Received: 25 May 2023 Revised: 19 September 2023 Accepted: 21 September 2023 DOI: 10.1002/vrc2.741 CASE REPORT Companion or pet animals Clinical and imaging features of a feline lumbar myelopathy resembling ossification of the posterior longitudinal ligament in humans Mar Reus-Coll Sònia Añor, Martí Pumarola Ignacio Montes de Oca Cristian de la Fuente, 1 Hospital Veterinari Montjuïc (Vetpartners), Abstract Barcelona, Spain A 9-year-old, male, neutered Russian blue cat was presented for a 2-week history of 2 Fundació Hospital Clínic Veterinari, Universitat pelvic limb weakness and reluctance to jump and climb stairs. Neurological exami- Autònoma de Barcelona, Bellaterra, Barcelona, Spain nation was consistent with an L4–L6 myelopathy. Ectopic calcification of soft tissues 3 within the vertebral canal involving the dorsal longitudinal ligament over the L5–L6 Departament de Medicina i Cirugia Animals, Facultat de Veterinària, Universitat Autònoma de vertebrae was diagnosed based on radiography, magnetic resonance imaging and com- Barcelona, Bellaterra, Barcelona, Spain puted tomography. The lesion was surgically excised, and histopathological examination 4 Unitat de Patologia Murina i Comparada, demonstrated fibrous connective tissue with calcification. The patient was neurologi- Departament de Medicina i Cirurgia Animals, cally normal 2 months after surgery, and no relapses have occurred in a follow-up period Facultat de Veterinària, Universitat Autònoma de Barcelona, Bellaterra, Barcelona, Spain of 2 years. The findings in this case had not been previously reported in cats, and share some similarities with a clinical entity in humans named ossification of the posterior Correspondence longitudinal ligament. Sonia Añor, Fundació Hospital Clínic Veterinari, Universitat Autònoma de Barcelona, 08193, K E YWOR D S Bellaterra, Spain. cats, dorsal longitudinal ligament, ectopic calcification, spinal cord Email: sonia.anor@uab.es BACKGROUND DISH in dogs and cats are reported to cause calcification of the dorsal longitudinal ligament. Ossification of the posterior longitudinal ligament (OPLL) is a The following case report describes the clinical features, clinical entity described in human medicine caused by ectopic multimodal imaging findings, surgical treatment and out- calcification within this ligament. This hyperostotic condition come of a feline patient with an L4–L6 myelopathy that shares causes spinal cord compression more commonly in the cervi- striking similarities with OPLL in humans. To the authors’ cal spine, but it can also affect the thoracic and lumbar spine. knowledge, this entity has not been previously described in Although 5% of human cases are asymptomatic, most show veterinary medicine. signs of myelopathy or radiculopathy that are variable depend- ing on the affected area and the severity of vertebral canal stenosis.1 The pathogenesis of OPLL remains obscure, and a CASE PRESENTATION multifactorial aetiology including genetic and environmental factors has been suggested. Some authors have also suggested A 9-year-old, male, neutered Russian blue cat was presented that the condition in humans is a variant of diffuse idiopathic for a 2-week history of progressive pelvic limb weakness and skeletal hyperostosis (DISH).1,2 reluctance to jump and climb stairs. According to the owner, In veterinary medicine, the most common hyperostotic the patient showed clinical signs of pain/discomfort. disorders that affect the vertebral column are spondylosis On presentation, the physical examination was consid- deformans and DISH.3 DISH is a systemic disease charac- ered normal. Neurological examination revealed a mild terised by ossification of soft tissues including the ventral ambulatory paraparesis, proprioceptive deficits and reduced longitudinal ligament and has been reported in one cat.4 patellar reflexes in both pelvic limbs. Pain was elicited upon Although DISH in dogs often remains clinically silent, the palpation of the caudal lumbar spine. The remaining neuro- only case reported in domestic cats showed signs consistent logical examination was considered normal. Neuroanatomic with a T3–L3 myelopathy. The dorsal longitudinal ligament in localisation was consistent with an L4–L6 myelopathy, and dogs and cats is the analogue of the posterior longitudinal lig- main differential diagnoses included intervertebral disc ament in human beings. Neither spondylosis deformans nor disease, neoplasia and inflammatory/infectious diseases. