1975
145
NEW ZEALAND VETERINARY JOURNAL
BILATERAL ASYMMETRY OF EQUINE LARYNGEAL MUSCLES T.
QUINLAN,
B. E.
GOULDEN, AND
INTRODUCTION
(Gunn, 1972, 1973; Duncan and Griffiths, 1973; Duncan et al., 1974) supports Cole's (1946) original findings that many apparently normal horses have changes in the histological structure. of intrinsic laryngeal muscles, such as would be expected fl10m a progressive, unilateral denervation atrophy. In this study,. equine larynges have been collected and examined to determine the presence of contralateral weight differences in those muscles.
RECENT WORK
MATERIALS AND METHODS
A. S.
DAVIES*
(2) Adductor muscles supplied by the re.current laryngeal nerve (lateral crico~arytenoid, transverse arytenoid, vocalis and ventricularis). (3) Crioothyroid. The mean weight, standard deviation and coefficient of variation were calculated for each muscle group. RESULTS
The wet and dry weights of muscles from both sides of the larynges are shown in Tables 1 and 2. The dry weights of the abductors, adductors supplied by the recurrent laryngeal nerve, and the cricothyroid muscles WeJ"e plotted on graphs to illustrate the relationship be.. tween the left and right sides (FigS. 1, 2, and 3). Statistical analysis on both wet and dry' weights demonstrated a highly significant difference in weight betwe.en sides of abductors and adductors supplied by the recurrent laryngeal nerve (P < 0.001). In both cases the muscles on the left side were: significantly lighter than those on the right.. In contrast there was no sig-
Twenty-four equine larynges Were obtained from animals of various ages, breeds, and from both sexes. All were randomly selected from post-mortem and knackery sources. An additional larynx was obtained from an animal in which left laryngeal dysfunction (roaring) had been diagnosed. This was examined for the. purpose of comparison. The larynges were prepared by strip.. ping them of their attachments to other structures, including the extrinsic laryngeal musculature. They were then placed 1: WEIGHTS OF INTRINSIC LARYNin 10% formalin for at least 7 days. The TABLE GEAL MUSCLES OF RANDOMLY SELECTED intrinsic laryngeal muscles were: dissected HORSES individually from their cartHaginous Freeze-dried attachments and weighed. Since it is Weight (g) Wet W ei~ht (g) possible that the use of wet, fixed tissue Right Left Left Right Muscle introduces an error in muscle weight measurement, the muscles were sub- Dorsal crico· x 8.17 2.12 2.36 8.70 0.67 SD 2.16 2.36 0.57 sequently freeze~dried and weighed again. arytenoid 28 CV 26 27 27 The significance of the: weight difference 3.04 0.97 0.78 3.79 between paired right and left muscles in Lateral crico- x 0.24 arytenoid SD 0.98 0.25 0.91 each horse was tested using Student's t 25 CV 32 24 32 test within the following groups: 2.38 0.67 0.60 x 2.72 Transverse ( 1) Abductor muscles arytenoid)
(dorsal
crico-
*T. Quinlan, B.V.Sc., D.V.C.S., and B. E. Goulden, B.V.sc., Ph.D., M.R.C.V.S., M.A.C.V.Sc., Department of Veterinary Clinical Sciences, Massey Uni· versity, Palmerston North. A. S. Davies, B.V.Sc., Ph.D., Department of Physiology and Anatomy, Massey Unive,rsity, Palmerston North.
arytenoid
SD CV
Vocalis
x
Ventricularis
x
Cricothyroid
x
SD CV SD CV
SD CV
0.81 34 5.10 1.71 34 2.23 0.60 27 5.76 1.74 30
0.73 27 5.73 1.45 25 2.65 0.51 19 5.74 1.72 30
0.19 32 1.29 0.36 28 0.54 0.11 20 1.62 0.58 36
0.19 28 1.46 0.40 27 0.62 0.13 21 1.58 0.50 32
146
NEW ZEALAND VETERINARY JOURNAL
VOL.
