arteriosclerosis

26 Pages·2005·5.09 MB·English

THE EYE IN EXPERIMENTAL
ARTERIOSCLEROSIS
BY JustinAlM. )onegan, M.D.
ARTERIOSCLEROSIS
in its various forms at present ranks first as a
cauise of (leatlh and disease in the United States. The octular mani-
festations of this disease not only constitute grave problems in
tlhemiiselves, buit also afford opportunities for observing directly
portions of the affected vital cardiovascular system. In the past
arteriosclerosis wvas reogarded as a degenerative disease-the in-
evitable wvearino' ouit of the arteries as a resuilt ofage. Accuimtulating
evidence tends to refuite this concept and to imply instead that
arteriosclerosis is a metabolic disease in wvrhichi disorders of the
blood lipids and lipoproteins are possibly of prime importance.
In disctussingy the senescenit clhanges in the retinal vascular tree,
Friedenwvald (i) lhas stated:
the atrophic and degenerative chaniges occur not oinly in the agedl but
also in presenile lersons. In fact, their occtcrrence without senility is so
striking that ophthalmologists have been forced to set up separate dis-
ease categories for these so-calledl juvenile cases. Histologically, these
atrophic andl degenerative conditions are associate(l with fibrosis and
hyalinization of the, choroiclal capillaries. The quiestion is still un-
settled whliether these hiistologic chlianges are specific evidence of
senescence of the aifflected tisstucs, inldepeident of systemic disease;
wlhether they represent a special vascular (lisease l)rocess, (listinct from
the four major categories p)reviously enumerated [i.e. (i) senescence, (2)
atherosclerosis, (3) the hypertensive arteriolosclerotic process andl (4)
diabetic vasctular (lisease], or whether they are the form that the
atherosclerotic process takes wlhein it involves vessels of capillary di-
iensions. The last possibility has certainly not beeni ruled out an(d is,
in fact, suibject to experimental approach, for a quite pure form of
atlherosclerosis can be pro(lduced in rabbits experimentally.
There lias been a tremilendouis amouint of researchi on the prob-
lenii and the acctiumtilated literature on the subbjecot iss t ex-
476 J46stin M. D)onegan
tensive, yet the eye hlas received btut scant attention in thle experi-
mental stuidies. If, tlherefore, it wNTere possible to produce in the
laboratory animal lesions, both general and octular, similar to those
of human arteriosclerosis, a valuiable tool for sttudying thle etiology,
patlhogenesis, and tlherapy of this condition wvouild have been
acquiired.
REVIEW OF THE I,ITERATURE ON EXPERINMENTAL ARTERIOSCLEROSIS
The earliest experiments wvere uindertaken witlhouit a clear idea
of exactly whliat lesions it wvas desired to produce. It lhas been pos-
sible to appraise correctly the experimental results in relation to
arterial changes clharacteristic of lhuman arteriosclerosis only since
the wvork of Jores (2; 190.3), Marclhand (3.<; 1904) and Asclhoff (4;
1908). These wvorkers determnined the different forims whliich are to
be distinguiislhed: atlherosclerosis, medial calcifications, etc. Athero-
sclerosis is probably the most significant of these and attempts to
produice these clhanges lhave been the goal of m-any investigators.
SPONTANEOUS ARTERIOSCLEROSIS IN ANINIALS
Before one can properly analyze any experimental restults it is
necessary to know the incidence and types of lesions wvlhich occur
spontaneouisly in the variotus animals tused in researcl.
The chicken has a hiighi incidence of spontaneouis arteriosclerosis
wvhiclh is first observed at five to six montlhs of age. After the first
year 45 percent of roosters and 40 percent of lhens are affected. The
lesions consist of small atlheromatous plaques in the thoracic and
abdominal aorta. They never uilcerate and there are no accomnpany-
ing medial changes, according to Datuber (5). Becauise of this lligh
incidence the chiicken is only suitable if used before six months of
age, or if employed for experiments on prevention of the disease.
