Sunday, October 20, 2019

PRIMARY CONGENITAL GLAUCOMA


Guest author

SABEEH

Ajmal Khan Tibbiya Colege
Aligarh, India



Introduction

Primary Congenital Glaucoma (PCG) is a potentially blinding disease of children, which if untreated, would result in a lifetime of blindness. It occurs due to obstruction of the drainage of the aqueous humor caused by a primary developmental anomaly at the angle of anterior chamber. Although PCG is the most common glaucoma seen in infancy, it is still an uncommon disease. The variable incidence in various ethnic groups points towards a genetic basis for the disease.



Epidemiology


  • The disease varies sustainability in different ethnic groups from 1:1250 births in Slovakian Roms to 1:20,000 in Scandinavian regions.
  • In the West, the average incidence is about 1:10,000 births, but appears to be higher in Asians.
  • In Saudi Arabia, it is reported to be 1:2500, while Indian's have incidence of 1:3300.
  • The disease was responsible for 4.2% of blindness in the pediatric population.


Inheritance


  • Most cases of PCG are sporadic in occurrence.
  • Recessive inheritance of some cases of PCG is proved by:
  1. A high frequency of parental consanguinity.
  2. The presence of disease in about 25% of sibs of probands.
  3. The presence of the disease in all children of a marriage between 2 affected persons.
  4. The occurrence of Glaucoma in collaterals of both parents some families.


Genetic defects


  • According to Human Genome Organization (HUGO) Nomenclature Committee, loci for congenital glaucoma are designated by GLC3 and letters are added to distinguish specific loci in order of their discovery.
  • Till date, 3 genetic loci have been linked to PCG:
  • GLC3A at chromosome locus 2p21.
  • GLC3B at chromosome locus 1p36.
  • GLC3C at chromosome locus 14g24.3
  • Of these only the GLC3A locus has been linked to a specific gene. This gene is called CYP1B1 and is the largest known enzyme of the human cytochrome p450 pathway.
  • There are several known missense mutations within the CYP1B1 gene.
  • Among these, the mutations identified are:
  • G61E, Ter223, P193L, E229K, R390C, R368H.
  • A study on the genotype-phenotype correlation of these patients identified the frame shift mutation and R390C homozygous mutation as being associated with very severe disease and poor prognosis regardless of any treatment.


Structural defects and clinical features


  • The glaucomas are a heterogeneous group of insidious diseases associated with elevated IOP and optic nerve atrophy.
  • Primary Congenital Glaucoma is a specific, inherited developmental defect in the trabecular meshwork and anterior chamber angle.
  • The developmental anomalies of the anterior chamber angle prevent drainage of aqueous humor, thereby elevating IOP.
  • Elevation of IOP in children younger than 3 years of age causes rapid enlargement of the globe, occurs primarily at the corneo-scleral junction.
  • As the cornea and limbus enlarge, the endothelium of the cornea and Descemets membrane are stretched. This stretching can result in a linear rupture of Descemet's membrane known as Haab's striae.
  • The Descemet's membrane ruptures may occur acutely causing an influx of aqueous into the stroma and epithelium resulting in sudden corneal edema.


Haab's striae

Clinical features of PCG typically include:

  1. Tearing
  2. Photophobia
  3. Buphthalmos (enlargement of the globe)
  4. Clouding of the cornea


More serious consequence of elevated IOP is that it can rapidly lead to axonal loss and permanent visual impairment in untreated children.

Buphthalmos


Management

The management of Congenital Glaucoma starts with parental counseling, which includes discussion for: need for surgery and possibilities of multiple surgeries; the need for lifelong follow up and the combination of problems to be tackled (IOP, amblyopia management, refractive correction, possible keratoplasty).

Panicker and associates have graded the severity of glaucoma depending upon the clinical features as given below in table.

SEVERITY INDEX FOR GRADING PCG

Clinical parameters used for grading
Normal
Mild
Moderate
Severe/Very severe
Corneal diameter
Upto 10.5mm
>10.5-12mm
>12-13mm
>13mm
IOP
Upto 16 mmHg
>16-20 mmHg
>2030 mmHg
>30mmHg
C/D ratio
0.3-0.4
>0.4-0.6
>0.6-0.8
>0.8
Last recorded VA
20/20


<20/400-NPL
Corneal opacity
No edema
Mild edema
Severe edema
Severe edema+Haab’s striae

Examination under anesthesia is an essential part of PCG management. It includes the following examinations:

EUA

Tonometry
General anesthesia usually lowers the IOP, except for ketamine.
The Tono-pen is convenient and easy to use.
The normal IOP in infants under anesthesia is usually in the low teens.
A pressure of 20 mmHg or more should be considered abnormal.

Corneal diameter
Using calipers the corneal diameter measurement should be taken from limbus to a similar point 180-degree away at the opposite limbus.
The 95% ranges of normal corneal diameters are: 9.4 mm to 11 mm at age 1 month, 10.5 mm to 11.7 mm at age 6 months, and 10.8 mm to 12 mm at age 12 months.
In PCG, the diameter of cornea may enlarge to as much as 17 mm.
Changes in the corneal diameter less than 0.5 mm in the follow up examination should be interpreted cautiously.

Gonioscopy
A 14 mm Koeppe lens provides a clear view of the angle of the eye and a hand held microscope with a Barkan light or any type of illuminator is necessary for gonioscopy during anesthesia.
In Congenital Glaucoma the Iris usually is inserted anterior to scleral spur and the angle recess is poorly formed.

Fundoscopy
Dilated fundus examination and disc evaluation are essential in diagnosing congenital glaucoma.
Optic nerve cupping larger than 30% of the disc diameter, especially if asymmetric between two eyes, is strong evidence that the disc is under pressure and may be glaucomatous.
Glaucomatous cupping in infants, unlike adults is usually reversible after normalization of IOP.
The younger the child, the faster is the reversibility.

Surgical treatment

The surgical options include:
Goniotomy
Trabeculectomy
Trabeculectomy with anti-fibrotic agents

Goniotomy

 
Trabeculotomy
A trabeculotomy-trabeculectomy combined surgery has been found to result in more favorable outcomes and many surgeons prefer that approach.
In case of refractory cases not responding to surgery, a repeat surgery is needed.


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