PENDAHULUAN
Tekanan intraokular adalah tekanan yang dihasilkan oleh isi bola
mata terhadap dinding bola mata. Tekanan ini dipengaruhi oleh
lapisan dinding bola mata dan volume bola mata yang terdiri dari :
aquos humor, korpus vitreus, pembuluh darah intraokular dan isinya.
Tekanan intraokular diharapkan berada dalam angka yang normal di
dalam dinamika cairan aquos humor, karena aquos humor sendiri
mempunyai fungsi sebagai media refraksi, pemberi nutrisi dan
mempengaruhi tekanan hidrostatik untuk stabilitas bola mata. Banyak
faktor yang mempengaruhi tekanan intraokular, antara lain : umur,
jenis kelamin, ras, genetik, waktu dan gangguan refraksi.Tekanan
intraokuler normal pada manusia dari data penelitian Becker dengan
menggunakan tonometer Shiotz pada 909 populasi adalah 16,1 mmHg
dengan SD 2,8 mmHg dan dari penelitian Leydecker dkk (1958) pada
10.000 populasi mendapatkan nilai tekanan intraokuler 15,8 mmHg
dengan SD 2,6 mmHg serta dari penelitian Goldmann pada 400 populasi
dengan menggunakan tonometer aplanasi mendapatkan nilai tekanan
intraokuler rata-rata 15,4 mmHg dengan SD 2,5 mmHg.Nilai tekanan
intraokuler pada setiap individu dipengaruhi oleh beberapa faktor
antara lain: usia, jenis kelamin, musim, variasi diurnal, ras,
kelainan refraksi, latihan, obat-obat anastesi, alkohol . Pada
beberapa penelitian dijumpai korelasi antara tekanan intraokuler
dengan usia, dimana dengan bertambahnya usia cenderung terjadi
peningkatan tekanan intraokuler, yang mungkin disebabkan oleh
faktor-faktor kardiovaskular, demikian juga yang berhubungan dengan
jenis kelamin dimana dari penelitian Armalys (1965) dengan
menggunakan tonometer applanasi mendapatkan tekanan intraokuler
pada wanita berusia lebih dari 40 tahun lebih tinggi dari pria yang
mungkin disebabkan oleh faktor-faktor hormonal (menstruasi).
TINJAUAN PUSTAKA
Tekanan IntraokularTekanan intraokular di tentukan oleh
kecepatan pembentukan aquos humor dan tahanan terhadap aliran
keluarnya dari mata. Tekanan intraokular diatur oleh dinamika
cairan aquos humor termasuk diantaranya : produksi cairan aquos,
aliran cairan dan tekanan vena episklera. Fungsi dari aquos humor
adalah sebagai media refraksi, pemberi nutrisi dan juga
mempengaruhi tekanan hiodrostatik untuk stabilitas bola
mata.Tekanan bola mata pada manusia normal yang diukur dengan
pemeriksaan Tonometer Aplanasi rata-rata berkisar 15,4 2,5 mmHg
pada posisi duduk dan pemeriksaan Tonometer Schiotz rata-rata
berkisar 16,1 2,8 mmHg pada posisi berbaring. Distribusi tekanan
intraokular rata-rata dari populasi umum berkisar antara 10-20
mmHg.Produksi aquos humor melalui dua mekanisme yaitu aktif dan
pasif. Aktif ( 80%) dari produksi aquos, dimana aquos humor
disekresi oleh epiel prosesus siliaris yang tidak berpigmen melalui
metabolisme yang aktif dan tergantung pada jumlah sistim enzim;
serta mekanisme pasif ( 20%) melalui proses ultrafiltrasi plasma
kapiler, kemampuan plasma melewati sawar epitel dan aliran komponen
plasma yang disebabkan adanya perbedaan tekanan osmotik dan tingkat
tekanan intraokular.Tingkat produksi aquos homor rata-rata adalah
2,0 3,0 ml/menit atau 1% dari volume aquos humor per menit dan
angkanya menjadi 2,4 0,6 ml/menit jika dilakukan pengukuran dengan
alat fluorofotometri. Keadaan produksi aquos humor ini bervariasi
sesuai dengan variai diurnal dan berkurang selama tidur. Seperti
pada aliran aquos humor, produksi aquos humor juga berkurang dengan
bertambahnya usia. Pada proses trauma atau peradanganserta
pemberian obat-obatan yang digunakan dalam anestesi umum, obat
penurun tekanan darah ; dapat menurunkan produksi aquos humor.