Others of the class Aves also lhave a highl incidence of spontaneouis
arteriosclerosis, but not as gTeat as the clhicken. Rats and guinea
pigs are rarely affected and slhowv only small fibrotus intimal plaqules
of the aorta. Rabbits are only occasionally involved, showing a
fibrous intimal thickening devoid of fat, localized in the tlioracic
or abdominal aorta. Dogs after the age of five years sometimes lhave
thiickening of thie intimua, buit no fat deposition or imiedial lesions.
The Eye in Experimental Arteriosclerosis 477
Monkeys will occasionally have some small atheromatous plaques
at the archof theaorta (6).
MECHANICAL INJURIES IN ARTERIES
Many of the earliest attempts to produce arteriosclerosis in
experimental animals were by means of localized trauma to vessels,
i.e., ligating, pulling, pinching, or cauterizing. These forms of
localized trauma incite a nonspecific repair response to the in-
jury (7) which has nosimilarity to human arteriosclerosis orathero-
sclerosis. However, this nonspecific repair response may occur
when there are repeated minor injuries to vessels, as might occur
in severe hypertension or the tugging of branchingvessels on their
orifices. Also, Petroff (8) and Glasunow (g) have shown that any
lesion of arteries, however slight, will promptly result in increased
permeability of the arterial wall.
INJURIES TO NERVES
Arterial changes reported after severing or irritating certain
nerves by Lewaschew (io) and Frankel ( 1i) are ofan inflammatory
character and quite unlike those observed in atherosclerosis.
ADRENALIN SCLEROSIS
Josue (12; 1903) produced medial calcification in aortas of
rabbits by repeated small injections of adrenalin. Since then there
have been many reports confirming this work. The morphological
picture of these changes never varies in its essential features. The
lesions consist first of necrosis of the inner medial fibers and then
calcium depositions in the necrotic areas. More and more calcium
is deposited until thin calcium plates form. Ossification or small
dissecting aneurysms may develop at the site of the lesions. All
these changes take place exclusively in the media of the rabbit's
aorta while the intima does not exhibit any primary changes,
except that some intimal thickening may develop when small
doses of adrenalin are given over a prolonged period of time.
There is no associated inflammatory reaction or fat deposition.
The guinea pig, rat, and dog are also susceptible to these lesions,
but to a lesser degree than the rabbit. Lesions similar to those
478 jiustin M. Donegan
caused by adrenalin have been produiced by highi (losages of para-
thyroid lhormone, ergosterol, nicotine, ergot, and ephedrinle. These
clhanges differ froiim those in man becatuse only the aorta is affected,
thiere are no significant intimal lesions, and there is no lipoid depo-
sition in the areas of iimedial calcification.
"The concltusion seeImis juistified," as Anitsclikowv (13) states,
"tthat nonie of these stubstances eimiployed lor the prodtiction of
experimlenital atlherosclerosis is in itself able to prodtuce any arterial
changes corresponding to those observed in humllan atlherosclerosis,
eitlher by imieans of toxi( effects or by raisinia the blood pressure."
CHOLESTIEROI ARTERIOSCLEROSIS
In io98 Ignatowski (14), experiimieutiino wNith variouis types of
diets for rabbits, observed a higli incidence of atherosclerosis in
those animlals on a diet wlhich is not nattural to theimi, stuch as meat,
mnilk, and eggs. He attributed these c(langes to the injutrious effects
of the hiiglh protein content buit this wvas never confirimied. WVorking,
independently of eachi otlher, Anitsclhkowsr and Clhalatowv (15) and
almost simutiltaneotusly NVacker and Htieck (16) reported in 1913.
that puLre cholesterol dissolved in vegetable oil administered by
miiotuth over a considerable period of tiime cotuld produice lesions
in the intimia of the aorta of the rabbit very similar to those of
arteriosclerosis of imani. The oeneral resemblance between these
lesions and the lesions typical of hutmilan arteriosclerosis wvas im-
mediately recognized and arioused great interest. Shortly nutimerouis
other investigators began to tuse this imetlhod and in the interven-
ing years literally reamns hiave been written on experimental
cholesterol atherosclerosis.
