1. Introduction

With the growing use of magnetic resonance imaging (MRI), neurosurgeons are increasingly faced with a scenario in which the clinical picture of the patients is inconsistent with their radiographic findings. One of these is acute spinal cord injury in patients with pre-existing silent ossification of the ligamentum flavum (OLF), a clinical scenario with only seven previously published examples within the medical literature [1-5]. Herein, we present a new example in a 58-year-old woman.

2. Case Presentation

A 58-year-old female presented with acute lower limb weakness following a trivial fall. Fourteen months before admission, an asymptomatic thoracic OLF at the T10-T11 spinal level was incidentally discovered on her thoracolumbar MRI. The initial presentation was due to a complaint of back and lower back pain, and periodic follow-ups were proposed (Figure 1 and 2). 







On admission, lower extremity weakness was evident with an American Spinal Cord Injury Association (ASIA) impairment score of C along with a vague sensory level up to the umbilicus. In cases where plain radiographs of the spine were normal, a new MRI showed an apparent myelopathic patch at the T10-T11 level without significant change in the size of OLF (Figure 3).




A computerized tomography scan of the affected region showed a bilateral OLF with significant canal compromise (Figure 4). 



Surgical intervention

In surgical intervention, following laminectomy of the affected region, the corresponding OLFs were excised. At an 8-week follow-up, she could ambulate with support. At a 6-month follow-up, her neurological status improved significantly with an ASIA impairment score of E and no canal compromise was observed at T10-T11 OLF in her post-operative MRI (Figure 5). 



3. Discussion 

Symptomatic OLF is a relatively rare condition that affects the thoracic, lumbar, and cervical spine with decreased frequency [6-8]. This pathology, which occurs mostly in the lower thoracic spine, is a well-known entity in East Asian countries, although it is reported with increasing frequency in the Iranian population [9, 10].

Prevalence

With the growing use of modern neuroimaging tools, the number of subjects with silent thoracic ossification of the ligamentum flavum (TOLF) is increasing. According to three broad epidemiologic studies, the prevalence rate of asymptomatic TOLF in Chinese, Japanese, and Korean populations was 63.9%, 36%, and 16.9%,  respectively [11-13]. T10-T11 was the most affected in all these surveys [11-13].

The natural history of asymptomatic TOLF 

Natural history data on asymptomatic patients with TOLF was reported by Zhai et al. based on the data from a prospective study [14]. Accordingly, the mean rate of OLF thickness progression has been 1.2 to 1.6 mm per year [14]. In a patient with an asymptomatic TOLF, even a minimal traumatic event may result in an acute spinal cord injury without radiological and tomographic  evidence of fracture /dislocation, a rare scenario confined to 8 cases, including the current case (Table 1).




The presented case is unique and different from the other previously published cases concerning thoracic MRI features before the trivial traumatic event.
Our review showed that the age of patients ranged between 30 and 77 years, with a mean of 59.1 years at the time of injury [1-5]. Accordingly, men were more likely to be affected by this rare complex injury (5 out of 8). The location of the corresponding OLFs was at the lower thoracic region in all cases [1-5]. One patient became paraplegic (ASIA: A) following the traumatic event and 7 became paraparetic (ASIA: C). However, emergent surgical intervention was performed in two cases with complete recovery and in the remaining 6 cases with an acceptable degree of neurological recovery [1-5]. 

Differential diagnosis

Acute spinal cord injury without radiological and tomographic evidence of fracture or dislocation is not confined to TOLF and has been described in acute traumatic disc herniation, traumas in patients with ossification of the posterior longitudinal ligament (OPLL), and in post-traumatic epidural hematomas [15-17].

Surgery 
 

Asymptomatic patients with thoracic stenosis due to silent OLF are at risk of acute spinal cord injury even after trivial traumatic events. For asymptomatic patients in whom silent TOLF is incidentally found, periodic clinical follow-up and avoidance of high-risk activities are highly indicated. However, prophylactic surgery should be proposed once canal compromise is around or more than 40% [17]. Furthermore, acute spinal cord injury in a patient with this combined scenario requires emergent surgical intervention. Finally, postoperative neurological recovery is not confined to patients with incomplete neurological deficits, but even those patients with paraplegia may benefit from surgery.