Penyakit oklusi karotis juga dapat menurunkan produksi aquos
humor.Aliran aquos humor dari bilik mata belakang melalui pupil
menuju bilik mata depan kemudian mengalir melalui dua jalur
trabekula dan kanal Schlemm, kanalis intra -sklera, vena episklera
untuk selanjutnya masuk kedalam sirkulasi, aliran ini meliputi 90 %
dari seluruh aliran aquos humor. Sedangkan 10 % aliran aquos humor
ini melalui jalur uveo-sklera yang melewati badan siliar menuju
ruangan suprakoroidal dan dialirkan oleh sirkulasi vena pada badan
siliar, koroid dan sklera dan sebagian kecil aliran-aliran aquos
humor ini juga melalui iris. Dilaporkan bahwa rata-rata kecepatan
aliran aquos humor berkisar dari 0,22 0,28 ml/menit/mmHg. Kecepatan
aliran ini berkurang sesuai dengan usia dan dipengaruhi oleh bedah,
trauma, obat -obatan serta faktor endokrin.
FISIOLOGI TEKANAN BOLA MATA
Didalam bola mata terdapat cairan intraokuler yang menjaga
tekanan yang cukup pada bola mata sehingga bola mata tetap
mengembang. Cairan intra okular terdiri dari aqueous humor dan
vitreus humor atau vitreus body. Aqueous humor terletak di depan
dan disamping lensa mata, terjadi aliran cairan yang bebas pada
aqueous humor. Vitreus body terletak diantara bagian posterior
lensa dan retina. Zat dan cairan berpindah dengan difusi pada
vitreus humor, dan terjadi sedikit aliran cairan. Aqueous humor
dibentuk dan diserap secara terus menerus. Keseimbangan pembentukan
dan penyerapan aqueous humor mengatur volume total dan tekanan
cairan intraokular.Aqueous humor terbentuk rata-rata 2 3 mikroliter
permenit. Diproduksi secara keseluruhan oleh prosesus ciliary pada
badan ciliary. Setelah terbentuk, aqueous humor akan mengalir
diantara ligamen lensa menuju ruang anterior melalui pupil. Di
ruang anterior, aqueous humor akan mengalir menuju ke sudut antara
kornea dan iris kemudian ke trabekular meshwork dan akhirnya menuju
canal schlemm sebelum dialirkan ke vena-vena ekstraokuler.Tekanan
bola mata berkisar antara 12 20 mmHg dengan rata-rata 15 mmHg.
Tekanan bola mata tetap konstan pada mata normal dengan lebih
kurang 2 mmHg dari batas rata-rata 15 mmHg. Tekanan bola mata
dipengaruhi terutama oleh tahanan aliran aqueous humor dari ruang
anterior mata ke canal schlemm. Aqueous humor yang masuk ke
meshwork dari trabekular harus merembes pelan-pelan selama
perjalanan karena pada trabekular meshwork hanya terdapat saluran
yang sangat kecil dengan diameter 23 mikrometer. Tingkat aliran
aqueous humor meningkat seiring peningkatan tekanan bola mata.
Dalam tekanan rata-rata 15 mmHg, rata-rata kecepatan aliran menuju
canal schlemm adalah 2,5 mickoliter permenit setara dengan produksi
aqueous humor pada badan ciliary. Hal ini menyebabkan tekanan dalam
bola mata tetap pada level 15 mmHg.
Sumber : Guyton & Hall Textbook Of Medical Physiology
pp.575-576Intraocular pressureAssessment of intraocular pressure by
palpation is useful onlywhen the intraocular pressure is
considerably raised, as in acuteclosed angle glaucoma. The eye
should be gently palpatedbetween two fingers and compared with the
other eye or withthe examiners eye. The eye with acute glaucoma
feels hard.Consider acute angle closure in any person over the age
of 50with a red eye.
