feet. This type of testing is not practical at the bedside, so charts have been developed to be presented at 14 in. (Fig. 3-6). These give an extrapolated visual acuity in terms of 20 ft. and are adequate to detect neurologic visual dysfunction, but may miss refraction errors, particularly nearsightedness (myopia). For quick screening, it is useful to know that recognition of small-case newsprint at 14 in. is equivalent to 20/30 vision.
Visual acuity can be depressed by changes in the refractive structures of the eye anterior to the retina. In neurologic practice, we are not concerned with refractive problems and, therefore, it is important to differentiat these patients requiring new glasses from those with damage to neural structures. If a person customarily wears glasses, s/he should be tested with them on. If difficulties still exist, then further testing is warranted. Ophthalmoscopic examination should detect corneal opacities or cataracts. It would also detect intraocular problems, such as hemorrhage, which could obscure vision. Refractive errors commonly affect visual acuity. However, conditions such as myopia (near-sightedness) or hyperopia (far-sightedness) can be corrected with a series of lenses. Alternatively, at the bedside, most simple refractive errors can be corrected by having the patient look through a cluster of pin-holes (Fig. 3-7). This works by only permitting parallel light rays to pass the pin-hole. This markedly increases the dept of focus since parallel rays do not have to be focused. Pinholes would make inexpensive but impractical glasses, however, because peripheral vision, most central vision, and a great amount of light are eliminated. If visual acuity is not substantially corrected by the pinhole and if no problems exist with the refractive media of the eye (on ophthalmoscopic examination), it can be assumed there is a neurological (i.e., cone system) central visual defect.
A further way to estimate refractive errors, which is particularly useful in the uncooperative patient, is to determine the diopters (e.g., +3, -3) of ophthalmoscope adjustment necessary to focus on the macula or optic nerve head. This assumes "0" to be equivalent to normal acuity.
After determining that the visual deficit is not a refraction or occlusive problem one can deduce, by default, it is a dysfunction in the neural visual apparatus. In order to localize the lesion, it is necessary to evaluate the visual field integrity. Visual field loss tends not to be an all-or-nothing phenomenon. The patient frequently has a partial deficit, particularly in the central field, and can see larger objects after small objects are no longer perceived. Testing that utilizes small objects is more sensitive.
Formal testing of visual field can be done in several ways. There are automated tests, such as "Goldman visual field testing" in which the patient is asked to push a button when a flash of light is detected. "Tangent screen" testing is an older method in which the patient fixates on a central target at a distance of 3 meters while objects are moved into the screen. Of course, each eye has to be tested independently (Fig. 3-8).
At the bedside, "confrontation" is the most commonly employed method for evaluating visual field. This can be quite accurate if carefully done. One eye is covered and the person is asked to fix their vision on the examiner's pupil at a distance of approximately 1.5 ft. A small, colored object (for example a 3 mm red object such as a fireplace match) is moved into the visual field in a plane halfway between the patient and the examiner. The patient is asked to indicate when s/he sees the object and when it turns red as well as whether it disappears or loses its color anywhere in the field. This technique allows the examiner to compare the patient's vision with their own (presumably normal). Additionally, it is critical to observe whether the patient is fixating on the central object (i.e., examiner's pupil) during the examination. A colored object is used because it defines the major extent of cone vision. On occasion, a partial loss of central vision manifests itself more in depression of color perception than in actual loss of visual acuity. For example, optic neuritis often diminishes the ability to see red objects ("red desaturation"). Screening to test all peripheral quadrants with both of the patient's eyes open and fixating on the examiner's nose reveals all peripheral defects except the rare cases of nasal hemianopias and hemianopias with temporal crescent sparing (see Figs. 3-3 and 3-4). Monocular testing does not miss any peripheral defects but takes twice as long.