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. © 2023 The Authors. Veterinary Record Case Reports published by John Wiley & Sons Ltd on behalf of British Veterinary Association. Vet Rec Case Rep. 2023;11:e741. wileyonlinelibrary.com/journal/vrc2  of  https://doi.org/10.1002/vrc2.741 20526121, 2023, 4, Downloaded from https://bvajournals.onlinelibrary.wiley.com/doi/10.1002/vrc2.741 by Readcube (Labtiva Inc.), Wiley Online Library on [27/11/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License VETERINARY RECORD CASE REPORTS  of  INVESTIGATIONS LEARNING POINTS/TAKE-HOME MESSAGES Complete blood cell count and serum biochemistry (including ionised calcium) did not reveal any abnormalities. Sero- ∙ Calcification of the dorsal longitudinal ligament logical testing for feline immunodeficiency virus and feline should be considered when calcified lesions are leukaemia virus were negative. Thoracic radiographs and seen within the vertebral canal. abdominal ultrasound were unremarkable. ∙ Combined magnetic resonance imaging and com- Lateral radiographs of the lumbar spine were performed puted tomography is a useful diagnostic imaging under general anaesthesia. The L5–L6 disc space was slightly approach to assess uncommon spinal disorders, as narrowed, and a thin double radiopaque line was observed they provide complementary information. within the vertebral canal overlying the same intervertebral ∙ Surgical treatment in this case resulted in a long- disc space (Figure 1). lasting good outcome. Magnetic resonance imaging (MRI) of the lumbar verte- bral column was performed with a low-field MRI (Airis Light 0.25T, Hitachi). Sequences included sagittal and transverse T2-weighted (T2W; TR = 3200 ms, TE = 80 ms), sagittal short tau inversion recovery (STIR; TR = 2800 ms, TE = 16 ms, TI = TREATMENT 90 ms), sagittal 3D heavily T2W sequence with myelographic effect (3DFSEMyelo; TE = 8000 ms, TE = 525 ms) and sagit- A right-sided hemilaminectomy was performed over the L5– tal and transverse T1-weighted images (T1W; TR = 512 ms, L6 vertebrae. Upon opening the vertebral canal, the spinal TE = 18 ms) before and after intravenous administration of cord was observed to be severely deviated dorsally, but no paramagnetic contrast (gadoteric acid, 0.1 mmol/kg body- obvious mass or extruded disc material were visible. Careful weight; Dotarem, Guerbet). On T2W images, all the nuclei inspection of the ventral epidural space was performed using pulposi of the lumbar intervertebral discs had normal hyper- Rothon microdissectors, and a very hard epidural mass was intense appearance except for the L5–L6 and L6–L7 discs that detected in the midline overlying the intervertebral disc space. had a partial and a total loss of the T2 hyperintense signal, The lesion was firmly attached to the spinal cord, and it was respectively. There was an extradural mass lesion overlying not possible to remove it without causing excessive traction the L5–L6 intervertebral disc space causing severe compres- over the nervous tissue. A no. 11 scalpel blade was used to sion of the spinal cord. On the sagittal plane, the lesion had a gently separate the mass from the dura mater and from the peripheral thin rim that was hypointense on T2W, T1W and floor of the vertebral canal. A plaque of white calcified material STIR sequences, consistent with calcification. The centre of attached to fibrous tissue debris was retrieved and submitted the lesion was hyperintense on T2W/STIR images and isoin- for histopathological analysis. CSF leakage was observed just tense on T1W images (Figure 2). On the transverse plane, after spinal cord decompression, and absorbable dura substi- the hypointense peripheral thin rim had a reverse C-shaped tute (Lyoplant; B. Braun Surgical) was used to repair the dural morphology (Figure 3). Moderate contrast enhancement of tear. The patient recovered uneventfully from the surgical the centre of the lesion was detected on post-contrast T1W procedure. images. Computed tomography (CT) of the lumbar spine using a two-slice scanner (Somaton Spirit, Siemens) was performed OUTCOME AND FOLLOW-UP immediately after MRI for further characterisation of the lesion. Bone and soft tissue