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76 Cards in this Set
- Front
- Back
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Three stages of prenatal development
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Pre-embryonic = fertilization - 3 weeks
Embryonic = 4 - 8 weeks post fertilization Fetal = 9 weeks - term (40 weeks) Premature = birth before 37th week |
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Week 1
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Female ovum fertilized by male sperm = creates diploid cell (zygote). Zygote has 22 autosomes and 1 sex chromosome from each parent (46 total). Genetic characteristics determined at this point. Zygote increases size by mitosis (cell division that results in genetically identical cells). By day 3, solid ball of ~ 12-16 cells reaches uterus and evolves into blastocyst. Between day 6 & 7 blastocyst superficially implants into wall of uterus.
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Blastocyst
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3 components:
Inner cell mass or embryoblast which develops into embryo. Large central filled cavity into which embryoblast projects. Thin outer layer of cells which encloses embryoblast & central cavity & contributes to formation of placenta. |
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Week 2
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Blastocyst firmly implants. Maternal blood supply to embryoblast is established & nourishes embryoblast prior to development of placenta. Embryoblast differentiates into flattened, circular embryonic disc with 2 layers.
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Week 3
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Gastrulation occurs = 2 layer embryonic disk grows & evolves into 3 layer disk: Ectoderm, Mesoderm, & Endoderm. Notochord also develops: structure that evolves into bony axial skeleton (vertebral column, ribs, sternum, & skull). Ectoderm overlaying notochord thickens to form neural plate, which eventually becomes CNS. Neural plate depresses inward to form neural groove. Thickening of ectoderm = auditor placode - this becomes the membranous labryinth. Primitive cardiovascular system also begins to form = 1st system to become functional in embryo.
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3 layers
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Ectoderm: gives rise to outer skin layers, nervous system and inner ear.
Mesoderm: associated with skeletal, circulation structures, kidneys & reproductive organs - gives rise to ossicles. Endoderm: epithelial lining of digestive & respiratory systems, including Eustachian tube & ME cavity, also liver & pancreas. |
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Week 4
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Primitive heart begins to beat by day 22 - has only one atrium & one ventricle. Edges of neural plate = neural folds which approach each other & fuse to form neural tube. If fusion of neural folds is disrupted & neural tube does not close completely = severe congenital abnormalities of brain & spinal cor. Auditory placode becomes depressed inward to form auditory pit or otic pit, which will develop into sensory structure of inner ear. Around day 30 otic pit closes to become auditory vesicle. Embryo develops into characteristic "C" shape at end of 4th seek.
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1st branchial arch
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Mandibular arch evolves into tragus (external ear), ossicles (malleus, body of incus), mandible (lower jaw), maxilla (upper jaw), zygomatic bone (cheekbone), & squamous portion of the temporal bone.
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2nd branchial arch
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Hyoid (suspension for the larynx), auricle, ossicles (stapes, lenticular process of incus, handle of malleus).
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3rd branchial arch
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Contributes to development of hyoid bone.
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4th branchial arch
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Contributes to formation of larynx.
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5th & 6th weeks
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Several small swellings (aricular hillocks) develop between 1st & 2nd branchial arches - hillocks will fuse & form external ear.
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End of 8th week
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Embryo appears human. All major organ systems have become established (at least in primitive state). During this embryonic period major congenital anomalies may arise from exposure to drugs, infections, or toxins that cause birth defects or from genetic or chromosomal abnormalities.
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Pharyngeal pouches
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Outpockettings of primitive pharynx; four of them separate branchial arches internally.
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Branchial groves
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Separate branchial arches externally.
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Branchial membranes
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Appear where branchial groves approximate pharyngeal pouches; approximation of ectoderm of groves w/ endoderm of pouches induces formation of mesenchyme.
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Fetal development
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9 weeks to birth. Fetus undergoes rapid growth of body structures, growth & differentiation of tissues & major organ systems. Fetus is less susceptible to death or major deformity than in embryonic period, but growth & normal functional development can still be interfered with .