23
lEFT
lEFT
5
RIGHT
FIG. 1: Dry weights, abductor muscles
crico-arytenoid muscle. R - Clinical roarer
dorsal
FIG. 2: Dry weights, total adductor muscles supplied
by recurrent laryngeal nerves. R - Clinical roarer
nificant difference in the weilghts of the be bilaterally symmetrical. This has been left and right cricothyroid muscles. Since 'shown to be the case with respect to the coefficient of variation of muscle weight weights of human laryngeal muscles are similar for wet and dry muscles the (Bowden and Scheuer, 1960). In the experimental error was similar for both horse, --however, many authors (Cole, methods. Variation was generally greater 1946; Gunn, 1972, 1973; Duncan and in the weight of muscles from the left Griffiths, 1973; Duncanet aI., 1974) have reported differences in the histological side. structure of intrinsic laryngeal muscles DISCUSSION Assuming that the muscles on the left between each side of the larynx. They and right sides of the larynx share similar embryological development, and that lEFT the nature and amount of work per-
formed by the laryngeal muscles is equal
on both sides, the intrinsic laryngeal musculature of the normal animal should
TABLE 2: WEIGHTS OF INTRINSIC LARYNGEAL MUSCLES FROM A CLINICAL CASE OF RECURRENT LARYNGEAL NERVE PARALYSIS
Muscle
Dorsal cricoarytenoid Lateral cricoarytenoid Transverse arytenoid Vocalis Ventricularis Cricothyroid
Wet Weight (g) Right Left
Freeze-dried Weight (g) Right Left
2.51
8.71
0.92
2.18
0.69
3.78
0.21
0.91
1.29 5.63 1.39 8.58
3.31 9.31 4.86 8.86
0.34 1.23 0.33 2.33
0.74 2.22 1.08 2.41
./.'
/. ~-/
.
0
5
GRAMS
FIG. 3: Dry weights, cricothyroid muscles.
RIGHT
1975
147
NEW ZEALAND VETERINARY JOURNAL
have shown that muscle fibres in the adduotors and abductors on the left side/ frequently show atrophic changes, whilst suoh ohanges are only rarely observed in most muscles on the right side of the larynx (Duncan and Griffiths, 1973). In the opinion of many authors (Cole, 1946; Gunn, 1-972, 1973; Dunoan and Griffiths, 1973; Duncan et al., 1974) the atrophy follows degenerative ohanges in the left recurrent laryngeal nerve. This nerve unilaterally supplies motor fibres to all the intrinsic laryngeal muscles with the exception of the cricothyroid (Sisson and Grossman, 1953). In the equine larynges examined the: marked weight difference between the muscles supplied by the recurrent laryngeal nerve contrasted with the bilaterally symmetrical weights of the cricothyroid muscles. The diffe;rence in the weight of affected muscles could result therefore from degenerative changes in the recurrent laryngeal nerve. The only previous author to report on the weight of equine intrinsic laryngeal muscles was Gunn (1972). He found nOi significant difference: between the dorsal crico.. arytenoid muscles on each side of the larynx in 12 animals. Moreover, in one case a markedly histologically atrophied left muscle was found to be; heavier than its partner. These findings are at variance with those of the present study in which, however, histological examinations were not made. The weight differences found in the clinically diagnosed case of recurrent laryngeal nerve paralysis affected similar muscles but the wasting was more extensive. It is possible, there£ore, that the other animals in which gross weight difference existed between right and left muscles are the subclinical "roarers" re'" ferred to by Duncan et ai. (1974). In the opinion of Cole (1946), horses required a 50% atrophy of the left dorsal cricoarytenoid muscle to show symptoms of laryngeal hemiplegia at exercise. When discussing this condition, Gunn (1973) suggested that it was congenital in character and progressive in development. The source of the animals studied
