Pedigree Analysis of Children with Phonology Disorders Barbara A. Lewis
This study examined 87 pedigrees of individuals with histories of preschool phonology disorders. Results confirmed previous reports that speech and language disorders aggregate in families, with a higher incidence of males affected than females. Significantly more family members with dyslexia and learning disabilities, but not stuttering or hearing impairment, were found in pedigrees of individuals with phonology disorders than in pedigrees of nondisabled individuals. Probands with and without additional language problems did not differ in the incidence of affected family members. Nuclear family members demonstrated a higher incidence of disorders than when all family members were considered, with brothers of probands most often affected. Pedigrees of female probands had more affected members in their nuclear families than pedigrees of male probands.
C
ontemporary investigators in the field of dyslexia have proposed that dyslexia may be viewed as a developmental language disorder characterized primarily by deficits in phonological processing skills (Catts, 1989, 1991; Kamhi & Catts, 1989; Liberman & Shankweiler, 1985; Pennington, 1990; Scarborough, 1990). Several recent studies that have followed preschool children with speech and language disorders into early grade school have reported that children with articulation disorders alone generally do not have difficulty learning to read, whereas children with more global language deficits as preschoolers may have later reading problems (Bishop & Adams, 1990; Catts, 1991; Magnusson & Naucler, 1990; Scarborough, 1990). The relationship between early language difficulties and later reading problems is not well understood. One explanation may be a general verbal trait deficit, including phonological coding
and phoneme awareness deficiencies (Catts, 1989; Olson, Wise, Conners, & Rack, 1989; Pennington, 1990), that affects both spoken and written language. Studies of the genetic basis of dyslexia have identified deficits in phonological processing skills as the heritable component to dyslexia (Olson et al., 1989). However, the genetic basis of speech and language disorders has been less extensively examined. Phenotypes for developmental phonology disorders into early grade school and beyond are not well defined (Lewis & Freebairn-Tarr, in press). Therefore, the phenotypic boundaries between dyslexia and developmental phonology disorders have not been identified, nor has the comorbidity of dyslexia and developmental phonology disorders been examined. The purpose of the present study is to examine the pedigrees of probands with preschool phonology disorders for affected family members with speech and language JOURNAL OF LEARNING DISABILITIES VOLUME 25, NUMBER 9, NOVEMBER 1992 Downloaded from ldx.sagepub.com at Harvard Libraries on April 23, 2015 PAGES 586-597
disorders, dyslexia, and learning disabilities.
Genetics of Dyslexia Versus Genetics of Speech and Language Disorders Anecdotal reports of "congenital word blindness" were found as early as the late 1890s (Kerr, 1897; Morgan, 1896). Extensive family studies of dyslexia have documented that dyslexia aggregates in families (Decker & DeFries, 1980; DeFries, Singer, Foch, & Lewitter, 1978; Finucci, 1978; Hallgren, 1950). Twin studies and adoption studies have established a genetic component to dyslexia and estimated heritability (Bawkin, 1973; DeFries, Fulker, & LaBuda, 1987). The most extensive study to date, the Colorado Family Reading Study (Decker & DeFries, 1980; Lewitter, DeFries, & Elston, 1980), included segregation and linkage analysis (Smith, Kimberling, Pennington, & Lubs, 1983; Smith, Pennington, Kimberling, & Ing, 1990). Results support genetic heterogeneity in the transmission of dyslexia (see DeFries, 1991, or Pennington, 1990, for a review). In contrast, the familial basis of developmental speech and language disorders has not been extensively studied, although such a basis to some speech and language disorders has long been proposed (Arnold, 1961; Eutis, 1947; Ingram, 1959; Lenneberg,
587
VOLUME 25, NUMBER 9, NOVEMBER 1992
1967; Luchsinger, 1970; Seemann, 1937). As early as the late 1930s, pedigrees of children with speech and language impairment with multiple affected family members were reported. Several recent studies (Lewis, Ekelman, & Aram, 1989; Lewis, 1990a; Neils & Aram, 1986; Parlour & Broen, 1989; Tallal, Ross, & Curtiss, 1989; Tomblin, 1989) have supported a familial and perhaps even a genetic basis to developmental speech and language disorders of previously unknown etiology. These studies reported an increased incidence of speech, language, and reading disorders among family members of children with speech and language impairments. Twin studies, segregation analysis, and linkage analysis have not been conducted to date. One possible explanation for the lack of genetic studies of speech and language disorders is that the phenotype for developmental speech and language disorders changes with development. Speech and language disorders are not as readily identifiable in older children and adults as dyslexia often is, and, thus, phenotypes of older individuals have not been described. Most individuals with histories of speech and language disorders as preschoolers are considered normal speakers as adults. Pedigrees of individuals with speech and language disorders have been constructed primarily by historical report. The following section provides a review of the familial studies of developmental speech and language disorders.