Since these lesions are so easy to prodtuce in rabbits the effects of
clholesterol feedings have been stuidied in a variety ofother animals.
Howvever, attempts to prodtuce arterial lesions by this methlod in
other animals have met witlh only limited suLccess. Dauber and
Katz (17) hlave been stuccessfuil in prodtucing arteriosclerosis in
chickens wvith cholesterol feedings, btut it vill be recalled that
there is a high incidence of the spontaneouis type ofarteriosclerosis
after the age of six months in these animals and it is probable
that these investigators are only accelerating the development of a
natturally occturring process. Rats and mice are practically resistant
The Eye in Experitmental Arteriosclerosis 479
to the developmient of any of these lesions (i8). Bailey (19; 1915)
rieported the productioni of experimiiental cholesterol arterio-
sclerosis in the aorta of guiinea pigs, l)ut these lesions vere only
vlil)le microscopically and conisisted of "simall patclhes of fatty
infiltration in the intimla and tipper Inedia. The clhaiacteristic
proliferation and stubsequient degeneration seen in the rabbit wvere
entirely lacking." Dogs are very resistant to the development of
lesions. Dragstedt (20) fed a smiall grouip of dogs 5 Gm. of clholes-
terol in stunflower oil daily for five years and was not able to pro-
dtuce any lesions. However, Steiner and Kendall (21) wvere stuccess-
fuil recently in produicinlg generalized arteriosclerosis in dcogs by
givinig thiouracil in addition to clholesterol. One Gmn. of thiouiracil
and io GIm1. of clholesterol daily were given for one year to produlce
these lesionis. Interestingly enouglh, twvo of these dogs lhad athiero-
sclerotic lesions in tile cerebral arteries, the first time thiat stuclh
lesions lhave been reported in any species fed clholesterol. Very
little wvork lhas been done with monkeys and the resuilts have been
negative todate.
Mlethod. A variety of clholesterol-riclh diets have been adminis-
tered lfor the prodtuction of experimiiental clholesterol arterio-
sclerosis. Egg yolks lhave been the miost commnonly uised, btut brain
tisstue, liver, and hydrotus wvool fat have been enmployed wvitlh stuc-
cess (Dtuff, i8). These substances wvere mixed wvitlh the ordinary
food or wvere tlinned ouit by miiixtuire wvitli somne liqtuid and ad-
ministered by mneans of a stomach tube. A variety of mediutms for
the soltution or stuspension of cholesterol have been tried wvith
success. Sunflover-seed oil and 5 percent sodiutm oleate lhave been
emiiployed suLccessfully. None of thiese solvents alone produiced
clhanges in the arteries. The clholesterol content of the preparations
ranged froM 2 to 7.5 percent. At present the tusuial metlhod is to
give a ma)bbit .5 to 1 Gim. of cholesterol in oil per (lay. Solutionis of
cholesterol in oil can be dissolved in etlher, potured over the food
and the etlher allowved to evaporate, tlhus producing an even dis-
tribution of clholesterol over all the food particles. The animnals
eat the diet avidly and gain weighlt. WVitlh the uise of the various
solvents the absorption of clholesterol is increased, the blood
clholesterol levels are hiiglher, and the lesions develop imore rapidly
and extensively than wvlhen pure cholesterol is uised. Puire clholes-
48o Justin M. Donegan
terol is effective but about five times as much is required to obtain
comparable results (22). Parenteral administration of cholesterol
has in the past consistently met with failure. Since cholesterol is
insoluble in any aqueous medium it has to be given in the form
of a colloid suspension. Recently Bevans (23) has been able to
produce typical atherosclerotic lesions in the aorta by giving
repeated injections of a colloidal suspension of cholesterol in
sodium stearate. These lesions were indistinguishable from those
produced bycholesterol feeding.
