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Osteopetrosis (Albers-Schonberg disease, marble bones) is a relatively rare disease that is characterised by increased skeletal mass and bone density.1 It results from a defect in the development or function of osteoclasts with consequent impairment of bone resorption. The defect may be intrinsic to the osteoclast lineage or the mesenchymal cells that support the development and activation of the osteoclasts. Osteopetrosis is inheritable, and four clinical forms have been distinguished: autosomal-recessive malignant, autosomal-dominant benign, mild autosomal-recessive, and autosomal-recessive osteopetrosis with renal tubular acidosis. Of the four, the first two are the most prevalent.1 The disease is characterised clinically by multiple fractures, abnormally shaped bone, and anaemia. Its neurological manifestations include cerebrovascular complications, cranial nerve palsies, papilloedema, and blindness from optic nerve atrophy.2-5 Optic nerve atrophy is common and can result from the chronic effects of papilloedema or compression by a narrowed optic canal. Optic neuropathy associated with papilloedema can be prevented by aggressive management of intracranial pressure (ICP), whereas that associated with narrowing of the optic canal is usually treated by neurosurgical decompression.4 5
A 19 year old man, diagnosed with autosomal recessive osteopetrosis at about 5 months of age, presented in March 1997 with a dramatic decline in vision. He previously had had visual acuity of 20/30 in his right eye, 20/50 in his left eye, and full visual fields for most of his life. A brain CT in 1986 showed no optic canal narrowing. In 1994, he developed increased ICP and underwent a left optic nerve sheath fenestration and placement of a lumboperitoneal shunt (LPS). His vision remained normal until August of 1996 when he began to experience declining vision. He was referred to the Johns Hopkins Hospital in March of 1997.
Visual acuity with correction was 20/200 in each eye. Near vision was 20/400 in each eye. Visual fields were limited in each eye to a tiny paracentral area of about 5 degrees. Colour vision was markedly impaired, with the patient being unable to identify any of the figures on the Hardy-Rand-Rittler (HRR) pseudoisochromatic plates. Pupils were equal and reactive to light, and there was a left relative afferent pupillary defect of 0.3 log units when measured using a neutral density filter. Extraocular movements were normal. Ophthalmoscopy disclosed bilaterally pale optic discs.
Non-contrast CT of the head showed marked diffuse thickening of the calvarium with a ground glass appearance. The bony dysplasia involved the skull base, and there was narrowing of both optic canals, the petrous carotid canals, the internal auditory canals, and the cochlear and vestibular apparatus (figure A). There was also ossification of the mastoid and frontal sinuses. The CT also showed evidence of increased ICP, including an effaced third ventricle. An indium radiotracer study showed that the LPS catheter was patent, but ultrasonography demonstrated bilateral enlargement of the retrobulbar optic nerves and a positive 30 degree test, consistent with increased ICP, and a lumbar puncture disclosed an opening pressure of 450 mm Hg, with normal CSF contents.
Consideration was given to treating the patient with acetazolamide, but because of the severity of visual loss associated with pale optic discs, and because it was unclear if his decreased visual function was caused by compression of the optic nerves by the narrowed optic canals or increased ICP, it was decided to perform bilateral non-simultaneous optic canal decompressions combined with a cranial vault expansion. A bicoronal incision was made, a full thickness scalp flap was turned down to the level of the superior orbital rims bilaterally, and a large bifrontal bone flap was removed. The roof of the right optic canal was then removed along its entire length using a high speed drill and curettes. The bone flap, which was 3 cm thick, was thinned to about 1 cm and replaced, producing a significant cranial expansion.
Four days after surgery, the patient's visual acuity had improved to 20/30 bilaterally, he could correctly identify figures on seven of 10 HRR plates with the right eye and six of 10 colour plates with the left eye, and his visual fields were markedly expanded, almost to normal. A postoperative CT confirmed complete unroofing of the right optic canal (figure B).
Osteopetrosis related visual loss is often ascribed to optic nerve compression secondary to the narrowing of the optic foramina. However, optic nerve dysfunction can also result from the effects of increased ICP. Because our patient's unilateral optic canal decompression resulted in bilateral improvement in visual acuity and visual fields, it is reasonable to conclude that increased ICP and not narrowing of the optic canals was the cause of his visual deterioration. Thus, the cranial vault expansion that was performed in addition to the unilateral optic canal decompression was responsible for the rapid and dramatic improvement in the patient's visual function.
This case provides an important lesson on the evaluation of any patient with optic neuropathy that is presumed to be secondary to narrowing of optic canals in the setting of one of the craniostenoses. Although direct compression may indeed be primarily responsible for visual deterioration in patients with osteopetrosis and related conditions, increased ICP, related to either thickening of the skull or secondary occlusion of one of the cerebral venous sinuses, should always be considered a potential aetiology, and aggressively treated when identified or suspected.