Family Studies of Developmental Speech and Language Disorders Generally, family studies of developmental speech and language disorders have taken one of two forms: Either they have reported on a single, extended pedigree (Arnold, 1961; Eutis, 1947; Gopnik, 1990; Hurst, Baraitser, Auger, Graham, & Norell, 1990; Lewis, 1990a; Saleeby, Hadjian, Martinkosky, & Swift, 1978; Samples & Lane, 1985), or these studies have pooled a large sample of children with speech and lan-
guage impairments and reported the percentage of affected first-degree relatives (i.e., parents and siblings) (Lewis et al., 1989; Neils & Aram, 1986; Robinson, 1987; Tallal, Ross, & Curtiss, 1989; Tomblin, 1989). Studies reporting on a single kindred usually have selected the pedigree for study because of the large number of affected family members and the severity of the disorder. In these families the disorder often appears to be inherited in an autosomal dominant mode (i.e., the disorder is usually present in at least one parent of each affected child, and about one half of children of an affected parent are also affected), with variable expressivity and penetrance. However, the disorder that the child presents may differ in severity, as well as in type, from those of the parent, siblings, and other family members. While providing support for a familial basis for some developmental speech and language disorders, such families are not representative of the vast majority of children with speech and language disorders. Studies reporting percentages of affected first-degree relatives of a large number of children with speech and language disorders have found a significant increased incidence of speech, language, reading, and learning disorders when compared to nondisabled families (Lewis et al., 1989; Parlour & Broen, 1989; Tallal et al., 1989; Tomblin, 1989). However, the incidence of these disorders is much less than of those reported in single-pedigree studies. In addition, an autosomal dominant mode of inheritance is not upheld for all families. In fact, in some families, other affected family members are not found at all. These family studies have differed in the procedures for ascertaining the proband, criteria for determining affection status of family members, and the inclusion of extended family members versus limiting the study to nuclear family members.
Proband Selection Proband selection criteria have been both narrowly defined (so that the pro-
Downloaded from ldx.sagepub.com at Harvard Libraries on April 23, 2015
band exhibits a specific speech and language disorder, such as a phonology disorder) as well as broadly defined (so that all individuals with specific language impairments are included). Lewis et al. (1989) selected probands with severe preschool phonology disorders and reported that 25.5% of firstdegree relatives were affected. Other researchers (Neils & Aram, 1986; Robinson, 1987; Tallal et al., 1989; Tomblin, 1989) have selected probands with specific language impairment, including phonology disorders, and report from 22.9% to 48.9% of firstdegree relatives affected. Silver (1971) employed an even broader criterion for proband selection, studying families of children with learning disabilities that included speech and language disorders, and found 39.4% of first-degree relatives affected. See Table 1 for a summary of studies of first-degree relatives. Although the probands were selected by differing inclusion/exclusion criteria, these families may be representative of the same speech and language disorders. The percentage of affected first-degree relatives may vary according to the criteria employed for determining if an individual family member is affected.
Affection Status Determining criteria for the affection status of family members is another issue that must be considered. In the study of speech and language disorders, the phenotype (i.e., the characteristics of the speech and language disorder) changes as the individual matures. A preschooler with a speech and language disorder may exhibit normal speech and language at school age, adolescence, and adulthood. Direct testing of older individuals may no longer elicit the disorder. Similar to the case of proband selection, affection status may be narrowly defined as an individual's having a specific speech and language disorder, or, in broad terms, as having a verbal trait deficit encompassing dyslexia and other learning disabilities. Tallal et al. (1989)
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JOURNAL OF LEARNING DISABILITIES
TABLE 1 Studies Reporting Percentages of First-Degree Relatives Affected Study Ingram (1959) Silver (1971) Neils & Aram (1986) Lewis (1987) Tallal, Ross, & Curtiss (1989) Tomblin (1989)
Inclusion criteria 75 319 74 20 76 54
Speech/language disorders only Any learning disability Speech/language disorders only Speech/language disorders only Speech/language disorders; belowaverage school achievement Speech/language disorders only
considered parents affected if they had (a) a history of language problems; (b) a history of below-average achievement in reading, writing, or both; or (c) been held back in school. Affected individuals included anyone exhibiting language learning deficits—verbal or written. Other researchers (Lewis et al., 1989; Neils & Aram, 1986; Tomblin, 1989) considered individuals as affected only if they presented a speech or language disorder. Individuals with articulation disorders, language disorders, and stuttering were all reported as affected. When determining affection status for nuclear family members, historical reports are fairly accurate; however, when constructing pedigrees of extended families, reporting is less reliable.