Arterial lesions of experimental cholesterol arteriosclerosis in
rabbits. Numerous authors (24-27) have described the gross appear-
ance and distribution of the arterial lesions which follow choles-
terol feeding in the rabbit. These lesions make their appearance
first in the aorta but only after the liver, spleen, bone marrow, and
suprarenal glands have become loaded with lipoids. The earliest
lesions occur in the arch and upper thoracic aorta, especially
around the orifices of various branches of the aorta. After longer
periods of feeding, experimental lesions develop in the lower
thoracic and abdominal aorta but with decreasing frequency
caudally. Eventually the atheromatous lesions may involve most of
the upper portion of the aorta and also develop in the innominate,
subclavian, common iliac, and femoral arteries. The remainder
of the large arteries, including the pulmonary and carotid, may
be involved. The coronary arteries of the heart are usually at-
tacked but the renal arteries are not frequently mentioned. These
lesions have never been reported in the cerebral or retinal arteries
(13). Grossly the lesions are first observed as pinhead sized, light-
yellow elevations of the intima. With the continuation of cho-
lesterol feeding the individual lesions enlarge and coalesce, form-
ing irregular plaques. This thickening may be sufficient to cause
asignificant narrowingoftheaortic lumen.
These lesions, wherever they occur in the arterial tree, seem
to be essentially of the same nature. On microscopic examination
the early lesions first present a small accumulation of lipid ma-
terial in the subendothelial space on top of the internal elastic
lamella. Originally there is no cellular reaction to this lipid ac-
cumulation, but in a few days macrophages appear around these
deposits. These macrophages ingest the fat and assume a character-
The Eye in ExperimtneWtal Arteriosclerosis 48i
istic foamy appearance, from wvhiclh the names "foam cell," "foamy
macrophage or lipoplhage" are derived. Duff (18) states "Tlhe
phagocytic foam cells lhave attracted special attention because of
their exact resemblance in form and apparently also in function
to the large fat holding cells found in the intima of arteriosclerotic
lhuman arteries." At about the same time there occurs a prolifera-
tion of a fewv fibroblasts in the endotlhelium. The exact source of
these is unknown. The amnount of lipid accumulation increases,
gradually breaks outwtrard tlhrouighl the internal elastic lamella,
and is deposited in the uipper layers of the media. Some necrosis
of the foaimiy macroplhages occurs and stubsequiently some deposi-
tion of smnall calciumll grantules. As the fibroblasts continue to pro
liferate from the edge of the lesion they gradtually cover over
the lipiddeposit. Tlhis fibrotus tisstuereaction mayvaryconsiderably
from one lesion or animnal to anotlher. Necrosis of this fibrous
tissue or tulceration of the atlheromiia lhas not been reported. Micro-
scopic lesions have been observed as early as 25 to 3o days after
clolesterol feeding lhas been started, buit the tusual initial appear-
ance is after abouLt six veeks. The first gross lesions becomie ap-
parent in abouit twvo and a lhalflmontlhs.
As long as the clholesterol feeding is continuied tile lesions will
continue to grow, btut wvrhen clholesterol is discontinued the
atheromas gradually decrease in size. The lipid mnaterial slowly dis-
appears and the end restult is a simall fibrous intimal plaque.
Several otlher plhenomena accompany this experimental pro-
duction of arteriosclerosis in rabbits. The normal blood cho-
lesterol of the rabbit averages abouit 45 to 75 mg. percent. After
tlhree to four weeks of clholesterol feeding the blood levels rise to
200 to 250 mg. percent. As the feedina is continuied, the blood
levels contintue rising until they reachi their maximum height at
tlhree to four montlhs, at whr}iclh time they wvill usually be 8oo to
1,200 Mg. percent or even as liglh as 2,500 mg. percent. Botlh the
free and esterified clholesterol are elevated, nmost of the rise being
in the esterified fraction. Also there is a Imarked increase in the
neutral fats, phosplholipids and total lipids of the blood, even
thouglh only pure clholesterol is given (28). WVith this marked
hypercholesterolemia the seruim is somletimnes milky in appearance.
