Orbit Anatomy

BONY ORBIT
The bony orbits are quadrangular truncated pyramids
situated between the anterior cranial fossa above and
the maxillary sinuses below (Fig. 16.1). Each orbit is
about 40 mm in height, width and depth and is formed
by portions of seven bones : (1) frontal, (2) maxilla,
(3) zygomatic, (4) sphenoid, (5) palatine, (6) ethmoid
and (7) lacrimal. It has four walls (medial, lateral,
superior and inferior), base and an apex.
The medial walls of two orbits are parallel to each
other and, being thinnest, are frequently fractured
during injuries as well as during orbitotomy
operations and, it also accounts for ethmoiditis being
the commonest cause of orbital cellulitis.
The inferior orbital wall (floor) is triangular in shape
and being quite thin is commonly involved in blowout
fractures and is easily invaded by tumours of the
maxillary antrum.
The lateral wall of the orbit is triangular in shape. It
covers only posterior half of the eyeball. Therefore,
palpation of the retrobulbar tumours is easier from
this side. Because of its advantageous anatomical
position, a surgical approach to the orbit by lateral
orbitotomy is popular.
The roof is triangular in shape and is formed mainly
by the orbital plate of frontal bone.
Base of the orbit is the anterior open end of the
orbit. It is bounded by thick orbital margins.
The orbital apex (Fig. 16.2). It is the posterior end of
orbit. Here the four orbital walls converge. It has two
orifices, the optic canal which transmits optic nerve
and ophthalmic artery and the superior orbital fissure
which transmits a number of nerves, arteries and veins
(Fig. 13.2).
ORBITAL FASCIA
It is a thin connective tissue membrane lining various
intraorbital structures. Though, it is one continuous
tissue, but for the descriptive convenience it has been
divided into fascia bulbi, muscular sheaths,
intermuscular septa, membranous expansions of the
extraocular muscles and ligament of Lockwood.
Fascia bulbi (Tenon’s capsule) envelops the globe
from the margins of cornea to the optic nerve. Its
lower part is thickened to form a sling or hammock on
which the globe rests; this is called ‘suspensory
ligament of Lockwood’.
CONTENTS OF THE ORBIT
The volume of each orbit is about 30 cc.
Approximately one-fifth of it is occupied by the
eyeball. Other contents of the orbit include: part of
optic nerve, extraocular muscles, lacrimal gland,
lacrimal sac, ophthalmic artery and its branches, third,
fourth and sixth cranial nerves and ophthalmic and
maxillary divisions of the fifth cranial nerve,
sympathetic nerve, orbital fat and fascia.
SURGICAL SPACES IN THE ORBIT
These are of importance as most orbital pathologies
tend to remain in the space in which they are formed.
Therefore, their knowledge helps the surgeon in
choosing the most direct surgical approach. Each orbit
is divisible into four surgical spaces (Fig. 16.3).
1. The subperiosteal space. This is a potential space
between the bone and the periorbita (periosteum).
2. The peripheral space. It is bounded peripherally
by the periorbita and internally by the four recti
with thin intermuscular septa. Tumours present
here produce eccentric proptosis and can usually
be palpated. For peribulbar anaesthesia, injection
is made in this space.
3. The central space. It is also called muscular cone
or retrobulbar space. It is bounded anteriorly by
the Tenon’s capsule lining back of the eyeball
and peripherally by the four recti muscles and
their intermuscular septa in the anterior part. In
the posterior part, it becomes continuous with
the peripheral space. Tumours lying here usually
produce axial proptosis. Retrobulbar injections
are made in this space.
4. Tenon’s space. It is a potential space around
the eyeball between the sclera and Tenon’s
capsule.

PROPTOSIS

It is defined as forward displacement of the eyeball
beyond the orbital margins. Though the word
exophthalmos (out eye) is synonymous with it; but
somehow it has become customary to use the term
exophthalmos for the displacement associated with
thyroid disease.
CLASSIFICATION
Proptosis can be divided into following clinical
groups:
Unilateral proptosis
Bilateral proptosis
Acute proptosis
Intermittent proptosis
Pulsating proptosis
ETIOLOGY
Important causes of proptosis in each clinical group
are listed here:
A. Causes of unilateral proptosis include:
1. Congenital conditions. These include: dermoid
cyst, congenital cystic eyeball, and orbital
teratoma.