Nuclear Versus Extended Families There are advantages as well as disadvantages in utilizing nuclear rather than extended families in family studies. Reporting of affection status is more likely to be accurate for firstdegree relatives than more distant relatives. Nuclear family members are often more available for direct testing. It is more likely that nuclear family members will be affected, because firstdegree relatives share more common genes than second- and third-degree relatives. However, nuclear family members also share a common family environment. Environmental conditions may exert a greater influence on the speech and language disorder. Affected distant relatives without a com-
Mothers
Fathers
Brothers
Sisters
Total
%
%
%
%
%
— 25.3 — 8.0 10.0 41.7 58.3 25.0 25.0 36.8 43.4
— 23.0 — — 15.0 — — 48.9 — 37.0 41.6 41.5 29.4
19.6
39.9
mon environment provide support for a genetic influence to the disorder. Finally, although reports of affected distant relatives may not be as accurate as reports of affected nuclear family members, incidence of the disorder will probably be underreported. Thus, a conservative estimate of the incidence of the disorder will be obtained. For the present study, probands were selected on the basis of their histories of preschool phonology disorders documented through review of clinical records. Pedigrees were examined for individuals with speech and language disorders, dyslexia, and learning disabilities, and compared to pedigrees of matched nondisabled subjects without histories of preschool speech or language difficulties. First, all pedigrees were considered together. Next, pedigrees of probands with phonology disorders alone were compared to pedigrees of probands with phonology disorders and other accompanying language disorders. Affection status of nuclear family members, as well as affection status of extended family members, was examined. Finally, pedigrees of female probands were compared to pedigrees of male probands. The specific research questions addressed were as follows: 1. Do pedigrees of individuals with histories of a preschool phonology disorder have more family members with speech/language disorders, dyslexia, learning disabilities, stuttering, and hearing impairment than pedigrees of matched nondisabled subjects? Downloaded from ldx.sagepub.com at Harvard Libraries on April 23, 2015
19.1
16.7
24.15
31.6 41.5 22.9
2. Does the incidence of the above disorders in pedigrees of individuals with a history of a preschool phonology disorder alone differ from that of individuals with histories of phonology disorders accompanied by additional language problems? 3. Does the incidence of disorders in the nuclear families differ from the incidence of the disorders in the extended families? 4. Does the incidence of disorders in pedigrees of female probands differ from the incidence of disorders in pedigrees of male probands?
Method Subjects The subjects in this study were participants in a large family study in which the goal was to define a phenotype for developmental phonology disorders in the following age groups: preschool, school-age, adolescence, and adulthood (Lewis & FreebairnTarr, in press). The probands were selected on the basis of their histories of a preschool phonology disorder. Subjects were identified through review of the past clinical records at Cleveland Hearing and Speech Center, Rainbow Babies and Childrens Hospital, and the current caseloads of speech/language pathologists in the greater Cleveland area. To be included in the study, subjects were required to meet the following criteria: 1. Documented history of a moderate to severe preschool articulation/ phonology disorder, as confirmed
VQLUME 25, NUMBER 9, NOVEMBER 1992
2.
3.
4.
5. 6.
7.
by a speech/language pathologist in their clinical records. Normal hearing acuity at follow-up, as defined by a pure tone audiometric screening test at 25 such as dB HL ISO for 500,1000, 2000, and 4000 Hz bilaterally; also, a report in their clinical record of normal hearing acuity as a preschooler. No history of chronic otitis media (fewer than six episodes prior to age 3 as reported by parent). Normal peripheral speech mechanism at follow-up, as evidenced by the Test of Oral Structures and Functions (TOSF) (Vitali, 1986), and for preschoolers, the Oral Speech Motor Control Protocol (Robbins & Klee, 1987). Standard English spoken in the home. Normal nonverbal intelligence at follow-up, defined as a standard score of 85 or above as measured by the Test of Nonverbal Intelligence (TONI) (Brown, Sherbenou, & Johnsen, 1982) or the Performance Scale of the Wechsler Preschool and Primary Scale of Intelligence (WPPSI) (Wechsler, 1963). Absence of frank neurological involvement as reported by the parent.