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Development of external ear
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Development is initiated by formation of branchial arches during 4th week. 1st & 2nd arches are critical to development of external ear.
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Development of auricle
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6 nodular swellings of tissue (hillocks) appear on 1st & 2nd arches. Hillocks grow & fuse into recognizable auricle by week 8. Mesenchymal layer (middle fibrous layer) changes to cartilage. Weeks 7-20 auricle continues to develop & begins to move laterally as it is displaced by mandible & face. Auricle is adult shape by week 20, but continues to grow until about 9 years.
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Development of external auditory meatus
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Develops from 1st branchial grove starting at week 4-5. Pharyngeal pouch (developing ME) deepens & comes in contact w/ primitive external auditory canal. Contact lost due to proliferation of embryonic connective tissue. By week 8 developing EAC deepens to approach ME space. Shortly thereafter, ectodermal lining of primitive EAC proliferates to a solid tissue = meatal plug, which fills the medial portion of the canal. Meatal plug stays until 21-28 weeks, then hollows out to become medial 2/3 of EAC.
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Development to tympanic membrane
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Outer layer of membrane arises from ectoderm of meatal plug. Inner layer comes from 1st pharyngeal pouch (endoderm lined). Middle layer develops from mesenchymal tissue that separates the inner & outer layers.
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Development of middle ear
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ME cavity & eustachian tube develop from endoderm lined 1st pharyngeal pouch (recall, separates 1st & 2nd branchial arches). ME develops from the inside outward.
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Weeks 5-6 ME development
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Tympanic cavity begins to develop. 1st pharyngeal pouches elongates. Elongation becomes tympanic cavity & auditory tube (later becomes Eustachian tube). 1st branchial grove widens - begins inward extension to become EAC. Ossicles begin to develop (cartilaginous form).
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Weeks 7-15 ME development
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By 8th week - tympanic cavity present in lower half of future ME; upper half filled w/ mesenchymal tissue. By 8 1/2 weeks - incus & malleus reach complete cartilaginous form. Stapes continues to develop in cartilaginous form until week 15.
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Weeks 16-32 ME development
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Ossification of malleus & incus begins - nearly complete by week 32. Stapes begins to ossify by week 18. Stapes develops further during life (e.g., child's stapes is less fragile than adult). As ossicles begin to ossify, surrounding mesenchymal tissue starts to become absorbed by surrounding tissue resulting in expansion of ME space.
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Development of inner ear
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Arise from otic placodes. Become apparent by end of 3rd week. Otic placodes depress inward to form otic or auditory pit. 4th week - edges of otic pit come together & fuse to form otic or auditory vesicle (otocyst). Otic vesicle cuts off from surface ectoderm & migrates inward.
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Development of membranous labryinth
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Weeks 5: auditory vesicle elongates shortly after closing off from surface.
Week 6: vestibular portion develops slightly before saccular portion (from which cochlea arises). Week 7: one cochlear coil present. Sensory cells 1st appear in utricle & saccule. Semicircular canal development continues. Week 8: 1 1/2 cochlear turns present. By end of week 8 (beginning of week 9): 2 1/2 cochlear turns present. Swift growth of labyrinth by week 10; cochlea is nearly adult form; this means that 2 1/2 turns develop in about 2 1/2 months. |
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Development of sensory structures of inner ear
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Weeks 7-20
By 11th or 12th week - incompletely differentiated hair cells apparent. IHCs present throughout cochlear duct. OHCs present only in basal half. By 16th week - differentiated HCs visible. By 17th week - full number of adult HCs. Development of supporting cells parallels development of sensory structures. |
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Innervation of cochlea
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7-20th week
Ganglion cells from spiral ganglion grow toward membranous labyrinth. Immediately after coiling of cochlea is complete (~week 9), nerve fibers begin developing in cochlea - afferent & efferent fibers form when HC differentiation begins (by 11th week). By week 20 - mature synaptic patterns on IHCs - OHC synaptic patterns continue to develop. By week 34 - cochlea reaches final size & structure - growth & development are complete. |
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Temporal bone
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Forms part of lateral wall & base of skull. Squamous - superior to external aud meatus. Mastoid - posterior to external aud meatus. Petrous - extends medially from external aud canal, most dense bone in body & its middle 3rd contains the structures of the ME & inner ear. Tympanic - form anterior, inferior, & part of posterior walls of external aud meatus.