in the present series tended to select a disproportionately high percentage of aged animals. If Gunn's theory is correct, this may have accentuated the weight differences re:corded. SUMMARY
The intrinsic laryngeal muscles from the larynges of 24 clinically normal horses and the larynx from a clinical case of left recurrent laryngeal neuropathy were weighe:d.. In the former animals the abductor and adductor muscles supplied by the recurrent laryngeal nerve were significantly lighter on the left side. In the clinical "roarer" gre.ater differences were observed. In neither case, however, was there a significant difference in weight of the right and left cricothyroid muscle. It is postulated that the left muscles were lighter because of atrophy resulting from recurrent laryngeal nerve dysfunction. ACKNOWLEDGEMENTS
The authors are indebted to K. Oulaghan, for obtaining the larynx from the clinical "roarer"; to Miss J. E. Chapman for freeze~drying and weighing the dried laryngeal muscles; and to R. Fletcher for help and advice on the statistical ana~ lyses. REFERENCES Bowden, R. E. H.; Scheuer, J. L. (1960): Weights of abductor and adductor muscles of the human larynx f. Laryng. Otol., 74: 971·80 Cole, C. R. (1946): Changes in the equine larynx associated with laryngeal hemiplegia. Am. ,. vet. Res., 8: 69·77. Duncan, I. D.; Griffiths, I. R. (1973): Pathological changes in equine laryngeal muscles and nerves. Proc. 19th Ann. ConI. Am. Ass. Equine Pract.: 97-113. Duncan, I. D.; Griffiths, I. R.; McQueen, A.; Backer, G. O. (1974): The pathology of equine laryn" geal hemiplegia. Acta neuropath. (Ber!.), 27: 337-48. Gunn, H. M. (1972): Histochemical observations on laryngeal skeletal muscle fibres in "normal horses. Equine vet. J., 4: 144-8. - - - (1973): Further observations on laryngeal skeletal muscle in the horse. Equine vet. J" 5: 77·84. Sisson, S.; Grossman, J. D. (1953): The Anatomy of the Domestic Animals, 4th ed. Saunders. London and Philadelphia.
(Received for publication October 16, 1974)
145
NEW ZEALAND VETERINARY JOURNAL
BILATERAL ASYMMETRY OF EQUINE LARYNGEAL MUSCLES T.
QUINLAN,
B. E.
GOULDEN, AND
INTRODUCTION
(Gunn, 1972, 1973; Duncan and Griffiths, 1973; Duncan et al., 1974) supports Cole's (1946) original findings that many apparently normal horses have changes in the histological structure. of intrinsic laryngeal muscles, such as would be expected fl10m a progressive, unilateral denervation atrophy. In this study,. equine larynges have been collected and examined to determine the presence of contralateral weight differences in those muscles.
RECENT WORK
MATERIALS AND METHODS
A. S.
DAVIES*
(2) Adductor muscles supplied by the re.current laryngeal nerve (lateral crico~arytenoid, transverse arytenoid, vocalis and ventricularis). (3) Crioothyroid. The mean weight, standard deviation and coefficient of variation were calculated for each muscle group. RESULTS
The wet and dry weights of muscles from both sides of the larynges are shown in Tables 1 and 2. The dry weights of the abductors, adductors supplied by the recurrent laryngeal nerve, and the cricothyroid muscles WeJ"e plotted on graphs to illustrate the relationship be.. tween the left and right sides (FigS. 1, 2, and 3). Statistical analysis on both wet and dry' weights demonstrated a highly significant difference in weight betwe.en sides of abductors and adductors supplied by the recurrent laryngeal nerve (P < 0.001). In both cases the muscles on the left side were: significantly lighter than those on the right.. In contrast there was no sig-
