Australian und New Zealaxd Jourxuf of Opbthalmology ( t 998) 26, 99- 100

Letters to the Editor

Po I y hexamet hy I en e biguan i de treatment for Acanthamoeba ke rati t is Dear Editor,

We, in the Corneal Unit a t the Royal Victorian Eye and Ear Hospital (Melbourne, Vic., Australia), have been using poly- hexamethylene biguanide (PHMB) since April 1993 t o treat Acanthamoeba keratitis. We have been very impressed with its efficacy and lack of toxicity.

Unfortunately, as noted in Kumar and Crawfords editor- ial,l there is n o commercially available preparation in Australia at the present time. O u r Pharmacy Department has been making up PHMB 0.02% eye drops from a commer- cially available PHMB (polyhexamide) 20% solution cur- rently marketed as Baquacil (ICI, Wilmington, DE, USA) concentrate for pool cleaning.

The solution is prepared by a two-step dilution and fil- tration method through a 0.22 p m filter under sterile condi- tions in a laminar flow hood. Each bottle of drops is used for a period of up t o 4 weeks. So far, 21 patients have been treated using this solution, of which 15 have been definitely culture positive for Acantbamoeba. Of the nine patients for which we have adequate hospital follow-up records, a medical cure has been attained in eight. O n e patient has required two therapeutic penetrating keratoplasties and another patient, whom we have classified as a medical cure, has extensive stromal scarring and may require an optical corneal graft.

O u r usual regime for commencement of therapy is to start with triple therapy of PHMB 0.02% drops q.1 h along with propamidine (Brolene) drops q . l h and Neosporin (gramicidin, neomycin sulphate and polymixin; Wellcome, Cabarita, NSW, Australia) q. 1 h after first obtaining speci- mens for culture and performing debridement of all involved corneal epithelium. We feel that removing the involved epithelium is quite important initially to decrease the amoebic load and t o increase stromal penetration of the topical therapy. After 2 or 3 days of intensive treatment we tend to discontinue the neosporin drops first and decrease the frequency of the other drops to 2-4 hourly, depending on the clinical response. After 1-2 weeks of therapy, propamidine is then discontinued and therapy with PHMB 0.02% drops only is continued for a period of 6-12 weeks.

Because of the intense pain associated with this condition, in some cases topical steroids may be introduced after a period of initial anti-amoebic treatment.

While we applaud the call for commercially available PHMB drops, we feel it is imperative that therapy for this potentially blinding condition not be compromised by the lack of such a preparation and we encourage the authors to have the drops made up by their Pharmacy Department.

REFERENCES 1 . Kurnar B, Crawford C. Should we be using polyhexamethyl-

ene biguanide to treat Acantbamoeba keratitis? Aust. N.Z. 1. Opbthalmol. 1997; 25: 94-5.

Dr Laurence Sullivan Corneal Unit

Royal Victorian Eye and Ear Hospital East Melbourne, Victoria

Australia

Evolution of oculomotor muscles Dear Editor,

An interesting comparison can be made between those cranial nerves with predominantly motor function: each seventh nerve controls all the muscles of facial expression on one side of the face and each twelfth nerve controls half the complex musculature of the tongue. In contrast, the fourth nerve and the sixth nerve each control one tiny oculomotor muscle.

H o w did this strange situation evolve in humans? Humans are essentially sighted animals depending on vision both for survival from predators and for food gathering.

The fourth nerve that moves the eye downwards and inwards (once each medial rectus has adducted the globe) can perhaps be regarded as the muscle of predation. T h e exquisitely accurate depth perception proceeding from binocular function in downgaze depends, in large part, on

the accurate contraction of each superior oblique muscle. This highly accurate judgement of direction and distance in downgaze would have been very useful t o early man in the hunting of ground hugging animals. To omnivorous man, these would have been a useful source of protein. So perhaps

100 Letters to the Editor

those early humanoids who happened to have (possibly by mutation) a single cranial nerve controlling the down and in movement of each eyeball were the fittest because they ate best.

As a predominantly sighted animal man has always had one grave defect; he is completely lacking in vision posteri- orly and must have been vulnerable to predators attacking from behind.

The three principal mechanisms by which this blind area is covered are: ( i ) the neck muscles, which rotate the head, (i i) the lateral rectus muscle on each side; and (iii) the Riddoch effect.

It is the Riddoch effect that enables man to perceive the smallest of moving objects in the periphery of his visual field which, theoretically, from the distribution and sensi- tivity of the receptors in the peripheral retina, would not be perceived. This extreme sensitivity to movement in the peripheral field is achieved by the complex neuronal con- nections of the peripheral ganglion cells. (The accuracy of early visual field testing was significantly negated, espe-

gave erroneous recordings when a moving test target was used.)

One can postulate that those early humans who had a single cranial nerve to control each lateral rectus muscle were examples of survival of the fittest so far as escape from carnivorous predators was concerned.

All theories of evolution, with the possible exception of the examination of vestigial (from 'vestigium': a footprint (Latin)) remnants, such as the uniformly occurring appendix and the occasionally occurring webbed toes, must of neces- sity result from retrospective speculation. 1 present the above hypotheses to my colleagues as totally unproven speculation.

Hugh Fraser Nundab, QLD 4 0 1 2

Australia

Dr Hugh Fraser died suddenly on 18 October 1997. After consultation with Dr Fraser's family, the Editor was encour- aged to publish this letter. Any correspondence would be

cially in the peripheral field, by the Riddoch effect, which welcomed.