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Location of cochlear portion of inner ear
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Cochlear portion of the inner ear is situated anteriorly & medially to internal aud meatus & vestibular portion of inner ear.
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Internal auditory meatus location
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Internal aud meatus houses facial nerve (7th), which lies superior to aud portion of acs nerve (8th) & anterior to superior vestibular branch of 8th cranial nerve.
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How many children are affected by congenital or early childhood onset SNHL/deafness?
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1 in 1000
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Congenital
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Present at birth - doesn't imply etiology (cause).
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Hereditary
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Inherited, passed down from previous generations through DNA, genes, &/or chromosomes.
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Genetic
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Caused by a gene, can be considered subset of hereditary (e.g., Alport syndrome).
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Familial
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Present in several related family members, does not imply etiology (e.g., families of 2 cousins w/HL of dissimilar etiology).
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When do the ossicles begin to develop?
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Weeks 5-6
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When are there 2.5 turns in the cochlea
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End of week 8, beginning of week 9
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Full # of adult hair cells
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17th week
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Time frame over which external ear develops
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Starts at week 4, adult shape by week 20. Continues to grow till age 9.
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Percent of hearing losses thought to have a genetic origin
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50%
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Percent of genetic hearing losses estimated to be autosomal dominant inheritance
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22%
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Percent of genetic hearing losses estimated to be recessive inheritance
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77%
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Percent of genetic hearing losses estimated to be sex-linked (or mitochondrial) inheritance
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1%
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Proportion of genetic hearing losses estimated to be associated with syndromes
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1/3
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Proportion of genetic hearing losses estimated to be not associated with syndromes
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2/3
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Sex linked (x-linked) inheritance
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Abnormal gene(s) on X sex chromosome. Most are recessive. Inheritance through maternal line. No male-to-male inheritance. 50% male offspring of carrier mother are affected. 50% female offspring of carrier female will also be carriers. Absence or milder form of disorder in carrier female.
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Factors complicating diagnosis of autosomal dominant inheritance
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Lack of penetrance: skipped generation.
Variability in expression: degree to which disorder is manifested in affected individual. |
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What percentage of identified types of non-syndromic genetic hearing loss of autosomal dominant inheritance show progressive hearing loss
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50%
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Rang of configuration of non-syndromic genetic hearing loss of autosomal dominant inheritance
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Congenital severe SNHL, congenital low-freq SNHL, progressive low-freq SNHL w/ childhood onset, mid-freq SNHL, progressive SNHL (abrupt high freq SNHL), progressive mixed HL, unilateral SNHL (mod-severe unilateral), progressive vestibulo-cochlear dysfunction & SNHL.
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4 types of aplasia
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Michel-Type
Mondini Schieve Alexander |
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Michel-Type Aplasia
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Most severe, complete failure of development of inner ear, complete absence of vestibule, cochlea & internal aud meatus.
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Mondini Aplasia
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Incomplete development of membranous & bony labyrinth of inner ear. Wide variations but typically only 1.5 turns of cochlea. Often middle & apical turns affected. Have minimal residual hearing with progressive component. Can be unilateral or bilateral. Often considered candidates for implantation (if there are still nerves preset that can be activated).
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Schiebe Aplasia
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Most common aplasia of inner ear. Involves membranous labyrinth. May show normal radiologic result with profound & total deafness.
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Alexander Aplasia
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Least severe. Only basal turn affected, only high-freq HL.
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What does the acronym CHARGE stand for?