Probands were also grouped on the basis of their preschool language testing into two categories: those with histories of preschool articulation/ phonology disorders alone (n=30), and those with histories of language problems in addition to the preschool articulation/phonology disorder (n = 45). Probands were considered to have additional language problems if clinical records showed test scores 1 standard deviation below the mean, or lower, on standardized language testing taken as a preschooler. Twelve probands could not be categorized on the basis of their preschool records and therefore were omitted when comparisons of pedigrees of children with preschool phonology and language disorders versus phonology alone were made. Probands with disorders were group-matched to individuals with normal phonology and language ac-
cording to age, sex, and socioeconomic status. Socioeconomic status was determined through the use of the Hollingshead Four Factor Index of Social Class (Hollingshead, 1975). Nondisabled subjects were recruited from private and public schools, as well as day care centers in and around the Cleveland area. All the nondisabled subjects conformed to the same inclusion criteria as the probands with disorders, with the exception of having a history of preschool phonology disorder. Table 2 displays the subject demographics for the probands with disorders and their nondisabled matched subjects. The mean age for probands with disorders was 12.42 years (SD = 7.01 years), and for the nondisabled subjects, 12.49 years (SD = 7.48 years). Fifty-four of the probands with disorders were males, 33 were females; 49 of the nondisabled probands were male, 30 were female. The mean IQ of the probands with disorders was 100.58 (SD = 17.16), and the mean IQ of the nondisabled probands was 107.01 (SD = 14.27). A t test revealed a significant difference between the IQs of the probands with disorders and those of the nondisabled probands (t = -2.53, p= .012). Socioeconomic status did not differ significantly between groups.
Construction of Pedigrees Family history information was collected through parent interviews or, in the case of adult subjects, from the subjects themselves. Information for the nuclear families, and the maternal and paternal lines, was recorded. Histories of speech/language disorders, as well as other developmental disorders (e.g., dyslexia, learning disabilities, and hearing impairment), were obtained. All information was procured first, through completion of a questionnaire, and then verified as the pedigree was drawn by the interviewer. Family members were classified as affected or unaffected employing the following operational definitions:
589
2.
3.
4.
5.
language disorder if (a) he or she had ever been enrolled in speech/ language therapy, (b) he or she was unable to be understood until 5 years or older, or (c) he or she currently demonstrated below-ageappropriate speech/language skills as measured by the Test of Early Language Development (Hresko, Reid, & Hammill, 1981) for preschoolers, and the Clinical Evaluation of Language FundamentalsScreening (Semel, Wiig, & Secord, 1989) for school-age children, adolescents, and adults. Dyslexia: An individual was classified as dyslexic if he or she (a) was labeled as dyslexic in school or (b) had received tutoring for reading. Learning disability: An individual was considered learning disabled if he or she was ever enrolled in special classes in school. Stuttering: An individual was coded as a stutterer if he or she (a) received speech therapy for stuttering, (b) reported stuttering past age 5, or (c) presently was considered to stutter. Hearing impairment: An individual was coded as hearing impaired if he or she (a) wore a hearing aid or (b) reported a congenital hearing loss.
Pedigrees were drawn employing standard symbols (Plomin, DeFries, & McClearn, 1990): Squares represented males; circles represented females; shaded circles or squares indicated individuals with speech and language disorders; an arrow signified the proband; R represented an individual with a reading problem; ST a known stutterer; LD an individual with a learning disability; and H a subject with a hearing impairment. Information for three generations of family members was recorded. Generations are designated by roman numerals. See Figure 1 for a sample pedigree.
Data Analysis
Data analyses were conducted to an1. Speech/language disorder: An individ- swer the four research questions posed ual was coded as having a speech/ earlier. First, affected family members Downloaded from ldx.sagepub.com at Harvard Libraries on April 23, 2015
JOURNAL OF LEARNING DISABILITIES
590
TABLE 2 Demographics for Disordered and Nondisabled Probands
Age Mean SD Range Males Females IQ Mean SD SESa 1 2 3 4 5
Disorders n=87
Nondisabled n=79
12.42 7.01 4.00-27.50 yrs. 54 33
12.49 7.48 4.00-27.50 yrs. 49 30
100.58 17.16
107.01 14.27 3 7 23 33 13
4 10 25 35 13
Socioeconomic status determined by Hollingshead (1975) scale.
family members in the pedigrees of probands with disorders than in the nondisabled probands' pedigrees for speech and language disorders (x2 = 170.68, p .50); learning disabilities (x2 = 1.80, p>.20); stuttering (x 2 =2.08, p>.20); and hearing impairment (x2 = .45, p> .50). Thus, grouping pedigrees by the probands' preschool language status did not result in significant differences in the percentages of affected family members.
