Gustatory Disturbances Dysgeusia

78 When smell is impaired, the patient loses the capacity for fine differentiation of tastes but is able to distinguish the primary tastes (sweet,

Ventral posteromedial -nucleus of the thalamus

Fibers to the salivatory nuclei

_ l-linnnramm ic

Postcentral gyrus

Salivatory nuclei (inferior and superior)

,- Nucleus of solitary tract

Ventral posteromedial -nucleus of the thalamus

_ l-linnnramm ic

Postcentral gyrus

Fibers to the salivatory nuclei

Pterygopalatine ganglion

Greater petrosal n.

Soft palate, uvula

Solitary Tract Nucleus Alone

,- Nucleus of solitary tract

Pterygopalatine ganglion

Greater petrosal n.

-Fibers to muscles of facial expression, ^ mastication, and deglutition

Geniculate ganglion (VII)

Inferior ganglion of X Inferior ganglion of IX

-Chorda tympani

Jugular foramen — Glossopharyngeal n. Superior laryngeal n. (X) -Lingual n. (V/3)

Soft palate, uvula

Retina. Visible light is electromagnetic radiation at wavelengths of 400-750 nanometers. The dioptric system (cornea, aqueous humor of the anterior and posterior ocular chambers, pupil, lens, vitreous body) produces a miniature, upside-down mirror image of the visual field on the retina. The fovea, located in the center of the macula at the posterior pole of the eyeball, is the area of sharpest vision in daylight. Blood is supplied to the eye by the ophthalmic artery via the ciliary arteries (supplies the choroid) and the central retinal artery (supplies the retina). The optic disk, the central retinal artery that branches from it, and the central retinal vein can be examined by ophthalmoscopy. Visual pathway. The visual pathway begins in ¡3 the retina (first three neurons) and continues C through the optic nerve to the optic chiasm, from which it continues as the optic tract to the "g lateral geniculate body. The optic radiation 'c arises at the lateral geniculate body and termi-2 nates in the primary (area 17) and secondary visual areas (areas 18, 19) of the occipital lobe. The fibers of the retinal neuronal network converge at the optic disk before continuing via the optic nerve to the optic chiasm, in which the medial (nasal) fibers cross to the opposite side. The right optic tract thus contains fibers from the temporal half of the right retina and the nasal half of the left retina. The lateral geniculate body is the site of the fourth neuron of the optic pathway. Its efferent fibers form the optic radiation, which terminates in the visual cortex (striate cortex) of the occipital lobe. The central foveal area has the largest cortical representation. The visual pathway is interconnected with midbrain nuclei (medial, lateral, and dorsal terminal nuclei of the pretectal region; superior colliculus), nonvisual cortical areas (somatosensory, pre-motor, and auditory), the cerebellum, and the pulvinar (posterior part of thalamus). Visual field. The monocular visual field is the portion of the external world seen with one eye, and the binocular visual field is that seen by both eyes. The visual fields of the two eyes overlap; the overall visual field therefore consists of a central zone of clear binocular vision produced by the left and right central foveae, a peripheral 80 binocular zone, and a monocular zone. Partial decussation at the optic chiasm brings visual information from the right (left) side of the world to the left (right) side of the brain. The visual field is topographically represented at all levels of the visual pathway from retina to cortex; lesions at any level of the pathway cause visual field defects of characteristic types. If the images on the two retinas are displaced by more than a certain threshold distance, double vision (dipl-opia) results. This is most commonly due to disturbances of the extraocular muscles, e. g., paralysis of one or more of these muscles (p. 86). Stereoscopic vision. Three-dimensional visual perception (stereoscopic vision) is produced by comparison of the slightly different images in the two eyes. Stereoscopic vision is very important for depth perception, though depth can be judged to some extent, through other cues, with monocular vision alone. Color vision. Testing of color vision requires standard definition of the colors red, blue, and green. The visual threshold for various colors, each defined as a specific mixture of the three primary colors, is determined with a standardized color perception chart. Disturbances ofcolor vision may be due to disturbances ofthe dioptric system, the retina, or the visual pathway. Cortical lesions cause various kinds of visual agnosia. Lesions of area 18 may make it impossible for patients to recognize colors despite intact color vision (color agnosia), or to recognize familiar objects (object agnosia) or faces (prosop-agnosia). Patients with lesions of area 19 have intact vision but cannot recognize or describe the objects that they see. Spatial orientation may be impaired (visuospatial agnosia), as may the inability to draw pictures. Persons with visual agnosia may need to touch objects to identify them.

