Medical Management Of Glaucoma

Duane Flemming, D.V.M., J.D., D.A.C.V.O.

 

I. GLAUCOMA 

A. Definition: A group of eye diseases characterized by an increase in the intraocular pressure, and subsequent pathologic changes produced by this increased pressure.

  1. Intraocular pressure is determined by the rate of aqueous humor production and by the resistance to aqueous humor outflow.
  2. Increased intraocular pressure is “ always” due to obstruction of aqueous flow.

II. TYPES OF GLAUCOMA

A. Primary Glaucoma:

The block in aqueous outflow is due to some inherent ocular defect.

There are two types:

  • Open angle
    • Rare in animals
    • Beagles and Miniature Poodles
  • Narrow Angle
    • Bassett Hounds, Cocker Spaniels, Norwegian Elkhounds, Siberian Huskies, Samoyeds, Dachshunds, Miniature Poodles, Welsh Terriers, Wire-haired Fox Terriers, Chihuahuas, SealyHam Terriers, Mixed Breeds.

B. Secondary Glaucoma

The block in aqueous outflow is due to one or more concurrent intraocular diseases.

Treatment should be directed toward:

  • Controlling the pressure followed by correction of the inciting factor.

C. Congenital Glaucoma

  • The block in aqueous outflow is due to a congenital abnormality of the drainage angle such as mesodermal dysgenesis or goniodysgenesis.
  • Seen mostly in Bassett Hounds and Samoyed

III. CLINICAL PRESENTATION

A. Signs:

  • Pain
  • Episcleral venous congestion
  • Corneal Edema
  • Pupillary Dilation
  • Increased intraocular pressure as measured by a TONOPEN or a Schiotz Tonometer
  • Buphthalmia
  • Blindness.

B. Classification of Clinical Cases- May be primary, secondary, or congenital in origin.

1. Advanced (absolute) Glaucoma in a Blind, Scarred Eye

  • NOT AN EMERGENCY
  • No immediate treatment necessary
  • Requires intraocular prosthesis or enucleation

2. Advanced (absolute)Glaucoma in a Cosmetically Acceptable Eye)

  • Urgent but still not an emergency
  • Primary effort directed toward maintenance of a cosmetically acceptable eye

3. Acute congestive Glaucoma, non-buphthalmic

  • THIS IS AN EMERGENCY
  • There may have been extensive damage, but vision or at least a cosmetically acceptable eye may be salvaged.
  • These cases are frequently refractory to medical therapy.

4. Early Glaucoma Non-Congestive

  • Unusual to see at this stage
  • Prompt and efficient medical or surgical intervention is required.

IV. MEDICAL THERAPY

A. Goal: To Increase the facility of outflow and decrease the production of aqueous humor by the use of:

B.  Miotics (Parasympathomimetics) Contract longitudinal ciliary muscles and iris sphincter muscles leading to enlargement of ciliary cleft (drainage angle)

1. Direct Acting Miotics (Cholenergics): Cause an increase in the A-P diameter of the lens and therefore decrease the depth of the anterior  chamber.

  • Pilocarpine

2. Indirect Acting Miotics (Cholinesterase inhibitors): Inhibit cholinesterase at neuromuscular junctions and allow accumulation of acetylcholine

  • Physostigmine salicylate (Eserine)
  • Echothiophate iodide (Phospholine Iodide)
  • Demecarium bromide (Humorsol)

3. Dual Acting Miotics (Direct and Indirect Acting)

  • Carbachol

C. Adrenergic Agents

1. Sympathomimetics: Increase facility of outflow by an alpha receptor mediated increase in the trabecular meshwork function.

  • Dipivefrin hydrochloride ( Propine, AKpro)
  • Epinephryl (Epinal, Eppy/N)
  • Epinephrine (Epifrin, Glaucon)

2. Beta-Adrenergic Blockers: Decreases aqueous humor production by inhibiting Beta receptors.

  • Betaxolol (Betoptic)
  • Timolol (Timoptic)

D. Carbonic Anhydrase Inhibitors: Inhibition of the enzyme carbonic anhydrase leads to a partial inhibition (40-60%) of aqueous humor production and results in metabolic acidosis.

1. Acetazolamide (Diamox)

  • 100 mg/kg reduces aqueous flow by 75%
  • Metabolic acidosis which is frequently offset by a respiratory alkalosis and lethergy
  • Hypokalemia

2. Dichlorphenamide (Daranide)

  • Well tolerated by dogs and is less likely than Acetazolamide to induce vomiting and diarrhea

3.Ethoxzolamide (Cardrase)

  • Twice as potent as acetazolamide therefor can use lower dosages.

4. Methazolamide (Neptazane)

  • Reduces IOP more rapidly with smaller doses but may produce greater malaise

 5. Dorzolamide (TruSopt/CoSopt)

  • Topical ophthalmic Solution
  • CoSopt is dorzolamide with Timolol

6. Brinzolamide (Azopt)

  • Topical ophthalmic suspension

E.Osmotic Agents: Raise the osmolality of the blood and thus remove fluid from the eye.

1. Glycerine

  • Rapidly absorbed after oral administration
  • Largely metabolized to C02 and H2O
  • Use extreme caution in diabetic dogs.

2. Mannitol

  • Given IV - requires time and cooperative patient
  • Decreases IOP within 30 min and lasts for 6 hrs.

F. Prostaglandins: reduces collagens within the uveoscleral outflow pathway which reduces hydraulic resistance and facilitates outflow.

  • Latanoprost (XALATAN)

G. Vasodilators: Systemic Anti-hypertensive. Preserves retinal blood flow.

  • Amlodipine (Norvasc, Lotrel)   

TOPICAL DRUGS IN GLAUCOMA THERAPY

MIOTICS                                              
CONCENTRATIONS   
DOSAGE
STABILITY
Pilocarpine                                               1.0-4.0%          BID-QID Good
Carbachol                                            0.75-3.0E% BID-QID   Good
Physostigmine  0.25-1.0%   BID-QID Limited
Demecarium  0.125-0.25% SID-BID Good
Echothiophate  0.06-0.25%  SID-BID Limited
ADRENERGICS      
Dipivefrin   0.1%  BID Good
Epinephryl  0.5-1.0% SID-BID Good
Epinephrine  0.5-2.0% SID-BID Good
BETA BLOCKERS      
Betaxolol 0.25-0.50% BID Good
Timolol 0.25-0.50% SID-BID Good
OTHERS      
Xalatan 0.005%  SID-BID  Limited
 Amlodipine 2.5 mg  SID  
       

HYPEROSMOTIC AGENTS IN GLAUCOMA THERAPY

AGENTS    
CONCENTRATIONS
DOSAGE   
ROUTE
Glycerine  50%  1-2 gm/kg (repeated to TID) PO
Mannitol   20% 1-2 gm/kg   IV

CARBONIC ANHYDRASE INHIBITORS IN GLAUCOMA THERAPY

AGENT 
CONCENTRATIONS  
DOSE 
ROUTE
Acetazolamide  3-10 mg/kg BID-TID PO
Dichlorphenamide  1-10 mg/kg BID-TID PO
Ethoxzolamide 1.5-7 mg/kg BID-TID PO
Methazolamide 2-8 mg/kg BID-TID PO
Dorsolamide 2.0% BID-TID Topical 
Brinzolamide  1% BID-TID Topical