Nuclear Family Members Versus Extended Family Members The third research question concerned affected nuclear family members, that is, first-degree relatives (parents and siblings), versus all family members, spanning three generations. As shown in Table 5, the data set included 323 nuclear family members— 82 mothers, 81 fathers, 67 sisters, and 93 brothers. Affection status on seven
VOLUME 25, NUMBER 9, NOVEMBER 1992
a
a
0H-T—0
0
0
11
591
6a 6
III
ID
0
This study examined 87 pedigrees of individuals with histories of preschool phonology disorders. Results confirmed previous reports that speech and language disorders aggregate in families, with a higher incidence of males affected than females. Significantly more family members with dyslexia and learning disabilities, but not stuttering or hearing impairment, were found in pedigrees of individuals with phonology disorders than in pedigrees of nondisabled individuals. Probands with and without additional language problems did not differ in the incidence of affected family members. Nuclear family members demonstrated a higher incidence of disorders than when all family members were considered, with brothers of probands most often affected. Pedigrees of female probands had more affected members in their nuclear families than pedigrees of male probands.
C
ontemporary investigators in the field of dyslexia have proposed that dyslexia may be viewed as a developmental language disorder characterized primarily by deficits in phonological processing skills (Catts, 1989, 1991; Kamhi & Catts, 1989; Liberman & Shankweiler, 1985; Pennington, 1990; Scarborough, 1990). Several recent studies that have followed preschool children with speech and language disorders into early grade school have reported that children with articulation disorders alone generally do not have difficulty learning to read, whereas children with more global language deficits as preschoolers may have later reading problems (Bishop & Adams, 1990; Catts, 1991; Magnusson & Naucler, 1990; Scarborough, 1990). The relationship between early language difficulties and later reading problems is not well understood. One explanation may be a general verbal trait deficit, including phonological coding
and phoneme awareness deficiencies (Catts, 1989; Olson, Wise, Conners, & Rack, 1989; Pennington, 1990), that affects both spoken and written language. Studies of the genetic basis of dyslexia have identified deficits in phonological processing skills as the heritable component to dyslexia (Olson et al., 1989). However, the genetic basis of speech and language disorders has been less extensively examined. Phenotypes for developmental phonology disorders into early grade school and beyond are not well defined (Lewis & Freebairn-Tarr, in press). Therefore, the phenotypic boundaries between dyslexia and developmental phonology disorders have not been identified, nor has the comorbidity of dyslexia and developmental phonology disorders been examined. The purpose of the present study is to examine the pedigrees of probands with preschool phonology disorders for affected family members with speech and language JOURNAL OF LEARNING DISABILITIES VOLUME 25, NUMBER 9, NOVEMBER 1992 Downloaded from ldx.sagepub.com at Harvard Libraries on April 23, 2015 PAGES 586-597
disorders, dyslexia, and learning disabilities.
Genetics of Dyslexia Versus Genetics of Speech and Language Disorders Anecdotal reports of "congenital word blindness" were found as early as the late 1890s (Kerr, 1897; Morgan, 1896). Extensive family studies of dyslexia have documented that dyslexia aggregates in families (Decker & DeFries, 1980; DeFries, Singer, Foch, & Lewitter, 1978; Finucci, 1978; Hallgren, 1950). Twin studies and adoption studies have established a genetic component to dyslexia and estimated heritability (Bawkin, 1973; DeFries, Fulker, & LaBuda, 1987). The most extensive study to date, the Colorado Family Reading Study (Decker & DeFries, 1980; Lewitter, DeFries, & Elston, 1980), included segregation and linkage analysis (Smith, Kimberling, Pennington, & Lubs, 1983; Smith, Pennington, Kimberling, & Ing, 1990). Results support genetic heterogeneity in the transmission of dyslexia (see DeFries, 1991, or Pennington, 1990, for a review). In contrast, the familial basis of developmental speech and language disorders has not been extensively studied, although such a basis to some speech and language disorders has long been proposed (Arnold, 1961; Eutis, 1947; Ingram, 1959; Lenneberg,
587
VOLUME 25, NUMBER 9, NOVEMBER 1992
1967; Luchsinger, 1970; Seemann, 1937). As early as the late 1930s, pedigrees of children with speech and language impairment with multiple affected family members were reported. Several recent studies (Lewis, Ekelman, & Aram, 1989; Lewis, 1990a; Neils & Aram, 1986; Parlour & Broen, 1989; Tallal, Ross, & Curtiss, 1989; Tomblin, 1989) have supported a familial and perhaps even a genetic basis to developmental speech and language disorders of previously unknown etiology. These studies reported an increased incidence of speech, language, and reading disorders among family members of children with speech and language impairments. Twin studies, segregation analysis, and linkage analysis have not been conducted to date. One possible explanation for the lack of genetic studies of speech and language disorders is that the phenotype for developmental speech and language disorders changes with development. Speech and language disorders are not as readily identifiable in older children and adults as dyslexia often is, and, thus, phenotypes of older individuals have not been described. Most individuals with histories of speech and language disorders as preschoolers are considered normal speakers as adults. Pedigrees of individuals with speech and language disorders have been constructed primarily by historical report. The following section provides a review of the familial studies of developmental speech and language disorders.