Limbic system. Connections with the limbic system (hippocampus, amygdala, parahippocampal gyrus; p. 144) account for the ability of visual input to evoke an emotional response.

Lateral geniculate body (right)

Lateral geniculate body (right)

Cross Section Human Cell
— Occipital cortex (left)

Optic n.

Course of the visual pathway

Left optic tract after chiasmal decussation Optic chiasm -

Cross section of retrobulbar fibers

Optic-

disk (blind spot)

Patient's-

right and left visual fields

Central fovea

Left optic tract after chiasmal decussation Optic chiasm -

Lateral Geniculate Body

Area 17 (striate cortex)

Retinal image of object

Optic radiation (left)

Ophthalmic a.

Area 17 (striate cortex)

Calcarine sulcus

Area 18 Area 19 Lateral geniculate body

Optic tract (posterior third)

(cross section)

Distribution of fibers along the visual pathway

Binocular portion of visual field

Superior colliculus

Pretectal region

Binocular portion of visual field

Superior colliculus

Pretectal region

Pretectal Region

Retinal fibers

Monocular visual field of nasal retina ("temporal sickle")-

Fibers of nasal retina (all decussate)

Retinal fibers

/Area 17 (striate cortex)

Pulvinar

Area 18

Area 19

Projections to visual cortex

Lateral geniculate body Terminal nuclei Macular fibers (papillomacular bundle) Projections of the visual pathway

Examination. The visual fields of both eyes should always be jointly assessed. The confrontation test, in which the examiner "confronts" the patient's visual field with his or her own, intact, contralateral visual field, is used to check for visual field defects. For the test to be performed correctly, the patient and the examiner must first fixate along the same line. The examiner then slowly moves a white or red object (at least 1 cm in diameter) from the periphery of the visual field toward the center in a number of different directions, and determines where the patient can and cannot see it. Alternatively, the examiner may raise one or more fingers and ask the patient to count them (a useful test for small children, and for persons whose vision is so poor that it ¡3 cannot be tested by the first method). The per-C ceived brightness (unequal in patients with hemianopsia) of the hand in the nasal and tem-"g poral portions of the visual field is also deter-'c mined. The red vision test enables the detection 2 of a central scotoma as an area in which the red color is perceived as less intense. More detailed information can be obtained by further oph-thalmological testing (Goldmann perimetry, automatic perimetry).

Visual field defect (scotoma). The thin myelinated fibers in the center of the optic nerve, which are derived from the papillomacular bundle, are usually the first to be affected by optic neuropathy (central scotoma). From the optic chiasm onward, the right and left visual fields are segregated into the left and right sides of the brain. Unilateral lesions of the retina and optic nerve cause monocular deficits, while ret-rochiasmatic lesions cause homonymous defects (quadrantanopsia, hemianopsia) that do not cross the vertical meridian, i.e., affect one side of the visual field only. Anterior retrochiasmatic lesions cause incongruent visual field defects, while posterior retrochiasmatic lesions lead to congruent visual field defects. Temporal lobe lesions cause mildly incongruent, contralateral, superior homonymous quadrantanopsia. Bitemporal visual field defects (heteronymous hemi-anopsia) have their origin in the chiasm. Unilateral retrochiasmatic lesions cause visual field defects but do not impair visual acuity. Organic 82 visual field defects widen pregressively with the distance of test objects from the eye, whereas psychogenic ones are constant ("tubular fields"). Prechiasmatic lesions may affect the retina, papilla (= optic disk), or optic nerve. Transient episodes of monocular blindness (amaurosis fugax) see p. 372 (table 22 a). Acute or subacute unilateral blindness may be caused by optic or retrobulbar neuritis, papilledema (intracranial mass, pseudotumor cerebri), cranial arteritis, toxic and metabolic disorders, local tumors, central retinal artery occlusion, or central retinal vein occlusion.