2. Traumatic lesions. These are: orbital haemorrhage,
retained intraorbital foreign body, traumatic
aneurysm and emphysema of the orbit.
3. Inflammatory lesions. Acute inflammations are
orbital cellulitis, abscess, thrombophlebitis,
panophthalmitis, and cavernous sinus thrombosis
(proptosis is initially unilateral but ultimately
becomes bilateral). Chronic inflammatory lesions
include: pseudotumours, tuberculoma, gumma and
sarcoidosis.
4. Circulatory disturbances and vascular lesions.
These are: angioneurotic oedema, orbital varix
and aneurysms.
5. Cysts of orbit. These include: haematic cyst,
implantation cyst and parasitic cyst (hydatid cyst
and cysticercus cellulosae).
6. Tumours of the orbit. These can be primary,
secondary or metastatic.
7. Mucoceles of paranasal sinuses, especially frontal
(most common), ethmoidal and maxillary sinus are
common causes of unilateral proptosis.
B. Causes of bilateral proptosis include:
1. Developmental anomalies of the skull:
craniofacial dysostosis e.g., oxycephaly (tower
skull).
2. Osteopathies: Osteitis deformans, rickets and
acromegaly.
3. Inflammatory conditions: Mikulicz’s syndrome
and late stage of cavernous sinus thrombosis.
4. Endocrinal exophthalmos: It may be thyrotoxic
or thyrotropic.
5. Tumours: These include symmetrical lymphoma
or lymphosarcoma, secondaries from neuroblastoma,
nephroblastoma, Ewing’s sarcoma and
leukaemic infiltration.
6. Systemic diseases: Histiocytosis, systemic
amyloidosis, xanthomatosis and Wegener’s
granulomatosis.
C. Causes of acute proptosis. It develops with
extreme rapidity (sudden onset). Its common causes
are: orbital emphysema fracture of the medial orbital
wall, orbital haemorrhage and rupture of ethmoidal
mucocele.
D. Cause of intermittent proptosis. This type of
proptosis appears and disappears of its own. Its
common causes are: orbital varix, periodic orbital
oedema, recurrent orbital haemorrhage and highly
vascular tumours.
E. Causes of pulsating proptosis. It is caused by
pulsating vascular lesions such as caroticocavernous
fistula and saccular aneurysm of
ophthalmic artery. Pulsating proptosis also occurs
due to transmitted cerebral pulsations in conditions
associated with deficient orbital roof. These include
congenital meningocele or meningoencephalocele,
neurofibromatosis and traumatic or operative hiatus.
Investigation of a case of proptosis
I. Clinical evaluation
(A) History. It should include: age of onset, nature of
onset, duration, progression, chronology of orbital
signs and symptoms and associated symptoms.
(B) Local examination. It should be carried out as
follows:
1. Inspection. (i) To differentiate proptosis from
pseudoproptosis which is seen in patients with
buphthalmos, axial high myopia, retraction of
upper lid and enophthalmos of the opposite eye;
(ii) to ascertain whether the proptosis is unilateral
or bilateral; (iii) to note the shape of the skull;
and (iv) to observe whether proptosis is axial or
eccentric.
2. Palpation. It should be carried out for retrodisplacement
of globe to know compressibility of
the tumour, for orbital thrill, for any swelling
around the eyeball, regional lymph nodes and
orbital rim.
3. Auscultation. It is primarily of value in searching
for abnormal vascular communications that
generate a bruit, such as caroticocavernous fistula.
4. Transillumination. It is helpful in evaluating
anterior orbital lesions.
5. Visual acuity. Orbital lesions may reduce visual
acuity by three mechanisms: refractive changes
due to pressure on back of the eyeball, optic
nerve compression and exposure keratopathy.
6. Pupil reactions. The presence of Marcus Gunn
pupil is suggestive of optic nerve compression.
7. Fundoscopy. It may reveal venous engorgement,
haemorrhage, papilloedema and optic atrophy.
Choroidal folds and opticociliary shunts may be
seen in patients with meningiomas.
8. Ocular motility. It is restricted in thyroid
ophthalmopathy, extensive tumour growths and
neurological deficit.
9. Exophthalmometry. It measures protrusion of the
apex of cornea from the outer orbital margin (with
the eyes looking straight ahead). Normal values
vary between 10 and 21 mm and are symmetrical
in both eyes. A difference of more than 2 mm
between the two eyes is considered significant.