Family Studies of Developmental Speech and Language Disorders Generally, family studies of developmental speech and language disorders have taken one of two forms: Either they have reported on a single, extended pedigree (Arnold, 1961; Eutis, 1947; Gopnik, 1990; Hurst, Baraitser, Auger, Graham, & Norell, 1990; Lewis, 1990a; Saleeby, Hadjian, Martinkosky, & Swift, 1978; Samples & Lane, 1985), or these studies have pooled a large sample of children with speech and lan-
guage impairments and reported the percentage of affected first-degree relatives (i.e., parents and siblings) (Lewis et al., 1989; Neils & Aram, 1986; Robinson, 1987; Tallal, Ross, & Curtiss, 1989; Tomblin, 1989). Studies reporting on a single kindred usually have selected the pedigree for study because of the large number of affected family members and the severity of the disorder. In these families the disorder often appears to be inherited in an autosomal dominant mode (i.e., the disorder is usually present in at least one parent of each affected child, and about one half of children of an affected parent are also affected), with variable expressivity and penetrance. However, the disorder that the child presents may differ in severity, as well as in type, from those of the parent, siblings, and other family members. While providing support for a familial basis for some developmental speech and language disorders, such families are not representative of the vast majority of children with speech and language disorders. Studies reporting percentages of affected first-degree relatives of a large number of children with speech and language disorders have found a significant increased incidence of speech, language, reading, and learning disorders when compared to nondisabled families (Lewis et al., 1989; Parlour & Broen, 1989; Tallal et al., 1989; Tomblin, 1989). However, the incidence of these disorders is much less than of those reported in single-pedigree studies. In addition, an autosomal dominant mode of inheritance is not upheld for all families. In fact, in some families, other affected family members are not found at all. These family studies have differed in the procedures for ascertaining the proband, criteria for determining affection status of family members, and the inclusion of extended family members versus limiting the study to nuclear family members.
Proband Selection Proband selection criteria have been both narrowly defined (so that the pro-
Downloaded from ldx.sagepub.com at Harvard Libraries on April 23, 2015
band exhibits a specific speech and language disorder, such as a phonology disorder) as well as broadly defined (so that all individuals with specific language impairments are included). Lewis et al. (1989) selected probands with severe preschool phonology disorders and reported that 25.5% of firstdegree relatives were affected. Other researchers (Neils & Aram, 1986; Robinson, 1987; Tallal et al., 1989; Tomblin, 1989) have selected probands with specific language impairment, including phonology disorders, and report from 22.9% to 48.9% of firstdegree relatives affected. Silver (1971) employed an even broader criterion for proband selection, studying families of children with learning disabilities that included speech and language disorders, and found 39.4% of first-degree relatives affected. See Table 1 for a summary of studies of first-degree relatives. Although the probands were selected by differing inclusion/exclusion criteria, these families may be representative of the same speech and language disorders. The percentage of affected first-degree relatives may vary according to the criteria employed for determining if an individual family member is affected.
Affection Status Determining criteria for the affection status of family members is another issue that must be considered. In the study of speech and language disorders, the phenotype (i.e., the characteristics of the speech and language disorder) changes as the individual matures. A preschooler with a speech and language disorder may exhibit normal speech and language at school age, adolescence, and adulthood. Direct testing of older individuals may no longer elicit the disorder. Similar to the case of proband selection, affection status may be narrowly defined as an individual's having a specific speech and language disorder, or, in broad terms, as having a verbal trait deficit encompassing dyslexia and other learning disabilities. Tallal et al. (1989)
588
JOURNAL OF LEARNING DISABILITIES
TABLE 1 Studies Reporting Percentages of First-Degree Relatives Affected Study Ingram (1959) Silver (1971) Neils & Aram (1986) Lewis (1987) Tallal, Ross, & Curtiss (1989) Tomblin (1989)
Inclusion criteria 75 319 74 20 76 54
Speech/language disorders only Any learning disability Speech/language disorders only Speech/language disorders only Speech/language disorders; belowaverage school achievement Speech/language disorders only
considered parents affected if they had (a) a history of language problems; (b) a history of below-average achievement in reading, writing, or both; or (c) been held back in school. Affected individuals included anyone exhibiting language learning deficits—verbal or written. Other researchers (Lewis et al., 1989; Neils & Aram, 1986; Tomblin, 1989) considered individuals as affected only if they presented a speech or language disorder. Individuals with articulation disorders, language disorders, and stuttering were all reported as affected. When determining affection status for nuclear family members, historical reports are fairly accurate; however, when constructing pedigrees of extended families, reporting is less reliable.