Normal Intraocular PressureMeasurement of the intraocular
pressure in alarge number of normal subjects reveals anormal
distribution extending from pressuresof 1012 mmHg to 2528mmHg. The
pattern ofdistribution fits a Gaussian curve, so that themajority
of subjects have a pressure of about16mmHg. For clinical purposes,
it is necessaryto set an arbitrary upper limit of normal. Byand
large, the eye can stand low pressuresremarkably well, but when the
pressure isabnormally high, the circulation of bloodthrough the eye
becomes jeopardised andserious damage can ensue. For clinical
purposes,an upper level of 21 mmHg is oftenaccepted. Above this
level, suspicions are raisedand further investigations
undertaken.
Maintenance ofIntraocular PressureIf the eye is to function as
an effective opticalinstrument, it is clear that the intraocular
pressuremust be maintained at a constant level. Atthe same time, an
active circulation of fluidthrough the globe is essential if the
structureswithin it are to receive adequate nourishment.The cornea
and sclera form a tough fibrous andunyielding envelope and within
this an evenpressure is maintained by a balance between
theproduction and drainage of aqueous fluid.Aqueous is produced by
the ciliary epitheliumby active secretion and
ultrafiltration.Acontinuous flow is maintained through thepupil,
where it reaches the angle of the anteriorchamber.On reaching the
angle of the anteriorchamber, aqueous passes through a grill
knownas the trabecular meshwork and then reaches acircular canal
embedded in the sclera known asSchlemms canal. This canal runs as a
ringaround the limbus (corneoscleral junction) andfrom it, minute
channels radiate outwardsthrough the sclera to reach the episcleral
circulation.These channels are known as aqueousveins and they
transmit clear aqueous to theepiscleral veins, which lie in the
connectivetissue underlying the conjunctiva. In actual fact,the
proof of the route of drainage of aqueouscan be verified by any
medical student itsimply entails examining the white of the eye
around the cornea with extreme care, using thehigh power of the
slit-lamp microscope. After atime, one can sometimes detect that
some of thedeeper veins convey parallel halves of blood andaqueous
in the region beyond the junction ofaqueous and episcleral vein.The
relative parts played by ciliary epitheliumand trabecular meshwork
in maintaining whatis a remarkably constant intraocular
pressurethroughout life are not fully understood. Itwould appear
that the production of aqueous isan active secretion, whereas the
drainage ismore passive, although changing the tone of theciliary
muscle can alter the rate of drainage. Innormal subjects, the
intraocular pressure doesnot differ in the two eyes by more than
about3 mmHg. Wider differences can lead one tosuspect early
glaucoma, especially if there is afamily history of the disease.
The normalintraocular pressure undergoes a diurnal variation,being
highest in the early morning andgradually falling during the first
half of the day.This diurnal change could become exaggerated as the
first sign of glaucoma.Measurement ofIntraocular PressureThe
gold-standard method of intraocular pressuremeasurement is Goldmann
applanationtonometry. The Goldmann tonometer is suppliedas an
accessory to the slit-lamp microscope.The principle of applanation
is as follows:when two balloons are pushed together so thatthe
interface is a flat surface, the pressure withinthe two balloons
must be equal. By the sameargument, when a fixed flat surface is
pressedagainst a spherical surface, such as the cornea,at the point
at which the spherical surface isexactly flattened, the intraocular
pressure isequal to the pressure being applied. The applanationhead
is a small Perspex rod with aflattened end,which is fitted to a
moveable arm.The tension applied to the moveable arm can bemeasured
directly from a dial on the side of theinstrument. The observer
looks through the rodusing the microscope of the slit-lamp, and
thepoint at which exact flattening occurs can thusbe gauged. For
applanation tonometry, the measurement of the intraocular pressure
is sucha basic requirement in any eye clinic, attemptshave been
made to introduce even more rapidand efficient devices. Perhaps the
most ingeniousto date is the tonometer, which measuresthe
indentation of the cornea in response to apuff of air by a
photoelectric method. This airpufftonometer is less accurate than
applanation,but it is useful for screening, althoughabnormal
results should be confirmed byGoldmann tonometry.patient is seated
at the slit-lamp and not lyingdown but it is still necessary to
instill a drop oflocal anaesthetic beforehand. Because the
measurement of the intraocular pressure is sucha basic requirement
in any eye clinic, attemptshave been made to introduce even more
rapidand efficient devices. Perhaps the most ingeniousto date is
the tonometer, which measuresthe indentation of the cornea in
response to apuff of air by a photoelectric method. This
airpufftonometer is less accurate than applanation,but it is useful
for screening, althoughabnormal results should be confirmed
byGoldmann tonometry.