Chiasmatic lesions. Lesions of the optic chiasm usually produce bitemporal visual field defects. Yet, because the medial portion of the chiasm contains decussating fibers while its lateral portions contain uncrossed fibers, the type of visual field defect produced varies depending on the exact location of the lesion. As a rule, anterior chiasmatic lesions that also involve the optic nerve cause a central scotoma in the eye on the side of the lesion and a superior temporal visual field defect (junction scotoma) in the contralateral eye. Lateral chiasmatic lesions produce nasal hemianopsia of the ipsilateral eye; those that impinge on the chiasm from both sides produce binasal defects. Dorsal chiasmatic lesions produce bitemporal hemianopic paracentral scotomata. Double vision may be the chief complaint of patients with bitemporal scotomata. Retrochiasmatic lesions. Depending on their location, retrochiasmatic lesions produce different types of homonymous unilateral scotoma: the defect may be congruent or incongruent, quadrantanopsia or hemianopsia. As a rule, temporal lesions cause contralateral superior quadran-tanopsia, while parietal lesions cause contralateral inferior quadrantanopsia. Complete hemianopsia may be caused by a relatively small lesion of the optic tract or lateral geniculate body, or by a more extensive lesion more distally along the visual pathway. Sparing of the temporal sickle (p. 80) indicates that the lesion is located in the occipital interhemispheric fissure. Bilateral homonymous scotoma is caused by bilateral optic tract damage. The patient suffers from "tunnel vision" but the central visual field remains intact (sparing of macular fibers). Cortical blindness refers to subnormal visual acuity due to bilateral retrogeniculate lesions. Bilateral altitudinal ho-monymous hemianopsia (i.e., exclusively above or exclusively below the visual equator) is due to extensive bilateral damage to the temporal lobe (superior scotoma) or parietal lobe (inferior sco-toma).

Visual field Directions tested Blind spot Test object — Macular region

Altitudinal Hemianopia

Line of fixation of eye

Patient ca. 50 cm from examiner

Line of fixation of eye

Patient ca. 50 cm from examiner

Confrontation test

Homonymous inferior quadrantanopsia

Homonymous hemianopsia (macular sparing)

Incongruent homonymous quadrantanopsia

Homonymous hemianopsia

Binasal homonymous defect

Homonymous superior quadrantanopsia

- Homonymous hemianopic central visual field defect (occipital pole lesion)

Sparing of contralateral "sickle" and macula (lesion of calcarine cortex on medial surface of

Monocular defect Right visual field

Meridian Left visual field

Homonymous superior quadrantanopsia

Homonymous hemianopsia (macular sparing)

Incongruent homonymous quadrantanopsia

Homonymous hemianopsia

Binasal homonymous defect

Monocular defect Right visual field

Meridian Left visual field

Anopsia Diagnosis

Bitemporal hemianopsia

"Junction scotoma"

Tunnel vision Cortical blindness

Inferior altitudinal hemianopsia

Bitemporal hemianopsia

"Junction scotoma"

Tunnel vision Cortical blindness

Types and localization of visual field defect

Inferior altitudinal hemianopsia

Bilateral homonymous visual field defect

The visual axes of the eyes are directed straight ahead on primary gaze (i.e., 23° inward from the more lateral axes of the orbits). Movements of the eyes are mediated by six extraocular muscles on each side. The lateral and medial rectus muscles are responsible for horizontal eye movements. Vertical eye movements are subserved by the superior and inferior rectus as well as superior and inferior oblique muscles. The rectus muscles elevate and depress the eye when it is abducted, the oblique muscles when it is adducted. The two muscles of each synergistic pair (e.g., the left lateral rectus and right medial rectus muscles) receive equal degrees of innervation (Hering's law).