If there has been damage to optic nerve fibers for more than a couple of weeks, ophthalmoscopic visualization of the optic disk may reveal evidence of "atrophy" of the temporal portion of the optic nerve head (optic disk). This part of the optic disk transmits the optic nerve fibers from the macula (representing central vision). Atrophy causes the disk to change from its slightly yellowish appearance to a brighter white owing to gradual replacement of myelinated nerve fibers by glial scarring. Comparison with the opposite disk is useful in borderline cases when changes are minimal and the problem is monocular.
There is a normal, oval scotoma (the "physiologic blind spot") in the temporal portion of the visual field (see Fig. 3-8). This is the visual representation of the optic nerve head (the disk), which does not contain rods or cones. When a 3 mm object is passed over this area, a person usually says it disappears. It is often helpful to suggest to the patient that the object will likely disappear in some part of the visual field since most of us are completely unaware of the presence of a blind spot. If the object is moving midway between the examiner and the patient and both are fixating on each other's pupils, their blind spots should be superimposed. The ability of the test to pick up the physiologic blind spot is a good check on the reliability of the mode of testing the visual field.
An early sign of edema or swelling of the optic disk (papilledema) is enlargement of the blind spot. This is because the retina is extremely sensitive to mechanical pressure and minimal, unobservable edema of the optic nerve head causes significant dysfunction in the bordering receptive retina and, therefore, enlargement of the blind spot. Also, glaucoma will tend to push out on the optic disk, producing expansion of the optic cup (at the center of the disk) and also some expansion of the blind spot. More severe glaucoma can destroy the retina thorough pressure.
Peripheral visual fields are formally tested using the tangent screen (Fig. 3-8) and more completely using a perimeter (Fig. 3-9), which takes into account that the total visual field is an arc (see Fig. 3-5). A tangent screen is flat and so cannot demonstrate the total extent of the peripheral visual field. A relatively large, white (color is not useful with rod vision testing) object (approximately 10 mm in diameter) is moved along the perimeter from the outside in and the peripheral field is mapped out in degrees. The outside limits represent where the patient, while fixing on a central target, first sees the moving object. A simple and more practical technique is used at the bedside. The patient is asked to fixate on the examiner's pupil as in testing central vision, and a large object, frequently the examiner's index finger, tip first, is moved into the patient's visual field (confrontation) from a position lateral to the patient's head. This is a rapid and easy way to approximate peripheral visual fields (Fig. 3-10).
It is useful at the bedside to use tachistoscopic double simultaneous stimulation (TDSS) of the visual fields. This entails the rapid momentary presentation of two objects simultaneously into opposite visual fields. In practice, a momentary movement of the tip of the index finger in both fields is suitable (see Fig. 3-10). TDSS testing is advantageous because minor partial field deficits, which may not be picked up on unilateral stimulation become apparent; the object in the abnormal field is extinguished. A rapid single excursion of the examiner's index fingers in the peripheral fields is adequate (see Fig. 3-10). Extinction in the visual field may represent either a partial dysfunction in the visual pathways or may be due to inattention phenomenon to one side of the body. This latter difficulty is usually part of a broader syndrome of hemispatial inattention (neglect), usually resulting from damage to the contralateral parietal (and occasionally frontal) association cortices.
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Define the following terms:anosmia, homonomous, hemianopsia, quadrantanopsia, scotoma, papilledema, papillitis, optic neuritis.
3-1. How do you test olfaction?
3-2. What is the most common cause of unilateral anosmia?
3-3. In whom is it particularly important to test olfaction?
3-4. How can you determine if visual acuity problems are due to refractive or to nerve problems?
3-5. What is the significance of finding a monocular visual loss?
3-6. What is the significance of finding a homonomous visual field deficit?
3-7. Where would a lesion that produced bitemporal hemianopsia be located?
3-8. How can lesions of the parietal or temporal lobes produce vision loss? What kind of loss would you expect to find?
3-9. Where on the visual cortex is the representation of the center of vision?
3-10. What artery supplies the visual cortex?
3-11. How can you distinguish papilledema from papillitis?