window settings were used. Microscopically, the lesion was a proliferation of fibrous An extradural lesion with a hyperattenuating peripheral rim connective tissue with low cellularity associated with a gran- and a central hypoattenuating core was detected. On sagit- ular basophilic material (calcified tissue). There were also tal reconstruction, a double-layer hyperattenuating rim was small fragments of cancellous bone and haemorrhagic foci clearly seen overlying the L5–L6 intervertebral disc space (Figure 4c). There were no intervertebral disc material, neo- (Figure 4a,b). No other ectopic calcification/ossification sites plastic cells or inflammatory reaction. These findings were were detected. suggestive of a diagnosis of calcification/ossification of the A cerebrospinal fluid (CSF) sample was collected from the dorsal longitudinal ligament. cisterna magna, and CSF analysis was unremarkable (three The patient was rechecked periodically by the neurology nucleated cells/µL, total protein concentration 19 mg/dL; personnel, and 2 months after surgery, the neurological exam- reference ranges: <5 nucleated cells/µL and <25 mg/dL). ination was normal. No relapses were observed in a follow-up period of 2 years. DIFFERENTIAL DIAGNOSIS DISCUSSION Based on the clinical and imaging findings, the presump- tive diagnosis was compressive myelopathy caused by ectopic The findings described in this case report are consistent with calcification/ossification of soft tissue within vertebral canal an ectopic calcification of the dorsal longitudinal ligament, involving the dorsal longitudinal ligament. Intervertebral causing a lumbar myelopathy in a feline patient. The imaging disc protrusion with atypical imaging findings, a calcified findings are very similar, if not identical, to those described in inflammatory/infectious granuloma or, less likely, a calcified OPLL in humans.1 According to the human classification sys- neoplasia could not be completely ruled-out. tem, which relies on lateral radiography, there are four OPLL 20526121, 2023, 4, Downloaded from https://bvajournals.onlinelibrary.wiley.com/doi/10.1002/vrc2.741 by Readcube (Labtiva Inc.), Wiley Online Library on [27/11/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License  of  VETERINARY RECORD CASE REPORTS F I G U R E  Lateral radiography of the lumbar vertebral column, showing a double radiopaque line overlying the L5–L6 intervertebral disc space within the vertebral canal (white arrows). F I G U R E  Magnetic resonance imaging of the lumbar spine: T2-weighted (T2W) sagittal (a) and T1-weighted (T1W) sagittal (b) image. Extradural mass lesion dorsal to the L5–L6 intervertebral disc space causing a severe compression of spinal cord. The lesion had a peripheral thin rim hypointense in T2W and T1W. The centre of the lesion is hyperintense in T2W and isointense in T1W images. F I G U R E  Magnetic resonance imaging of the lumbar spine: T2-weighted (T2W) transverse (a) and T1-weighted (T1W) transverse (b) image. The extradural mass lesion has a peripheral thin rim hypointense in T2W and T1W images and a reverse C-shaped morphology. variants: continuous, segmental, localised and mixed.1 As the phied non-ossified ligament).6 Surgical treatment of OPLL dorsal longitudinal ligament in dogs and cats is the analogue may result in a dural defect that often causes CSF fistulas, of the posterior longitudinal ligament in humans, the case because the ossified ligament is firmly attached to or can reported here would be similar to the localised form of the even penetrate the dura mater. The presence of a double- disease in which ligament ossification is distributed over the layer sign is important, because it is associated more often intervertebral disc space without involvement of the vertebral with the presence of a dural defect than the single-layer body. sign.1,5–7 Furthermore, OPLL can be subclassified into single-layer In the case reported here, the double-layer sign was clearly and double-layer type depending on the morphology assessed identified on CT images, and the lesion was confirmed to be by CT scan.5,6 The single-layer sign consists of a single homo- tightly adherent to dura mater during surgery. geneous ossified posterior longitudinal ligament mass and the The main imaging differential diagnosis for a calcified double-layer sign describes