Critical Period In Humans

Von Noorden (him again) demonstrated that human amblyopia was accompanied by structural changes in the visual pathway. The LGN of a human amblyope was examined and cells in the parvocellular layers innervated by the amblyopic eye were about 18% smaller than the equiv from the other eye. The decrease in size was more pronounced in the layers receiving crossed fibres (1,4 and 6)

Preferential looking techniques have been used in thee past to monitor VA and stereo in both human and monkey infants w/convergent strabismus. Recent studies suggest VA is originally equal and coarse stereopsis is present. If strabismic infants below 2 are tested wearing prisms to correct the deviation then coarse stereopsis can be demonstrated but it can't without the treatment. The deviating eye doesn't actually develop amblyopia until after 6 months of age. Esotropia can be detected early w/a cover test.

Few deficits lead to amblyopia before six months of age. Emmetropisation sorts out refractive errors as long as they are modest within 6-12mths. An Rx needs to be persistent for two years or more from an early age before a permanent VA deficit occurs.

Cataract and other forms of stimulus deprivation (ptosis?) have a much more severe effect than anisometropia/astigmatism over a shorter period. If your baby has a congenital cataract then it needs to be removed pronto and the removal needs to be followed by a period of vigorous therapy. Weeks of deprivation can have a substantial effect between 6-18 months and months of dep can have an effect until 8 years of age.

In strabismic patients amblyopia and binocular function (w/stereopsis) must be considered. Amblyopia doesn't seem to develop in congen. esot until close to 1yr of age. Acuity differences didn't exceed normal limits until 9-11 months. The period of peak sensitivity lies between nine months and two years and then sensitivity declines until around the age of 8. Stereopsis in untreated esotropes is crude at best. If it's at worst alignment before 1.5 years can get it up to the dizzy heights of 'very crude'. Alignment after 1.5 years of age won't provide stereoscopic vision.

Occlusion therapy is the most common therapy in the treatment of amblyopia but its efficacy is unknown. Not good really. Retrospective studies have shown that 50% of patients show a marked improvement in VA following occlusion. These outcomes were independent of age. Occlusion must be administered with care to avoid the dreaded 'reverse amblyopia'. If the good eye is patched 50-70% of the time and both eyes open the rest of the time it can be avoided. The critical period for the cure of amblyopia seems to be longer than the critical period for its creation.

Summary

3 periods of development in the visual system

• Prestereoscopic (0-4 months) - acuity develops and direction and orientation specificity are refined
• Onset of Stereopsis (4-6 months) - stereopsis goes from zero to adult-like levels within a month at the same time as ocular dominance columns segregate
• Poststereoscopic (6-24 months) - acuity continues to develop

Restoration of BV may only be possible in the first 18 months of life although less dramatic improvement may be seen in px up to 3.

Restoration of VA depends on who you ask; Von Noorden thinks it's only really successful up to 5 years and Birbaum thinks 16! Experience suggests <6>10 little success.