книги студ / Color Atlas of Pathophysiology (S Silbernagl et al, Thieme 2000)

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A. Refraction Abnormalities

Short-sightedness (myopia)

Long-sightedness (hyperopia)

Long-sightedness of the elderly (presbyopia)

Diseases of the Optical Apparatus

Diseases of the Retina

preserved. In retinal detachment from the pigment epithelium no deflections are registered in the ERG. If the retina is completely detached, the patient is totally blind.

Diabetic retinopathy (→B) is the most common disease of the retina. The cells around the thin retinal blood vessels (pericytes) produce sorbitol from the increased supply of glucose (→p. 290), swell up, and thus narrow the vessels. Additionally, the vessel walls are thickened by glycosylation (AGE; →p. 290). This results in ischemia of the tissues, formation of angiotrophic mediators, increase in vascular permeability, formation of new vessels, and hemorrhages. This bleeding opacifies the vitreal body, the ischemia destroys the retina and may ultimately lead to blindness.

Night blindness. The visual pigment consists of 11-cis-retinol, a metabolite of vitamin A and a protein that is different in the rods and the three types of cones (→C1). In vitamin A deficiency the formation of visual pigment in rods and cones is impaired, resulting in reduced light perception especially at low light intensity.

The function of the cones is to provide color vision. The pigments of the red, green, and blue cones each have different spectral sensitivities. Mutations of the genes for the respective pigments impair color vision. Partial or complete loss of the particular pigment (→C2) leads to weak red color vision or red color blindness (protanomaly or protanopia, respectively), green color weakness or blindness (deuteranomaly or deuteranopia), or blue color weakness or blindness (tritanomaly or tritanopia). As the genes for the red and green pigments are located on the X chromosome, many more men than women suffer from red or green color blindness.

If there are no cones, not only is there no color vision, but visual acuity is also greatly reduced, because the person can see only with much fewer rods in the fovea (rod monochromasia).

Color vision can be tested e.g. with tables in which the numbers can be correctly recognized only by means of the corresponding cones (→C3).

A. Diseases of the Retina

Retinitis pigmentosa

Capillary

Glucose

Sorbitol AGE

Pericyte

Vessel narrowing

Ischemia

Vessel proliferation

Bleeding

Retinopathy (loss of vision)

Photo: Hollwich F. Taschenatlas der Augenheilkunde. 3rd ed. Stuttgart: Thieme; 1987

Plate 10.14 Diseases of the Retina

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Hearing Impairment

Sound waves are transmitted from the eardrum (tympanum) via the ossicles to the fenestra vestibuli (vestibular window) (→A). The transmitting apparatus in the middle ear acts as an impedance converter. Without it 98% of sound energy would be reflected away because of the markedly different resistances to the sound waves in the air and in the fluid of the inner ear. Invagination of the fenestra vestibuli results in simultaneous evagination of the fenestra cochleae (cochlear window). The eardrum normally protects the latter against external sound waves and conducts the sound energy specifically toward the fenestra vestibuli. Sound waves can also be transmitted to the bones of the skull and can thus stimulate the inner ear. However, this requires a much greater energy of sound.

The oscillation of the fenestra vestibuli produces traveling waves in the inner ear, at first spreading along the scala vestibuli. The stereocilia of the outer and inner hair cells are bent by evagination of the cochlear septum with the basilar membrane and the organ of Corti at a frequency-dependent location (→B1). This leads to the opening of K+ channels in the cell membrane. The endolymph in which the stereocilia of the hair cells are suspended (→B2) has a very high K+ concentration (ca. 150 mmol/L). K+ is secreted by the epithelial cells of the stria vascularis, by Na+-K+-2 Cl– cotransport and by Na+/K+-ATPase in the antiluminal membrane, as well as by a luminal K+ channel (→B3). When the K+ channels in the membrane of the hair cells are opened, K+ enters the cells and depolarizes them. This depolarization then triggers the release of the transmitter, especially in the inner hair cells. By contracting, the outer hair cells increase the local traveling wave and thus the amount of stimulation of the hair cells.

Causes of deafness. A tear in the eardrum, a lesion in the ossicles, or immobilization of the conduction apparatus, for example, caused by a purulent middle ear infection, dampen transmission to the fenestra vestibuli. Furthermore, if there is a hole in the drum, the fenestra cochleae is no longer protected. This results in middle ear hearing loss. While conduction

through the air is impaired, bone conduction remains normal (→A).