Twenty-four equine larynges Were obtained from animals of various ages, breeds, and from both sexes. All were randomly selected from post-mortem and knackery sources. An additional larynx was obtained from an animal in which left laryngeal dysfunction (roaring) had been diagnosed. This was examined for the. purpose of comparison. The larynges were prepared by strip.. ping them of their attachments to other structures, including the extrinsic laryngeal musculature. They were then placed 1: WEIGHTS OF INTRINSIC LARYNin 10% formalin for at least 7 days. The TABLE GEAL MUSCLES OF RANDOMLY SELECTED intrinsic laryngeal muscles were: dissected HORSES individually from their cartHaginous Freeze-dried attachments and weighed. Since it is Weight (g) Wet W ei~ht (g) possible that the use of wet, fixed tissue Right Left Left Right Muscle introduces an error in muscle weight measurement, the muscles were sub- Dorsal crico· x 8.17 2.12 2.36 8.70 0.67 SD 2.16 2.36 0.57 sequently freeze~dried and weighed again. arytenoid 28 CV 26 27 27 The significance of the: weight difference 3.04 0.97 0.78 3.79 between paired right and left muscles in Lateral crico- x 0.24 arytenoid SD 0.98 0.25 0.91 each horse was tested using Student's t 25 CV 32 24 32 test within the following groups: 2.38 0.67 0.60 x 2.72 Transverse ( 1) Abductor muscles arytenoid)
(dorsal
crico-
*T. Quinlan, B.V.Sc., D.V.C.S., and B. E. Goulden, B.V.sc., Ph.D., M.R.C.V.S., M.A.C.V.Sc., Department of Veterinary Clinical Sciences, Massey Uni· versity, Palmerston North. A. S. Davies, B.V.Sc., Ph.D., Department of Physiology and Anatomy, Massey Unive,rsity, Palmerston North.
arytenoid
SD CV
Vocalis
x
Ventricularis
x
Cricothyroid
x
SD CV SD CV
SD CV
0.81 34 5.10 1.71 34 2.23 0.60 27 5.76 1.74 30
0.73 27 5.73 1.45 25 2.65 0.51 19 5.74 1.72 30
0.19 32 1.29 0.36 28 0.54 0.11 20 1.62 0.58 36
0.19 28 1.46 0.40 27 0.62 0.13 21 1.58 0.50 32
146
NEW ZEALAND VETERINARY JOURNAL
VOL.
23
lEFT
lEFT
5
RIGHT
FIG. 1: Dry weights, abductor muscles
crico-arytenoid muscle. R - Clinical roarer
dorsal
FIG. 2: Dry weights, total adductor muscles supplied
by recurrent laryngeal nerves. R - Clinical roarer
nificant difference in the weilghts of the be bilaterally symmetrical. This has been left and right cricothyroid muscles. Since 'shown to be the case with respect to the coefficient of variation of muscle weight weights of human laryngeal muscles are similar for wet and dry muscles the (Bowden and Scheuer, 1960). In the experimental error was similar for both horse, --however, many authors (Cole, methods. Variation was generally greater 1946; Gunn, 1972, 1973; Duncan and in the weight of muscles from the left Griffiths, 1973; Duncanet aI., 1974) have reported differences in the histological side. structure of intrinsic laryngeal muscles DISCUSSION Assuming that the muscles on the left between each side of the larynx. They and right sides of the larynx share similar embryological development, and that lEFT the nature and amount of work per-
formed by the laryngeal muscles is equal
on both sides, the intrinsic laryngeal musculature of the normal animal should
TABLE 2: WEIGHTS OF INTRINSIC LARYNGEAL MUSCLES FROM A CLINICAL CASE OF RECURRENT LARYNGEAL NERVE PARALYSIS
Muscle
Dorsal cricoarytenoid Lateral cricoarytenoid Transverse arytenoid Vocalis Ventricularis Cricothyroid
Wet Weight (g) Right Left
Freeze-dried Weight (g) Right Left
2.51
8.71
0.92
2.18
0.69
3.78
0.21
0.91
1.29 5.63 1.39 8.58
3.31 9.31 4.86 8.86
0.34 1.23 0.33 2.33
0.74 2.22 1.08 2.41
./.'
/. ~-/
.
0
5
GRAMS
FIG. 3: Dry weights, cricothyroid muscles.