Measurement of Intraocular PressureWith the patients eyes
closed, the examiner places his or her hands on thepatients head
and palpates the eye through the upper eyelid with both
indexfingers (Fig. 1.15). The test is repeated on the contralateral
eye for comparison. Ref e-book 3
Palpation (Fig. 1.15, p. 15): Comparative palpation of both
eyeballs is a preliminaryexamination that can detect increased
intraocular pressure.! If the examiner can indent the eyeball,
which fluctuates under palpation,pressure is less than 20mm Hg.! An
eyeball that is not resilient but rock hard is a sign of about
6070mmHg of pressure (acute angle closure glaucoma).Schitz
indentation tonometry (Figs. 10.4a and b): This examination
measuresthe degree to which the cornea can be indented in the
supine patient. Thelower the intraocular pressure, the deeper the
tonometer pin sinks and thegreater distance the needle
moves.Indentation tonometry often provides inexact results. For
example therigidity of the sclera is reduced inmyopic eyes, which
will cause the tonometerpin to sink more deeply for that reason
alone. Because of this, indentationtonometry has been largely
supplanted by applanation tonometry.Applanation tonometry: This
method is the most common method of measuringintraocular pressure.
It permits the examiner to obtain a measurementon a sitting patient
within a few seconds (Goldmanns method, see Fig. 10.5ac) or on a
supine patient (Draegers method). A flat tonometer tip has
adiameter of 3.06mmfor applanation of the cornea over a
corresponding area(7.35mm2). This method eliminates the rigidity of
the sclera as a source oferror (see also tonometric
self-examination).Intraocular pressure of 22mm Hg is regarded as
suspicious. Caution:Infection is possible in the presence of
conjunctivitis.Pneumatic non-contact tonometry: The electronic
tonometer directs a3 ms blast of air against the cornea. The
tonometer records the deflection ofthe cornea and calculates the
intraocular pressure on the basis of this deformation.Schitz
indentation tonometry. a The tonometer is placed onthe anesthetized
cornea. The examiner retractsboth eyelids and the patient focuseson
his or her thumb with the other eye.Advantages:! Does not require
the use of a topical anesthetic.! Non-contact measurement
eliminates risk of infection (may be used tomeasure intraocular
pressure in the presence of conjunctivitis).Disadvantages:!
Calibration is difficult.! Precise measurements are possible only
within low to middle range pressures.! Cannot be used in the
presence of corneal scarring.! Examination is unpleasant for the
patient.! Air flow is loud.! The instrument is more expensive to
purchase than an applanationtonometer.
Goldmann applanation tonometry. Slit-lamp measurementof
intraocular pressure:After application of anesthetizingeyedrops
containingfluorescein, thetonometer tip is placed onthe
cornea.Measuring the twenty-four-hour pressure curve (Fig. 10.6):
This examinationis performed to analyze fluctuations of the
pressure level over a 24-hourperiod in patients with suspected
glaucoma.A single measurement may not be representative. Only a
24-hour curveprovides reliable information about the pressure
level.Intraocular pressure fluctuates in a rhythmic pattern. The
highest valuesfrequently occur at night or in the early morning
hours. In normal patients,these fluctuations in intraocular
pressure rarely exceed 46mm Hg.Pressure is measured on the ward at
6:00 a.m., noon, 6:00 p.m.,9:00 p.m., and midnight. Outpatient
24-hour pressure curves without nighttimeand early morning
measurements are less reliable.
Tonometric self-examination. The patient places the tonometer on
his or her forehead and uses thefixation light to align it in the
proper position. The head of the tonometer then
automaticallypresses against the cornea, measures intraocular
pressure, and retracts.Pressure is indicated in a digital
display.