The hair cells can be damaged by sound stress (i.e., impingement of too loud a sound for too long) and ischemia. But thanks to its high glycogen content they can survive short periods of ischemia by anaerobic glycolysis. Hair cells can also be damaged by certain drugs—such as aminoglycoside antibiotics and the chemotherapeutic agent cisplatin—that, via the stria vascularis, are accumulated in the endolymph. This results in inner ear hearing loss that affects air and bone conduction equally (→B4). Both the hearing threshold and the active component of basilar membrane displacement are affected, so that discrimination of different higher-frequency tones is impaired (→B5). Lastly, inadequate depolarization of the inner hair cells can produce an unusual and disturbing sound sensation (subjective tinnitus). This can also be caused by inadequate excitation of neurons in the auditory pathway or the auditory cortex.

Stiffening of the basilar membrane disturbs the micromechanics and thus probably contributes to hearing loss in the elderly (→B1).

Inner ear deafness can also be the result of abnormal endolymph secretion. Thus loop diuretics at high dosage not only inhibit renal but also auditory Na+-K+-2 Cl– cotransport. In addition, there is a known (but rare) genetic defect of the luminal K+ channel. The channel, which consists of two subunits (IsK/KvLQT1), is also expressed in the heart (as well as other organs), where it participates in repolarization. A

defect of KvLQT1 or IsK results not only in deafness but also in delayed myocardial repolari-

zation (long QT interval [Jervell, Lange–Nielsen syndrome]). Abnormal absorption of endolymph can also cause deafness. The endolymph space becomes evaginated, distorting the relationship between hair cells and tectorial membrane (endolymph edema; →B6). Finally, increased permeability between the endolymph and perilymph spaces may be responsible for Ménière’s disease, which is characterized by attacks of deafness and vertigo (→B7).

Plate 10.16 Hearing Impairment

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Vestibular System, Nystagmus

False information leads to inadequate movement of the eye muscles (nystagmus), and thus to roving of the surrounding objects on the retina (the room spins). Dizziness occurs and, via connections with autonomic neurons, nausea and vomiting. However, these disturbances are usually quickly compensated if there is prolonged loss of one of the organs in the vestibular system.

Olfaction

Sensory cells in the olfactory mucosa transmit different qualities of odor, namely flowery, ethereal, musky, camphoric, foul, sweaty, and stinging. Their axons pass through openings in the cribriform plate to the olfactory bulb (→B). From there the information reaches the olfactory cortex via the olfactory tract and is then

330 transmitted to the hypothalamus, the amygdaloid bodies, and, via the thalamus, to the cortex

(frontal lobe and insular cortex). The olfactory sense may be lost in circulatory disorders, for example, in a nasal cold, nasal malformation, foreign body, tumor, hematoma, or abscess (conductive hyposmia). The sensitivity of the sensory cells is increased by estrogens and decreases in the elderly. It is reduced by genetic defects, some drugs (e.g., cocaine, morphine), and toxins (e.g., cement dust, lead, cadmium, cyanide, chlorine compounds). The axons of the sensory cells can be torn by fracture in the region of the cribriform plate. The central processing of olfactory sensations is impaired by neurodegenerative disease (Alzheimer’s disease [→p. 348], Parkinson’s disease [→ p. 312ff.]), inflammation, tumors, alcohol, epilepsy (→p. 338) and schizophrenia (→p. 352). This results in reduced (hyposmia) or absent (anosmia) sense of smell, or increased (hyperosmia), inadequate (parosmia), or unpleasant (cacosmia) olfactory sensation.

Taste

Taste receptors in the tongue, palate, and throat transmit the modalities sweet, sour, salty, and bitter. The information is transmitted to the solitarius nucleus via the facial (VII), glossopharyngeal (IX), and vagus (X) nerves (→C). After connecting with second-or- der neurons, the afferent fibers pass via the thalamus to the primary taste cortex in the region of the insula. Taste receptors may be genetically defective or damaged by radiation or some drugs (e.g., local anesthetics, cocaine, penicillamine, streptomycin). Their sensitivity is reduced in hyperthyroidism. Patients with diabetes mellitus suffer from a reduction in the ability to sense sweet tastes; those with an aldosterone deficiency cannot sense salty tastes. The chorda tympani of the facial nerve may be damaged by a skull fracture or inflammation as well as damage to or operation on the ear, while the glossopharyngeal nerve may be damaged during tonsillectomy. Central conduction and processing can be affected by tumors, ischemia, and epilepsy, causing a reduction or loss of gustatory sense (hypogeusia or ageusia, respectively). The sense of taste may also be increased (hypergeusia), inadequate (parageusia), or unpleasant (dysgeusia).