Title: Cobalt-Chromium Metallosis With Normal Electroretinogram
The popularity of Metal-on-Metal (MoM) hip implants, which are typically made out of cobalt and chromium, has made cobalt poisoning of the eye more common. This condition is called ocular cobalt toxicity. This study focuses on a 66-year-old-man whose vision was damaged because he got cobalt poisoning from a MoM hip implant. The authors tried to determine if his vision loss was caused by damage to the optic nerve, the retina, or both.
The authors used multiple types of eye exams, including the Humphrey Visual Field (VF) exam, an electroretinogram (ERG), a multifocal electroretinogram (mfERG), multifocal visual evoked potentials (mfVEP), and an optical coherence tomography (OCT) scan.
The patient had blurred vision, poor color vision, and a type of abnormal blind spot called cecocentral scotoma. Because he had problems with his right eye since childhood, only the test results from his left eye were considered in the study. His ERG results were normal, his mfERG results were normal, and his mfVEP results were normal. However, his mfVEP exam showed a type of visual malfunction called decreased central amplitudes. The OCT scans also appeared to be fully normal.
Decreased color vision and blind spots are indicators of nerve damage caused by chemical poisoning. Additionally, decreased central amplitudes are also a sign of nerve damage due to chemical poisoning. Therefore, the doctors believe the patient had an early stage of optic nerve poisoning that could not yet be detected by looking at the retina through an OCT scan.
It is not yet known how exactly a failed MoM implant causes eye damage. Cobalt and chromium leak into the bloodstream and the tissue surrounding the implant, causing high levels of metal in the blood. Typically, within 2 years, these patients develop hearing loss, visual loss, hypothyroidism, and heart disease.
Among other visual impairments, patients with cobalt poisoning have experienced damage to the optic nerve and problems with blood flow in the retina. Additionally, these patients’ ERGs usually yield abnormal results. An ERG is an eye exam that measures how the retina responds to light stimulation.
A 66-year-old man received a ceramic-on-ceramic hip replacement. Three years later, his implant broke and was replaced with a metal on metal hip. Nine months later, he realized that traffic lights looked orange to him. He had a hard time seeing at a distance and close-up, and he had to get a hearing aid. He also had swelling in his abdomen, ankle swelling, left hip pain, pale diarrhea, a rash, poor coordination, and a loss of mental clarity.
One year after his revision surgery, vision in his left eye was 20/30. He could not correctly identify any colors, but a funduscopy (an examination of the inner structure of the eye) showed that everything about his physical eye structure was normal.
Many tests and scans were performed on the patient. OCT and ERG scans all showed normal results. However, the mfVEP exam showed a type of visual impairment called central depression of amplitudes. Testing of his visual field showed another type of visual impairment called cecocentral depression.
The patient’s cobalt levels in his blood were 1,078 μg/L; acceptable levels are less than .3 μg/L. He received another revision surgery for his hip implant because he evidently had cobalt poisoning. During surgery, the doctors found ceramic fragments in his joints, and there was significant wear on the implant. The cobalt levels around his joint were above the maximum recordable level of 1,000.0 μg/L.
Following this revision surgery, the patient’s cobalt levels got progressively lower, and his vision continually improved until going back to normal.
This case is different from other reports of cobalt toxicity in the retina. In other cases, abnormalities were detected in the ERG scans. However, this patient had normal ERG and OCT results.
It is unclear whether the patient’s problem was fundamentally caused by the retina or the optic nerve. It is possible that the patient was experiencing poisoning of his retina that could not yet be detected by ERG scans. On the other hand, some of his symptoms suggest that his optic nerve was damaged due to cobalt poisoning.
In conclusion, the authors believe they may have detected a type of optic nerve poisoning that could not yet be physically detected on an OCT scan. Therefore, just because a patient with a metal on metal hip implant has normal ERG results does not mean he or she does not have cobalt poisoning, especially when other symptoms are present. Other symptoms to look out for include ringing in the ears (tinnitus), deafness, poor coordination, depression, heart failure, and hypothyroidism. It is important that eye doctors perform various types of tests and scans on patients with suspected cobalt poisoning.
Grillo, L., Nguyen, H., Tsang, S., Hood, D., Odel, J. Journal of Neuro-Opthalmology 2016;36:383-388.
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