After the lheight of this elevation of blood clholesterol is reaclhed
489 justin M. Donegan
in thiree to fotir miiontlhs, it tends to decrease slighltly anid remains
at a fairly stable level as long as the animal continues to receive
cholesterol. XVhen the clholesterol is stopped, the 1)lood levels
decrease slowvly until finally they restumiie a norimial valtue at the end
of three to fouir montlhs.
Accoimipanying this imiarked hyperclholesterolemiia there is a
large aimotuint of fat deposition tlhrotuohlot the lbody. The entire
retictuloendotlhelial systeimi of the rabbit is literally sattirated wvith
fat. Marked fat depositioln occturs in the spleei, lyIm1phi n1odes, bone
imiarrowv, anid adrenal cortex. Mlost of the fat depositioni in the liv,er
and adrenal cortex is in thie forimi of chlolesterol esters. On tlle pavs
subcutaneous xantlhomas freqtuently develop. These are essentially
large accumuittlations of lipid nmaterial containiing imiostly choles-
terol.
The deposition of lipids also goes on at the saime tiime in the
intracelltular portions of organs suchl as the tympanitic imieiilbrane
(Berbericlh, 29), in tenidons (Ktusnetzowvsky, 3o) and in the sclera,
the cornea, and the ciliary body (V'erse, [3.1]; Rolhrsclhnieider, [32];
Kolen, .])
If simall cdoses of clholesterol are given the blood clholesterol
level wvill rise to only 200 to 300 m1lg. perceint. There wvill be soimie
fat deposition in the reticuiloendotlhelial systemn and typical atlhero-
iiatous lesions wvill develop, buit onily after a conisiderally longer
period of tiimie (34).
Age appears to play soimie role in the prodtmctioni ol tlhese lesions.
In all the reports in the literatture youing aniimials, lotur to five
imontlhs old, wvere used. Taylor (35) lhas attempted to produce these
lesions in rabbits betwveen twvo and a lhalf and fouir years of age,
but even after being on a diet lligh in cholesterol for six monthls
and developing markedly elevated levels of clholesterol in the
blood these animals lhave developed only miniimial degrees of
atherosclerosis. Numlerouis other factors lhave been introdtuced
in these sttudies in an attemipt to iiiodify or accelerate thle develop-
ment of arteriosclerosis in rabbits fed clholester-ol. Tlhyroid lhor-
mone given witlh the clholesterol will delay the onset and decrease
the extent of the lesions. It wvill also imiarkedly inhiibit the rise of
the blood cholesterol levels. Thyroidectoiniy has an opposite effect.
The lesions wvill appear sooner and be more extensive, despite the
The Eye in Experimnenital Arteriosclerosis 483
fact that it cauises only a slight rise in the blood clholesterol levels.
Iodides, especially KI, ivill lhave the same effect as tlhyroid in the
intact animal, but lhave no effect in the tlhyroidectomized rabbit
(i8). Various lipotropic ag-ents, especially clholine, have been used
in an attemlpt to modify the effects of the clholesterol feedings,
btut opinion is divided as to the efficacy of these produicts.
Thiere is considerable confusion in the literatture in regard to
the effect of experimental hypertensioni on these lesions. It ap-
pears that experimental lhypertension lhas Ino effect as long as
kidney fuinction is normal, but wivitlh impaired renal ftunction
hiypertension does accelerate the development of experimental
arteriosclerosis (36). Cholesterol feeding per se lhas no effect on
the blood pressure, according to Duff (i8).