RIGHT
1975
147
NEW ZEALAND VETERINARY JOURNAL
have shown that muscle fibres in the adduotors and abductors on the left side/ frequently show atrophic changes, whilst suoh ohanges are only rarely observed in most muscles on the right side of the larynx (Duncan and Griffiths, 1973). In the opinion of many authors (Cole, 1946; Gunn, 1-972, 1973; Dunoan and Griffiths, 1973; Duncan et al., 1974) the atrophy follows degenerative ohanges in the left recurrent laryngeal nerve. This nerve unilaterally supplies motor fibres to all the intrinsic laryngeal muscles with the exception of the cricothyroid (Sisson and Grossman, 1953). In the equine larynges examined the: marked weight difference between the muscles supplied by the recurrent laryngeal nerve contrasted with the bilaterally symmetrical weights of the cricothyroid muscles. The diffe;rence in the weight of affected muscles could result therefore from degenerative changes in the recurrent laryngeal nerve. The only previous author to report on the weight of equine intrinsic laryngeal muscles was Gunn (1972). He found nOi significant difference: between the dorsal crico.. arytenoid muscles on each side of the larynx in 12 animals. Moreover, in one case a markedly histologically atrophied left muscle was found to be; heavier than its partner. These findings are at variance with those of the present study in which, however, histological examinations were not made. The weight differences found in the clinically diagnosed case of recurrent laryngeal nerve paralysis affected similar muscles but the wasting was more extensive. It is possible, there£ore, that the other animals in which gross weight difference existed between right and left muscles are the subclinical "roarers" re'" ferred to by Duncan et ai. (1974). In the opinion of Cole (1946), horses required a 50% atrophy of the left dorsal cricoarytenoid muscle to show symptoms of laryngeal hemiplegia at exercise. When discussing this condition, Gunn (1973) suggested that it was congenital in character and progressive in development. The source of the animals studied
in the present series tended to select a disproportionately high percentage of aged animals. If Gunn's theory is correct, this may have accentuated the weight differences re:corded. SUMMARY
The intrinsic laryngeal muscles from the larynges of 24 clinically normal horses and the larynx from a clinical case of left recurrent laryngeal neuropathy were weighe:d.. In the former animals the abductor and adductor muscles supplied by the recurrent laryngeal nerve were significantly lighter on the left side. In the clinical "roarer" gre.ater differences were observed. In neither case, however, was there a significant difference in weight of the right and left cricothyroid muscle. It is postulated that the left muscles were lighter because of atrophy resulting from recurrent laryngeal nerve dysfunction. ACKNOWLEDGEMENTS
The authors are indebted to K. Oulaghan, for obtaining the larynx from the clinical "roarer"; to Miss J. E. Chapman for freeze~drying and weighing the dried laryngeal muscles; and to R. Fletcher for help and advice on the statistical ana~ lyses. REFERENCES Bowden, R. E. H.; Scheuer, J. L. (1960): Weights of abductor and adductor muscles of the human larynx f. Laryng. Otol., 74: 971·80 Cole, C. R. (1946): Changes in the equine larynx associated with laryngeal hemiplegia. Am. ,. vet. Res., 8: 69·77. Duncan, I. D.; Griffiths, I. R. (1973): Pathological changes in equine laryngeal muscles and nerves. Proc. 19th Ann. ConI. Am. Ass. Equine Pract.: 97-113. Duncan, I. D.; Griffiths, I. R.; McQueen, A.; Backer, G. O. (1974): The pathology of equine laryn" geal hemiplegia. Acta neuropath. (Ber!.), 27: 337-48. Gunn, H. M. (1972): Histochemical observations on laryngeal skeletal muscle fibres in "normal horses. Equine vet. J., 4: 144-8. - - - (1973): Further observations on laryngeal skeletal muscle in the horse. Equine vet. J" 5: 77·84. Sisson, S.; Grossman, J. D. (1953): The Anatomy of the Domestic Animals, 4th ed. Saunders. London and Philadelphia.
(Received for publication October 16, 1974)