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Coloboma of the eye
Heart defect Atresia (complete closure) of the nasal choanae Retardation of growth Genital anomalies Ear anomalies |
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Stichler Syndrome
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Autosomal dominant. Cherubic features. Mild to moderate SNHL. Conductive loss secondary to chronic ear infections associated with cleft palate or defect of the ossicles. Types 1 & 2 progressive. No mental retardation.
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Pendred Syndrome
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Autosomal recessive. Goiter, Severe bilateral SNHL. No conductive loss. Progressive. No mental retardation.
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CHARGE Syndrome
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Autosomal dominant possible (mainly sporadic). Keyhole eye, short stature common, malformed auricle (often unilateral). No characteristic HL (asymmetric). Progressive. Sometimes mental retardation.
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Goldenhar Syndrome
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Etiologic heterogeneity (spontaneous), autosomal dominant in some families. Craniofacial, ocular and vertebral abnormalities. Conductive, sensorineural or mixed HL. Not progressive. Small % have mental retardation.
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Branchio-oto-renal syndrome
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Autosomal dominant. Ear, eye and kidney abnormalities. Conductive, sensorineural or mixed. Mild to profound. Usually symmetric. Occasionally progressive. No mental retardation.
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Charcot-Marie-Tooth Syndrome
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Type 1 & 2: autosomal dominant. Type 3 & 4: autosomal recessive. Type X: X-linked dominant. Weakness in extremities and family history. Sensorineural hearing loss in types 1, 2, & X. No conductive component. Can be sudden of progressive. No mental retardation.
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Waardenburg Syndrome
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Type 1, 2 & 3: autosomal dominant. Type 4: autosomal dominant or recessive. Brilliant blue or different colored eyes. White forelock. Skin depigmentation/ hyperpigmentation. SNHL of varying degree. No conductive loss. Type 2 progressive. No mental retardation.
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Treacher Collins Syndrome
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Autosomal dominant. Underdeveloped supraorbital rims, hypoplastic zygomas, narrow face, micrognathia. Flat conductive bilateral HL. Not progressive. Usually normal intelligence.
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Usher Syndrome
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Autosomal recessive. SNHL of varying degrees. No conductive component. Type 3 progressive. No mental retardation.
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Hurler Syndrome
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Autosomal recessive. Coarse facial features, thick lips, macrocephaly, claw hands, cloudy corneas, halted growth. Mostly conductive, some SNHL. Progressive. Mental retardation.
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Down Syndrome
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Trisomy 21. One of most common congenital disorders. Extra copy of chromosome 21 present in individual's karyotype. Characteristic facial appearance w/ flat face, protrusion of tongue, upper slant of palpebral fissures, fine, straight hair, hypotonia, small hangs w/ short fingers, small unusually shaped auricles w/ very narrow external auditory meati, mild to severe mental retardation, conductive HL 2nd to otitis media, also SNHL or mixed.
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TORCH
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Toxoplasmosis
Other bacterial infections Rubella virus Cytomegolovirus Herpes simplex virus |
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Which of the TORCH diseases are associated with progressive hearing loss?
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Toxoplasmosis, Rubella virus, Cytomegolovirus
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Which of the TORCH diseases are associated with delayed onset hearing loss?
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Other bacterial infections, Rubella virus
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Which of the TORCH diseases are associated with mental retardation?
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Other bacterial infections, Rubella virus, Cytomegolovirus, Herpes simplex virus
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Which of the TORCH diseases are associated with visual problems?
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Toxoplasmosis, Rubella virus, Cytomegolovirus
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How often does hearing loss occur with Rubella?
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50%
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Which of the TORCH diseases is most common in the general population?
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Cytomegolovirus
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Causes of post-natal hearing loss
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Atresia (complete closure) or stenosis (narrowing of canal)
Collapsed ear canals Aural discharge Cerumen & foreign bodies Bony growths – exostoses (outgrowths in the canal), osteomas (one single growth) Inflammatory conditions – otitis externa, perichondritis – after trauma, furuncle – boil or pimple inside ear Bullous myringitis TM perforations Otitis media Clesteatoma Mastoiditis Cleft palate |