Nuclear Versus Extended Families There are advantages as well as disadvantages in utilizing nuclear rather than extended families in family studies. Reporting of affection status is more likely to be accurate for firstdegree relatives than more distant relatives. Nuclear family members are often more available for direct testing. It is more likely that nuclear family members will be affected, because firstdegree relatives share more common genes than second- and third-degree relatives. However, nuclear family members also share a common family environment. Environmental conditions may exert a greater influence on the speech and language disorder. Affected distant relatives without a com-
Mothers
Fathers
Brothers
Sisters
Total
%
%
%
%
%
— 25.3 — 8.0 10.0 41.7 58.3 25.0 25.0 36.8 43.4
— 23.0 — — 15.0 — — 48.9 — 37.0 41.6 41.5 29.4
19.6
39.9
mon environment provide support for a genetic influence to the disorder. Finally, although reports of affected distant relatives may not be as accurate as reports of affected nuclear family members, incidence of the disorder will probably be underreported. Thus, a conservative estimate of the incidence of the disorder will be obtained. For the present study, probands were selected on the basis of their histories of preschool phonology disorders documented through review of clinical records. Pedigrees were examined for individuals with speech and language disorders, dyslexia, and learning disabilities, and compared to pedigrees of matched nondisabled subjects without histories of preschool speech or language difficulties. First, all pedigrees were considered together. Next, pedigrees of probands with phonology disorders alone were compared to pedigrees of probands with phonology disorders and other accompanying language disorders. Affection status of nuclear family members, as well as affection status of extended family members, was examined. Finally, pedigrees of female probands were compared to pedigrees of male probands. The specific research questions addressed were as follows: 1. Do pedigrees of individuals with histories of a preschool phonology disorder have more family members with speech/language disorders, dyslexia, learning disabilities, stuttering, and hearing impairment than pedigrees of matched nondisabled subjects? Downloaded from ldx.sagepub.com at Harvard Libraries on April 23, 2015
19.1
16.7
24.15
31.6 41.5 22.9
2. Does the incidence of the above disorders in pedigrees of individuals with a history of a preschool phonology disorder alone differ from that of individuals with histories of phonology disorders accompanied by additional language problems? 3. Does the incidence of disorders in the nuclear families differ from the incidence of the disorders in the extended families? 4. Does the incidence of disorders in pedigrees of female probands differ from the incidence of disorders in pedigrees of male probands?
Method Subjects The subjects in this study were participants in a large family study in which the goal was to define a phenotype for developmental phonology disorders in the following age groups: preschool, school-age, adolescence, and adulthood (Lewis & FreebairnTarr, in press). The probands were selected on the basis of their histories of a preschool phonology disorder. Subjects were identified through review of the past clinical records at Cleveland Hearing and Speech Center, Rainbow Babies and Childrens Hospital, and the current caseloads of speech/language pathologists in the greater Cleveland area. To be included in the study, subjects were required to meet the following criteria: 1. Documented history of a moderate to severe preschool articulation/ phonology disorder, as confirmed
VQLUME 25, NUMBER 9, NOVEMBER 1992
2.
3.
4.
5. 6.
7.
by a speech/language pathologist in their clinical records. Normal hearing acuity at follow-up, as defined by a pure tone audiometric screening test at 25 such as dB HL ISO for 500,1000, 2000, and 4000 Hz bilaterally; also, a report in their clinical record of normal hearing acuity as a preschooler. No history of chronic otitis media (fewer than six episodes prior to age 3 as reported by parent). Normal peripheral speech mechanism at follow-up, as evidenced by the Test of Oral Structures and Functions (TOSF) (Vitali, 1986), and for preschoolers, the Oral Speech Motor Control Protocol (Robbins & Klee, 1987). Standard English spoken in the home. Normal nonverbal intelligence at follow-up, defined as a standard score of 85 or above as measured by the Test of Nonverbal Intelligence (TONI) (Brown, Sherbenou, & Johnsen, 1982) or the Performance Scale of the Wechsler Preschool and Primary Scale of Intelligence (WPPSI) (Wechsler, 1963). Absence of frank neurological involvement as reported by the parent.
