1. Introduction

Bone fluorosis due to high fluoride contents in water and soil is endemic in North Africa and India. Neurological complications are rare. They consist of nerve root or spinal cord compression by bony excrescences, which predominate at the cervical spine. We report a new case of spinal cord compression due to ossification of the posterior vertebral ligament in a patient with bone fluorosis.

2. Case-report

This 49-year-old woman from southern Tunisia experienced increased difficulty in walking, bony pain in the limbs, and urinary incontinence. Her medical history was unremarkable. Physical examination showed brownish discoloration of the teeth and marked pyramidal signs in all four limbs. Standard radiographs of the spine disclosed diffuse vertebral sclerosis and ossification of the posterior longitudinal ligament along the cervical, thoracic, and lumbar spine. Ossification of the ligament was confirmed by computed tomography. Magnetic resonance imaging (MRI) disclosed compressionof the cervical spinal cord (Fig. 1aec). A radiographic survey showed ossification of the interosseous membranes of the forearms and legs. Fluoride poisoning was suspected based on the geographic origin of the patient, brownish discoloration of the teeth, and imaging findings. Fluoride assays using a fluoride ion electrode showed marked elevation in serum (253 mg/L; normal, <30 mg/L) and urine (22.9 mg/g creatinine; normal, <0.5 mg/g creatinine). Findings were normal from tests for renal and endocrine function. The extensive lesions and major risk of spinal instability prompted multilevel decompression laminectomy at the cervical spine. After surgery, the pain abated but the neurological impairments remained unchanged.

3. Discussion

Fluorosis is endemic in several areas of central and southern Tunisia. The diagnosis relies on the presence of diffuse bone sclerosis; ossification of interosseous membranes, tendon insertion sites, and intervertebral disks; and high levels of fluoride in serum (>0.2 mg/L) and/or urine (“1.8 mg/24 h). Bony pain predominating in the lower limbs followed by the spine is a common presenting syndrome. The pain is probably caused by microcracks, which may be visible by radionuclide scanning or MRI. The presenting symptom in our patient, however, was neurological impairment caused by spinal cord compression. This complication occurs in about 10% of patients with bony fluorosis, usually at the most advanced stage of the disease after prolonged exposure to very high fluoride doses. The cervical spine is the usual site of compression. The largest reported series of cervical myelopathy caused by bony fluorosis was established in India. Spinal cord compression is less common at the thoracic and lumber levels.

Fluoride replaces the hydroxide ion within the hydroxyapatite crystal. In addition, fluoride stimulates osteoblast activity, markedly increasing the formation of bone. Subperiosteal bone formation, a hallmark of the disease, causes bone thickening and may narrow the spinal canal and foramina. Ossification may occur at sites of tendon and ligament attachment, most notably at the spine. Neurological impairments complicating chronic fluorosis are often irreversible. Early in the disease, however, a favorable outcome may be achieved provided fluoride exposure is stopped. In our patient, factors involved in chronic fluoride poisoning and myelopathy included residence in an endemic area, strenuous manual labor in a hot climate leading to ingestion of large amounts of fluoride-laden water, and carrying heavy loads. Compression of the spinal cord and nerve roots in patients with advanced bony fluorosis is best visualized by MRI. High-intensity signal from the spinal cord on T2 images and low-intensity signal on T1 images may predict poor recovery after surgery. Surgery is mandatory in patients with spinal cord compression due to fluorosis. The outcome depends on the extent of the lesions, duration of poisoning, and risk of decompensation in the segments above and below the surgical site. Diagnostic delay and prolonged fluoride exposure are associated with poor outcomes.