Anatomy Flashcards
Spaeth
Spaeth notation for angle classification.
A number to indicate the approximate angular width, and another number to indicate the amount of pigment (from 0 to 4+) in the trabecular meshwork:
Iris insertion: (capital letter)
A = anterior to Schwalbe’s line
B = between Schwalbe’s line and scleral spur
C = scleral spur visible
D = ciliary body visible
E = greater than 1 mm of ciliary body visible
Curvature of iris: (lowercase letter) b = bowing anteriorly p = plateau configuration f = flat c = concave posterior bowing
The capital letter IN the parentheses indicates the pre-indentation appearance of the iris insertion while the un-parenthesed capital letter is the iris insertion appearance after indentation with the gonioscopy lens.
scanning laser polarimeter
scanning laser polarimeter (e.g. GDx) is a scanning laser ophthalmoscope that takes advantage of the birefringent properties of the retinal nerve fiber layer (RNFL). Basically when light passes through the parallel structure of the RNFL, its polarization state changes. The degree to which this light’s polarization state is changed is recorded and quantified by a detector. Thus, this machine is measuring the relative (not absolute) RNFL thickness. Other ocular structures that could affect this polarization phase shift include the crystalline lens (small effect) and cornea (larger effect). This is why modern scanning laser polarimeters typically have a variable corneal compensator which subtracts the cornea’s influence on the light’s phase shift.
Plateau iris
To definitely diagnose plateau iris syndrome, one must document a lack of change in the angle configuration after LPI. Therefore, a LPI should be performed first.
If this is not effective, cataract surgery with or without a secondary glaucoma procedure (e.g. trabeculectomy) can help lower IOP in this situation.
Confocal scanning laser ophthalmoscopy
Confocal scanning laser ophthalmoscopy machine is the HRT (Heidelberg Retina Tomograph). This machine basically takes tomographic slices of the structure being imaged and then uses a computer program to reconstruct the 3-d object of interest. The HRT is able to measure the absolute RNFL thickness.
Cyclodialysis cleft
The UBM is demonstrating the presence of a cyclodialysis cleft which is defined as a separation of the ciliary body from its attachment to the scleral spur. This allows aqueous to directly flow into the suprachoroidal space resulting in chronic hypotony. This condition is typically caused by some type of trauma (including intraocular surgery).
Cyclodialysis clefts can be diagnosed with gonioscopy, UBM, and/or anterior segment OCT (e.g. Visante). Chronically-low IOPs can result in hypotony maculopathy (i.e. macular folds) which can decrease the patient’s visual acuity.
The treatment of a cyclodialysis cleft typically begins with medical treatment, in the form of cycloplegia (e.g. atropine). Some providers will also use topical corticosteroids in the hopes of inducing an IOP rise. If this is unsuccessful after 6-8 weeks, argon laser treatment can be attempted. This consists of applying laser energy to the ciliary body and scleral spur to induce scarring of the cleft. Another potential option is transscleral diathermy and/or cryotherapy. If these are unsuccessful, incisional surgery is an option (e.g. direct cyclopexy, vitrectomy with gas tamponade, etc).
Another pearl to remember is to watch out for IOP spikes after any attempt to close these clefts. Oftentimes, the IOP may spike very high (e.g. 50 mmHg) after successful closure of a cyclodialysis cleft since the trabecular meshwork has not been used recently and/or has been damaged from the initial trauma.
The term “cyclodialysis” is sometimes confused by trainees with the terms, “iridodialysis” and “angle recession”.
Iridodialysis is tearing of the iris near its root/insertion. This condition is occasionally caused by surgeons who try to phaco through a wound that is incarcerated with iris.
Angle recession is due to a tear in the ciliary body, typically between its longitudinal and circular muscle fibers. On gonioscopy, this appears as an overly-wide ciliary body band. The angle recession itself does not directly cause increased IOP, but it is commonly associated with damage to the trabecular meshwork. This condition is also associated with blunt trauma.
What is the maximum amount of luminance that can be projected by the Humphrey Visual Field Analyzer?
The apostilb is a unit of “luminance” which itself is defined as the luminous intensity per unit area of light traveling in a given direction. Another unit of luminance is candela per square meter (cd/m2). The maximum luminance that can be projected by the Humphrey machine is 10,000 apostilbs. Another bit of trivia is that the maximum luminance that can be projected by the Goldmann perimeter is 1,000 apostilbs.
Automated perimeters make use of the decibel (dB) unit which is a relative term with NO absolute value in of itself. Instead, its specific value (i.e. in terms of luminance) is dependent on the maximum illumination that can be projected by the specific perimeter being used. Thus, a threshold of “x” dB would equate to different detected light sensitivities depending on which perimeter is used.