Vestibulo-ocular reflex (VOR). Impulses arising ¡3 in the semicircular canals in response to rapid C movement of the head induce reflex movement of the eyes in such a way as to stabilize the visual "g image (p. 26). For example stimulation of the 'c horizontal semicircular canal activates the ipsi-2 lateral medial rectus and contralateral lateral C rectus muscles, while inhibiting the ipsilateral lateral rectus and contralateral medial rectus muscles. The VOR makes the eyes move in the direction opposite to the head movements, at the same angular velocity. Optokinetic reflex. Optokinetic nystagmus (OKN) is triggered by large-scale, moving visual stimuli and serves to stabilize the visual image during slow head movement. OKN is characterized by slow, gliding conjugate movement of the eyes in the direction of an object moving horizontally or vertically across the visual field, in alternation with rapid return movements in the opposite direction (saccades). OKN is intact in psychogenic (pseudo) blindness. Fixation. Fixation is active adjustment of the gaze (with or without the aid of eye movement) to keep a visualized object in focus. Saccades. Saccades are rapid, jerky conjugate movements of the eyes that serve to adjust or set the point of fixation of an object on the fovea. Saccades may be spontaneous, reflexive (in response to acoustic, visual, or tactile stimuli), or voluntary; the rapid phase of nystagmus is a saccade. The speed, direction, and amplitude of a saccadic movement are determined before it is 84 carried out and cannot be influenced voluntarily during its execution. Shifts of visual fixation by more than 10° are accompanied by head move ments.

Slow ocular pursuit. Voluntary ocular pursuit can occur only when triggered by a moving visual stimulus (e. g., a passing car). Conversely, fixation of the gaze on a resting object while the head is moving leads to gliding eye movements. Fixation-independent ocular pursuit also occurs during somnolence and the early stages of sleep ("floating" eye movements). Vergence movements (convergence and divergence) are mirror-image movements of the two eyes toward or away from the midline, evoked by movement of an object toward or away from the head in the sagittal plane. They serve to center the visual image on both foveae and are accompanied by an adjustment of the curvature of the lens (accommodation) to keep the object in focus.

Neural pathways. The medial longitudinal fasciculus (MLF) interconnects the nuclei of cranial nerves III, IV, and VI. The MLF also connects with fibers conveying information to and from the cervical musculature, vestibular nuclei, cerebellum, and cerebral cortex and thus mediates the coordination of eye movements with movements of the body and head. Saccades are produced by two parallel systems: Voluntary eye movements are subserved by the frontal system, which consists of the frontal eye fields (areas 4, 6,8,9), the supplementary eye field (area 6), the dorsolateral prefrontal cortex (area 46), and a portion of the parietal cortex (area 7). It projects to the contralateral paramedian pontine reticular formation (PPRF), which coordinates vertical and horizontal saccades. Vertical and torsional eye movements are controlled by the rostral interstitial nucleus of the MLF and by the interstitial nucleus of Cajal. Reflex eye movements are initiated in the visual cortex (area 17) and temporal lobe (areas 19, 37, 39) and modulated in the superior colliculus (collicular system). Vergence and accommodation are mediated by the pretectal area in the vicinity of the oculomotor nucleus.

Inferior oblique m. (III)

Secondary position c •

Inferior oblique m. (III)

Secondary position

Pretectal Area
Peripheral Neuropathy Natural Treatment Options

Peripheral Neuropathy Natural Treatment Options

This guide will help millions of people understand this condition so that they can take control of their lives and make informed decisions. The ebook covers information on a vast number of different types of neuropathy. In addition, it will be a useful resource for their families, caregivers, and health care providers.

Get My Free Ebook


Responses

  • melissa
    How to test hemianopia?
    8 years ago

Post a comment