RADIATION/LASER 23

TABLE 1–8

Tissue Effects of Lasers (Mnemonic: ABCD)

PREGNANCY MAY INCREASE THE RISK OF, EXACERBATE, OR PRECIPITATE

Cortical blindness; idiopathic central serous retinopathy (ICSR); ischemic optic neuropathy (ION); toxemia of pregnancy with potential serous choroidal or retinal detachments (frequently bilateral); progression of diabetic retinopathy (DR) (5% risk of proliferative diabetic retinopathy in patients with moderate nonproliferative disease); Graves’ disease; pituitary adenoma; and meningioma.

Radiation/Laser

LASER PRINCIPLES Amplification of narrow wavelength (monochromatic) light, emitting intense and coherent (synchronous with respect to time and space) radiation. Power time ¼ energy. See Table 1–8 for tissue effects of lasers.

Chromophores: substances that absorb laser energy; in the retina mainly RPE melanin, but also xanthophyll (intraretinal, peak with blue light), hemoglobin (peak with yellow light), lipofuscin, and visual pigments.

Parameters for retinal treatments

Exposure time: 0.1 seconds for most treatments. A shorter time increases the blast effect with increased risk of tissue rupture, hemorrhage, and increased temperature. A longer time increases the ‘‘cooking’’ of tissue and increases overflow with less defined burn.

Power: start low and increase.

Size: for panretinal photocoagulation (PRP) and to close tears, use 250–500 mm or larger spot (must increase power if the size increases to maintain the same energy density); use smaller 50– 100 mm in macula to limit scotoma size.

Argon and krypton lasers are generated from a high voltage passed across gas-filled tubes and emit continuously.

Blue (488 nm): not used often, as it is absorbed by xanthophylls in the macula.

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24 GENERAL TOPICS IN OPHTHALMOLOGY

TABLE 1–9

The Electromagnetic Spectrum

Shorter wavelengths

Gamma rays 10 14 m, x-rays 10 10 m, ultraviolet 10 6 m, UVA 150–200 nm Visual spectrum

Violet 400 nm, blue 450–500 nm, green 500–550 nm, yellow 600 nm, red 650 nm Ophthalmic lasers

Excimer 193 nm, argon green 514 nm, HeNe 633 nm, diode 800 nm, YAG 1064 nm Longer wavelengths

Radar 10 2 m, FM radio, TV, shortwave radio 102 m, AM radio 104 m

Green (514 nm): superficial RPE; good for most conditions, highly absorbed by hemoglobin; may cause more direct closure of choroidal neovascular membrane (CNVM) vessels.

Krypton red (647 nm): deeper RPE; choriocapillaris; hurts more; use red or diode to treat through hemorrhage or nuclear sclerotic cataract; low scatter.

Dye yellow (577 nm): minimal xanthophyll absorption; low scatter, high hemoglobin absorption; useful in vascular lesions.

Diode (800 nm): solid-state continuous-wave laser (do not need a large cooling bath as with argon lasers); portable and highly efficient. Treats deeper RPE and choriocapillaris (can always see the lesion; hurts more, spares inner retina more).

ULTRAVIOLET RADIATION (MNEMONIC: ABC):

Ultraviolet A (UVA); longest wavelength; ages skin, blocked by lens

Ultraviolet B (UVB); 150–200 nm; burns skin and responsible for most skin cancer

Ultraviolet C (UVC); < 150 nm; most powerful and carcinogenic, but largely blocked by ozone

ELECTROMAGNETIC SPECTRUM See Table 1–9.

Surgery

PREOPERATIVE EVALUATION Overall perioperative mortality of eye surgery is 0.06 to 0.18%.

Anticoagulants: for most surgeries, except topical clear-corneal cataract surgery, have patient stop aspirin 7 to 10 days prior, stop other nonsteriodal anti-inflammatory drugs (NSAIDs) 1 or 2 days prior, and stop warfarin (Coumadin) 3 to 5 days before surgery.

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SURGERY 25

Patients at high risk for thromboembolic disease (prosthetic heart valves, prior embolic stroke, recent deep venous thrombosis, or pulomary embolus) may need to discontinue warfarin and be hospitalized for 24 to 48 hours prior to surgery for heparin, which can then be stopped 4 to 6 hours prior to surgery. For emergent surgery, reverse warfarin with 2 to 4 units fresh frozen plasma or vitamin K 10 mg intramuscularly (IM) or subcutaneous (SQ ) every 12 hours.

One month after deep venous thrombosis (DVT) there is a 1%

risk of a DVT-related event without warfarin. The risk decreases even further with time; thus, can usually stop anticoagulation 3 months after a DVT for outpatient surgery unless the patient is hospitalized or otherwise immoblized.

Cardiac disease: accounts for half of postoperative complications; avoid elective surgery within 6 months of a myocardial infarction. In general, patients are okay to proceed with elective surgery without further testing if they have mild and stable angina, have had a heart attack more than 6 months prior, have stable congestive heart failure (CHF) and can climb one flight of stairs without stopping. Otherwise refer for patient evaluation and clearance.

Cerebrovascular disease: avoid elective surgery within 3 months of a transient ischemic attack or stroke.

Chronic obstructive pulmonary disease (COPD): aggressive preoperative treatment to prevent coughing; patient may not be able to lie flat for surgery.

Diabetes: ask about prior history of ketoacidosis; oral hypoglycemics should be stopped the morning of surgery; insulin patients should receive 1/2 dose before surgery and be maintained on intravenous fluids containing glucose until they are taking orals.

Hepatic disease: avoid elective surgery until 1 month after liver function tests have normalized.

Hypertension: in general, avoid surgery if blood pressure is > 200/ 110 mmHg; patients may take their usual antihypertensive dose on the morning of surgery.

Platelets: transfuse for < 30,000; each unit adds 10,000.

Renal failure: dialyze the day before surgery to optimize fluid and electrolyte balance and to eliminate anticoagulants used in dialysis; beware of associated qualitative platelet defects (assess with bleeding time; greater risk with blood urea nitrogen [BUN] > 60).

Steroids: patients taking more than 7.5 mg of prednisone 3 months or 40 mg of prednisone 1 week are at risk for adrenal suppression and may need a ‘‘stress’’ steroid dose (for most ophthalmic surgeries, 25 mg hydrocortisone on the day of surgery is sufficient).

Valvular heart disease: clean ophthalmologic procedures pose little risk for endocarditis; thus, no antibiotic prophylaxis is typically necessary.

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SURGERY 27

PRINCIPLES OF SURGERY Understand that surgery is iatrogenic trauma and that most eye tissues can only repair themselves and not regenerate.

Determine goal; have a clear knowledge of surgery purpose.

Develop a well-defined plan.

Be adaptable and flexible.

Maintain good visualization of surgical field.

Minimize trauma.

Restore tissues to normal states.

Practice economy and control.

Continue with development and improvement.

SUTURE MATERIALS See Table 1–10.

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