anterior and posterior hyperat- extradural mass overlying the intervertebral disc space is tenuating rims (corresponding to ossification), separated by intervertebral disc extrusion. The presence of a double-layer a central hypoattenuating area (corresponding to hypertro- sign on the CT images, as well as the presence of a calcified 20526121, 2023, 4, Downloaded from https://bvajournals.onlinelibrary.wiley.com/doi/10.1002/vrc2.741 by Readcube (Labtiva Inc.), Wiley Online Library on [27/11/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License VETERINARY RECORD CASE REPORTS  of  F I G U R E  Computed tomography (CT) of the lumbar spine: sagittal reconstruction (a) and transverse CT image at the level of the L5–L6 vertebrae (b). An extradural lesion with hyperattenuating peripheral rim and central hypoattenuation (double layer sign). (c) Microscopic appearance of fragments of the dorsal longitudinal ligament. Note the presence of several ossified focal areas between fascicles of collagen fibres (white asterisks). Haematoxylin–eosin stain; bar: 200 µm. thin rim extending over the vertebral bodies beyond the F U N D I N G I N F O R M AT I O N vertebral epiphyses, could help to differentiate between cal- The authors received no specific funding for this work. cification/ossification of the dorsal longitudinal ligament and calcified intervertebral disc material. E T H I C S S TAT E M E N T The multimodal imaging diagnosis approach used in this The presented case report fulfils the high ethical standards case is also used in human medicine. MRI is helpful in concerning animal welfare. All investigations and treatments detecting presence of myelomalacia secondary to compres- were performed with the client’s consent. sion, the extent of spinal cord compression and the presence of foraminal stenosis.1 ORCID In human medicine, the pathogenesis of OPLL is poorly Sònia Añor https://orcid.org/0000-0002-1099-7698 understood, and some authors have suggested that OPLL is REFERENCES a variant of DISH.1,2 There is only one case of feline DISH 1. Abiola R, Rubery P, Mesfin A. Ossification of the posterior longitu- reported, and the imaging findings are the same as those dinal ligament: etiology, diagnosis, and outcomes of nonoperative and described in dogs.4 The case reported here does not fulfil operative management. Global Spine J. 2016;6:195–204. the diagnostic criteria for DISH, and to date, involvement of 2. Bakhsh W, Saleh A, Yokogawa N, Gruber J, Rubery P, Mesfin A. 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Bone window computed unlikely because the cat was on a well-balanced diet, the lesion tomography for detection of dural defect associated with cervical ossified was focal and there has been a long follow-up period without posterior longitudinal ligament. Neurol Med Chir. 1997;37:173–76. signs of relapse or other clinical signs. 6. Min, JH, Jang, JS, Lee, SH. Significance of the double-layer and single- To the authors’ knowledge, this case, supported by the layer signs in the ossification of the posterior longitudinal ligament of the cervical spine. J Neurosurg Spine. 2007;6:309–12. imaging and histological findings, is the first description of 7. Epstein, NE. Identification of ossification of the posterior longitudi- a feline lumbar compressive myelopathy resembling OPLL in nal ligament extending through the dura on preoperative computed humans. tomographic examinations of the cervical spine. Spine. 2001;26:182–86. 8. Seawright A, English PB. Deforming cervical spondylosis in the cat. J AU T H O R C O N T R I B U T I O N S Pathol Bacteriol. 1964;88:503–9. Mar Reus-Coll wrote the manuscript. Ignacio Montes de Oca, Mar Reus and Cristian de la Fuente acquired and interpreted the imaging part. Marti Pumarola performed histopatholog- How to cite this article: Reus-Coll M, Añor S, ical study. Sonia Añor and Cristian de la Fuente conceived, Pumarola M, Montes de Oca I, de la Fuente C. Clinical designed and supervised the project. All authors read and and imaging features of a feline lumbar myelopathy approved the final version of the manuscript. resembling ossification of the posterior longitudinal ligament in humans. Vet Rec Case Rep. 2023;11:e741. C O N F L I C T O F I N T E R E S T S TAT E M E N T https://doi.org/10.1002/vrc2.741 The authors declare they have no conflicts of interest.