The simplest instrument to measure proptosis is
Luedde’s exophthalmometer (Fig. 16.4). However,
the Hertel’s exophthalmometer (Fig. 16.5) is the
most commonly used instrument. Its advantage
is that it measures the two eyes simultaneously.
(C) Systemic examination. A thorough examination
should be conducted to rule out systemic causes of
proptosis such as thyrotoxicosis, histiocytosis, and
primary tumours elsewhere in the body (secondaries
in orbits). Otorhinolaryngological examination is
necessary when the paranasal sinus or a
nasopharyngeal mass apears to be a possible
etiological factor.
II. Laboratory investigations
These should include:
Thyroid function tests,
Haematological studies (TLC, DLC, ESR, VDRL
test),
Casoni’s test (to rule out hydatid cyst),
Stool examination for cysts and ova, and
Urine analysis for Bence Jones proteins for
multiple myeloma.
III. Imaging Technique
(A) Non-invasive techniques
1. Plain X-rays. It is still the most frequently used
initial radiological examination. Commonly required
exposures are in the Caldwell view, the Water’s
view, a lateral view and the Rhese view (for optic
foramina). X-ray signs of orbital diseases include
enlargement of orbital cavity, enlargement of optic
foramina, calcification and hyperostosis.
2. Computed tomography scanning. It is very useful
for determining the location and size of an orbital
mass. A combination of axial (CAT) and coronal
(CCT) cuts enables a three-dimensional
visualisation. CT scan is capable of visualising
various structures like globe, extraocular muscles
and optic nerves. Further, this technique is also
useful in examining areas adjacent to the orbits
such as orbital walls, cranial cavity, paranasal
sinuses and nasal cavity. Its main disadvantage
is the inability to distinguish between
pathologically soft tissue masses which are
radiologically isodense.
3. Ultrasonography. It is a non-radiational noninvasive,
completely safe and extremely valuable
initial scanning procedure for orbital lesions. In
the diagnosis of orbital lesions, it is superior to
CT scanning in actual tissue diagnosis and can
usually differentiate between solid, cystic,
infiltrative and spongy masses.
4. Magnetic resonance imaging (MRI). It is a major
advance in the imaging techniques. It is very
sensitive for detecting differences between normal
and abnormal tissues and has excellent image
resolution. The technique produces tomographic
images which are superficially very similar to CT
scan but rely on entirely different physical
principles for their production.
(B) Invasive procedures
1. Orbital venography. It is required in patients
who are clinically suspected of having orbital
varix. It confirms the diagnosis and also outlines
the size and extent of the anomaly which facilitates
proper surgical planning.
2. Carotid angiography. It is now performed only
in cases of pulsating exophthalmos and in those
associated with a bruit or thrill. The principal role
of carotid angiography in orbital diagnosis is to
identify the location and extent of ophthalmic
artery aneurysms, and the pathologic circulation
associated with various arteriovenous
communications along the ophthalmic artery–
cavernous sinus complex. It is also useful to
identify the feeding vessels prior to undertaking
surgery in patients with vascular orbital tumours.
3. Radioisotope studies. These are, nowadays,
sparingly employed. Radioisotope arteriography
has been found useful in proptosis of vascular
lesions. In this technique, sodium pertechnetate
Tc 99 m is injected intravenously and its flow is
visualised by a gamma scintillation camera.
IV. Histopathological studies
The exact diagnosis of many orbital lesions cannot
be made without the help of histopathological studies
which can be accomplished by following techniques:
1. Fine-needle aspiration biopsy (FNAB). It is a
reliable, accurate (95%), quick and easy technique
for cytodiagnosis in orbital tumours. The biopsy
aspirate is obtained under direct vision in an
obvious mass and under CT scan or
ultrasonographic guidance in retrobulbar mass
using a 23-gauge needle.
2. Incisional biopsy. Undoubtedly, for accurate
tissue diagnosis a proper biopsy specimen at
least 5 to 10 mm in length is required. However,
the scope of incisional biopsy in the diagnosis of
orbital tumours is not clearly defined. It may be
undertaken along with frozen tissue study in
infiltrative lesions which remain undiagnosed.
3. Excisional biopsy. It should always be preferred
over incisional biopsy in orbital masses which
are well encapsulated or circumscribed. It is
performed by anterior orbitotomy for a mass in
the anterior part of orbit and by lateral orbitotomy
for a retrobulbar mass.