The dB unit is actually a logarithmic unit whereby 10 dB is equivalent to a light intensity that is 1/10th as bright as the maximum possible light intensity. Thus, if a patient exhibits a threshold of 10 dB in a specific area of his visual field, then he can detect a light stimulus that is 1/10th as bright (or brighter) as the maximum light intensity. In the case of a Humphrey visual field, his threshold would be 1,000 apostilbs at this area of the visual field. Similarly, if the patient shows a threshold of 30 DB in a specific part of his visual field, then he can detect a light stimulus that is 1/1000th as bright as the maximum light intensity (i.e. 10 apostilbs). The Humphrey machine can test for threshold sensitivities from 0 dB to 50 dB (i.e. 10,000 apostilbs to 0.1 apostilbs). However, most humans cannot detect light less than 1 apostilbs; thus any sensitivities 40 dB or greater typically indicate a “trigger-happy” patient.
As an aside, 31.5 apostilbs is the background luminance that is projected by both the Humphrey and Goldmann perimeters. This amount of luminance has been shown to saturate the rod photoreceptors so that the cone photoreceptors are primarily being tested.
IOP decreases with…
IOP typically decreases during pregnancy. Other factors associated with a decrease in IOP include: aerobic exercise, certain drugs/medications (e.g. alcohol, cannabis, heroin), and certain systemic conditions (e.g. metabolic acidosis).
Schwartz-Matsuo
Schwartz-Matsuo syndrome (often just called “Schwartz syndrome”) is a rise in intraocular pressure in the setting of a chronic rhegmatogenous retinal detachment. Matsuo demonstrated the presence of photoreceptor outer segments in patients with this condition. In theory, the photoreceptors are liberated following retinal detachment and escape the subretinal space through the tear. These photoreceptors then migrate into the anterior chamber and occlude the trabecular meshwork. Repairing the retinal detachment leads to resolution of the intraocular pressure elevation.
Things that increase/decrease IOP
There is a classic table that lists which factors classically are associated with increases and decreases in IOP. This is an abbreviated list of these factors:
Things that increase IOP
Valsalva maneuver wearing necktie elevated central venous pressure orbital venous outflow obstruction hypothyroidism LSD ketamine Things that decrease IOP: aerobic exercise alcohol pregnancy metabolic / respiratory acidosis heroin cannabis
Tonography
Tonography is the most common method used to measure the facility of aqueous humor outflow in the eye. The average outflow facility of the normal eye ranges from 0.22-0.30 µL/min/mmHg. This value decreases with age and after intraocular surgery. Though tonography is not routinely performed in the clinic anymore, its steps are summarized here:
Place a Schiotz tonometer (of known weight) on the cornea for a few minutes. This causes an acute increase in intraocular pressure (IOP)
Measure the subsequent decline in IOP over time. This is typically performed by connecting the tonometer to a continuous recording device to generate a decay tracing.
Use this data and plug it into “Grant’s equation.” For your sanity, we would not recommend memorizing Grant’s equation which we will not detail here.
Fluorophotometry
Fluorophotometry is mainly used to measure the rate of aqueous formation, but can also be used to derive outflow facility. However, it is very time-consuming (e.g. 6 hours) and thus much less popular than tonography for this purpose. This procedure involves giving fluorescein either topically or systemically and measuring the decline of concentration of fluorescein in the anterior chamber by optical means.
Pupil block
If a patient who underwent a GDI had a shallow or flat anterior chamber, then two basic possibilities exist. Either there is too much aqueous being drained by the GDI or there is something posterior to the iris pushing it forward. If there was too much aqueous being drained, then the IOP would be low. Thus for this patient, there must be something pushing the iris forward. The possible mechanisms for this are: pupillary block, aqueous misdirection, choroidal effusion, or choroidal hemorrhage.
Typically in pupillary block, the anterior chamber is not completely flat, but instead is shallow peripherally with some space centrally. The anterior chamber can be flat with aqueous misdirection (aka “malignant glaucoma”), but this was not given as an answer choice. The anterior chamber can be very shallow/flat in choroidal effusion and choroidal hemorrhage, but the IOP is typically low if choroidal effusions are present. Thus, choroidal hemorrhage is the appropriate answer for this question.
Uveal tract attached to the sclera at what 3 points?
attached to sclera at: scleral spur, internal ostia of vortex veins, and the peripapillary tissue –> explains the dome-shaped configuration of choroidal hemorrhages.
Pv elevated in?
Sturge Weber
CC fistula
Thyroid orbitophathy
Goldmann Equation?
P = (F/C) + Pv P = IOP (mmHg) F = rate of aqueous formation (2.5 ul/min) C = facility of outflow (0.25 ul/min/mmHg) Pv = episcleral venous pressure