Probands were also grouped on the basis of their preschool language testing into two categories: those with histories of preschool articulation/ phonology disorders alone (n=30), and those with histories of language problems in addition to the preschool articulation/phonology disorder (n = 45). Probands were considered to have additional language problems if clinical records showed test scores 1 standard deviation below the mean, or lower, on standardized language testing taken as a preschooler. Twelve probands could not be categorized on the basis of their preschool records and therefore were omitted when comparisons of pedigrees of children with preschool phonology and language disorders versus phonology alone were made. Probands with disorders were group-matched to individuals with normal phonology and language ac-
cording to age, sex, and socioeconomic status. Socioeconomic status was determined through the use of the Hollingshead Four Factor Index of Social Class (Hollingshead, 1975). Nondisabled subjects were recruited from private and public schools, as well as day care centers in and around the Cleveland area. All the nondisabled subjects conformed to the same inclusion criteria as the probands with disorders, with the exception of having a history of preschool phonology disorder. Table 2 displays the subject demographics for the probands with disorders and their nondisabled matched subjects. The mean age for probands with disorders was 12.42 years (SD = 7.01 years), and for the nondisabled subjects, 12.49 years (SD = 7.48 years). Fifty-four of the probands with disorders were males, 33 were females; 49 of the nondisabled probands were male, 30 were female. The mean IQ of the probands with disorders was 100.58 (SD = 17.16), and the mean IQ of the nondisabled probands was 107.01 (SD = 14.27). A t test revealed a significant difference between the IQs of the probands with disorders and those of the nondisabled probands (t = -2.53, p= .012). Socioeconomic status did not differ significantly between groups.
Construction of Pedigrees Family history information was collected through parent interviews or, in the case of adult subjects, from the subjects themselves. Information for the nuclear families, and the maternal and paternal lines, was recorded. Histories of speech/language disorders, as well as other developmental disorders (e.g., dyslexia, learning disabilities, and hearing impairment), were obtained. All information was procured first, through completion of a questionnaire, and then verified as the pedigree was drawn by the interviewer. Family members were classified as affected or unaffected employing the following operational definitions:
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4.
5.
language disorder if (a) he or she had ever been enrolled in speech/ language therapy, (b) he or she was unable to be understood until 5 years or older, or (c) he or she currently demonstrated below-ageappropriate speech/language skills as measured by the Test of Early Language Development (Hresko, Reid, & Hammill, 1981) for preschoolers, and the Clinical Evaluation of Language FundamentalsScreening (Semel, Wiig, & Secord, 1989) for school-age children, adolescents, and adults. Dyslexia: An individual was classified as dyslexic if he or she (a) was labeled as dyslexic in school or (b) had received tutoring for reading. Learning disability: An individual was considered learning disabled if he or she was ever enrolled in special classes in school. Stuttering: An individual was coded as a stutterer if he or she (a) received speech therapy for stuttering, (b) reported stuttering past age 5, or (c) presently was considered to stutter. Hearing impairment: An individual was coded as hearing impaired if he or she (a) wore a hearing aid or (b) reported a congenital hearing loss.
Pedigrees were drawn employing standard symbols (Plomin, DeFries, & McClearn, 1990): Squares represented males; circles represented females; shaded circles or squares indicated individuals with speech and language disorders; an arrow signified the proband; R represented an individual with a reading problem; ST a known stutterer; LD an individual with a learning disability; and H a subject with a hearing impairment. Information for three generations of family members was recorded. Generations are designated by roman numerals. See Figure 1 for a sample pedigree.
Data Analysis
Data analyses were conducted to an1. Speech/language disorder: An individ- swer the four research questions posed ual was coded as having a speech/ earlier. First, affected family members Downloaded from ldx.sagepub.com at Harvard Libraries on April 23, 2015
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TABLE 2 Demographics for Disordered and Nondisabled Probands
Age Mean SD Range Males Females IQ Mean SD SESa 1 2 3 4 5
Disorders n=87
Nondisabled n=79
12.42 7.01 4.00-27.50 yrs. 54 33
12.49 7.48 4.00-27.50 yrs. 49 30
100.58 17.16
107.01 14.27 3 7 23 33 13
4 10 25 35 13
Socioeconomic status determined by Hollingshead (1975) scale.
family members in the pedigrees of probands with disorders than in the nondisabled probands' pedigrees for speech and language disorders (x2 = 170.68, p .50); learning disabilities (x2 = 1.80, p>.20); stuttering (x 2 =2.08, p>.20); and hearing impairment (x2 = .45, p> .50). Thus, grouping pedigrees by the probands' preschool language status did not result in significant differences in the percentages of affected family members.
Nuclear Family Members Versus Extended Family Members The third research question concerned affected nuclear family members, that is, first-degree relatives (parents and siblings), versus all family members, spanning three generations. As shown in Table 5, the data set included 323 nuclear family members— 82 mothers, 81 fathers, 67 sisters, and 93 brothers. Affection status on seven
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