Biomedical Engineering Reference
In-Depth Information
application of an antibiotic such as Zymar
(Allergan, Inc., Irvine,
CA) or Vigamox
®
(Alcon Laboratories, Fort Worth, TX) [
7
]. Fol-
lowing this technique small-molecule drugs and linear and globular
shaped macromolecules with high molecular weights may be
injected. Intravitreal elimination was found to depend on the
molecular weight of drug candidates. Macromolecules such as
proteins (40 and 70 kDa) may tend to cause longer retention in
vitreous body leading to slower elimination relative to small mole-
cules [
8
]. In spite of offering high drug concentrations in the back
of the tissues (vitreous humor, retina, and choroid), this technique
is associated with various short-term complications. Ocular com-
plications such as endophthalmitis, retinal detachment, and intravi-
treal hemorrhages have been reported.
™
Periocular is a broad term which refers to the region surrounding
the eye. Periocular route includes subconjunctival, peribulbar, pos-
terior juxtascleral, retrobulbar, and subtenon routes (Fig.
2
). Drug
administration via this route is considered as one of the most
promising mode for posterior ocular drug delivery, i.e., to retina.
Solutions are administered in close proximity to sclera which results
in high drug concentrations in deeper neural retina and vitreous
body. Sclera is a fibrous tissue made of collagen fibers derived from
dura mater of central nervous system. This tissue offers very low
resistance to drug permeability [
9
]. Hence, increased drug perme-
ability into deeper ocular tissues is achievable.
2.3
Periocular Route
Conjunctiva is a thin, semitransparent, mucous-secreting tissue
which forms the loose inner lining of the eye. It is continuous
with cornea and forms a thin membranous layer (bulbar conjunc-
tiva) above the white part of the eye called “sclera.” Administration
into the space between conjunctiva and sclera, i.e., beneath the
conjunctiva, is called as “subconjunctival injection.” This mode of
drug administration is considered minimally invasive. In general, a
25-30 gauge and 30 mm long needle is selected for drug adminis-
tration. During administration the beveled edge of needle is faced
towards sclera (
2.3.1 Subconjunctival
Injection
3 mm beyond sclera) and is slowly penetrated
across bulbar conjunctiva until the fold appears [
10
]. A volume of
500
L drug solution may be administered by this method.
μ
Subtenon injection implies administration of solutions into tenon's
capsule. It is a fibrous membrane that envelopes the eye from
limbus to the posterior optic nerve [
11
]. Subtenon's space is a
virtual cavity that is bound by tenon's capsule and sclera [
12
].
Initially a small surgical dissection is made into conjunctiva and
subtenon space. To administer drug solution into subtenon's space,
the individual is directed to look downwards with simultaneous
retraction of upper eyelid. A sharp-tipped 26-gauge and ~1.6 cm
long needle is inserted into posterior subtenon space [
12
]. Vision
2.3.2
Subtenon Injection