If the feeding of cholesterol-riclh diets is combined wvitlh somle
otlher procedure whilich is known to injuire thie arteries, the develop-
ment of experimental clholesterol arteriosclerosis is greatly ac-
celerated (37). This factor of traumina does not hlave to be as severe
as pinclhing, clamping, or cauiterizing a vessel, but may be repeated,
small trauimatic incidents suiclh as the blood impinging against the
wvall of the vessel wvitlh eaclh cardiac systole. This repeated type of
trauima to the orifices of the different branclhes of the aorta is
believed to be the reason for the predisposition of arteriosclerotic
lesions at these sites. Moreton (38) recently reported on the clhanges
in the physical state of the blood seruLm wvitlh fat ingestion. By
variouis physical-clhemical means lhe wvas able to demonstrate the
transient presence of ntumerous large lipoid particles after fat in-
gestion. It is conceivable that the repeated bomibarding of the
intimiial endotlhelitumn by these particles may have a deleterious
effect on the vessel wvall overa period of tiIme.
In summary, it wvould appear fromn a review of the literatuire
on the subject that lhyperclholesterolemnia is necessary for the de-
velopment of these atlherosclerotic lesions, btut that it is not neces-
sarily the sole factor.
RELATIONSHIP OF EXPERINIENTAL, ARrERIOSCLEROSIS
TO HUMIAN ARTERIOSCLEROSIS
Muich of this wvork on clholesterol-induiced arteriosclerosis in
experimental animlals lhas been excellent researclh, buit the quies-
484 Juistin M. Donegan
tion arises as to whethier these studies can be applied to the prob-
lem in man. First it muist be recognized that there are general
differences betveen clholesterol-induced arteriosclerosis in rabbits
and arteriosclerosis in muan. The main lesions in the rabbit are in
the ascending arclh and tlhoracic aorta, wxvhile in man the principal
lesions are located in the abdominal aorta. The pulmonary artery
is involved mnuclh imore frequently in the rabbit than in man. The
blood clholesterol levels are elevated to a degree that is rarely
seen in man. There is an extensive amouint of fat deposition in
the retictuloendothelial system in the rabbit that is very rarely
seen in man. The rabbit lesions never ulcerate. Also of significance
is the fact that the cerebral and retinal arteries have never been
reported to be involved in the rabbit, lblut are freqtuently involved
in man, and finally, the rabbits' arteriosclerosis is prodtuced by
adding a substance to the diet that hie never encouinters naturally.
These differences, howvever, may be explained to a considerable
extent. In regard to the differences in distribution of the aortic
lesions, differences in postture may be quite significant. Wilens
(39) showed that wvlhen cholesterol-fed rabbits were kept in an up-
right position for long periods of time, ntumerouis lesions wvould
occur in the abdomninal aorta. Altlouighi the usual methods of
feeding clholesterol prodtuce elevations of the blood cholesterol
levels in the rabbit mutclh greater than those in man, it vill be
remembered that it lhas been demonstrated that it is possible to
develop arteriosclerosis in rabbits wvitlh only a moderate hyper-
cholesterolemia and sliglht visceral fat deposition by giving small
doses of clholesterol over a prolonged period. The reason that the
atheromatous plaqutes on the aorta of the rabbit fail to ulcerate
may be the fact that they never exist for as long a time as they do
in man.
In disctussing this problemn Hass (40) hias recently stated
it is generally accepte(l that arteriosclerotic lesions similar to those often
found in human arteriosclerosis can be p)roduced in experimental ani-
mals by different methodls. As a rule, these methods lead to the pro-
duction of either a lesion which is most conspicuous in the media of
the vessel or a lesion which is principally in the intima. The complete
pictire of the arteriosclerosis commonly fotind in the media and intima
of age(l human vessels has not been redtuplicated. This shoul(d incite no
adverse criticism of the exlerilmental findings, for no disease, bacterial