Abstract
Snakebite is an important cause of morbidity and mortality in Nigeria, as in many parts of the tropics. However, blindness is a rare complication. Reported cases have usually followed bites by neurotoxic snakes such as the cobra. Reports of blindness produced by the bite of haematotoxic snakes are extremely rare. We report the case of bilateral blindness following a bite from a carpet viper. The literature on the subject matter is reviewed and the possible mechanisms of snakebite-induced blindness are discussed.
Introduction
Poisonous snakebite is a serious public health problem in Nigeria, as in many tropical countries. The carpet viper (Echis ocellatus) is responsible for the majority of cases of snakebite-induced mortality and morbidity in Nigeria. 1 Blindness is a rare complication and is usually caused by a bite from neurotoxic snakes such as the cobra. 2 We report the case of bilateral blindness following the bite of a haematotoxic snake, the carpet viper.
Case report
A 15-year-old boy was admitted in July 2009 to hospital six days after he had been bitten on the left foot by a snake while gathering firewood. The snake, which had been killed and brought with the patient, was identified as a carpet viper.
A few hours after the bite, he noted a painful swelling on the bite site. The following day he had spontaneous bleeding from the gums. Three days later he noted painless, progressive and fluctuant swellings on his forehead and the parietal areas of the head. At the time of presentation he had no visual symptoms.
On examination he was pale (packed cell volume = 16%) with a mildly swollen and tender left foot. There were no blisters or areas of necrosis. He had a sub-galeal haematoma involving the frontal and parietal areas bilaterally. His pulse rate was 90/min and his blood pressure was 110/70 mmHg. The clotting time was prolonged (>20 min).
He was given 20 mL of polyvalent anti-snake venom and transfused 3 units of whole blood. He was also given anti-tetanus prophylaxis and placed on antibiotics. The clotting test was repeated the following day. The blood still did not clot after 20 min. So he was given another 20 mL of the anti-snake venom.
On the second day of admission, the patient complained of blurred vision in his left eye and gradually lost vision in it. The following day, he lost vision in the other eye as well. On ophthalmological review, perception of light was absent in both eyes and the pupils were fixed and dilated. The globes were proptosed with dystopia and periorbital fullness superiorly. The conjunctivae were chymosed and prolapsed inferiorly. The anterior chambers were of normal depth but the lenses were hazy. The optic discs were pale and there were areas of pre-retinal haemorrhage and cotton wool spots. An impression of bilateral optic atrophy with possible retrobulbar haemorrhage was made.
On the third day of admission the patient's clotting time was still prolonged, although he was no longer bleeding from the gums. He was given a further 20 mL of the anti-snake venom and the clotting time subsequently normalized.
Over the next month the subgaleal haematoma subsided; the proptosis, conjunctival chemosis and retinal haemorrhage all resolved. However, the optic atrophy persisted and the perception of light was absent in both eyes. No specific treatment could be offered, so the patient and relatives were counselled and discharged.
Discussion
Three species of snakes are responsible for the majority of deaths and injuries associated with snakebites in Nigeria. These include carpet viper (E. Ocellatus), the black-necked spitting cobra (Naja nigricolis) and the puff adder (Bitis arietans). The incidence of bites has been reported to be 497 per 100,000 with a 12% natural mortality: E. ocellatus accounts for up to 66% of bites in the northern Savannah region of Nigeria. 1
Ocular complications following snakebite are rare. However, painful conjunctivitis and corneal ulceration, sometimes with sight loss, have been reported following venom sprayed into the eyes by a spitting cobra. Snake venoms are complex heterogeneous poisons with multiple effects. The carpet viper venom contains a prothrombin activating procoagulant, haemorrhagin and cytolytic fractions which cause haemorrhage, incoagulable blood and local necrosis.
The ophthalmic effects of envenomation are varied and depend on the species of snake involved. Some of the effects described include ptosis, extraocular muscle palsies, haemorrhage into the conjunctiva, anterior chamber, vitreous or retina, lid oedema, retinal and optic nerve oedema, pupillary changes, optic neuritis, optic atrophy, cataract and venom ophthalmia. Optic atrophy following a bite from a viper is extremely rare. The literature search revealed only one case from Nigeria which was reported in 1956. 3 No case of blindness was seen in a series of 115 cases of carpet viper bites reported by Warrell et al. in Nigeria. 4
Possible causes of optic neuritis following a viper bite include: direct effect of snake venom; allergy to the anti-snake venom; and multiple and extensive haemorrhage following the bite. However, there is no evidence that viper venom does have a direct toxic effect on the optic nerve, nor has it been reported as a manifestation of allergy to anti-snake venom. It is possible that, in this case, the optic nerve changes resulted from a severe haemorrhage. It is known that haemorrhage in other parts of the body can result in optic neuritis and, in such cases as in the present one, optic neuritis appears 3–7 days after the haemorrhage. For example, Mofredj et al. reported a case of loss of vision due to ischaemic anterior optic neuropathy following massive gastrointestinal bleeding from a peptic ulcer. 5 Another possibility, in this patient, is that a retrobulbar haemorrhage caused optic nerve compression. A computed tomography scan of the orbit could have confirmed or ruled out this possibility, but this facility was not available at our centre.
This case highlights the need for prompt conveyance of snakebite victims to hospital. Early administration of anti-snake venom and blood transfusion could have saved this boy's sight.