Spinal epidural lipomatosis (SEL) is a disease in which fat tissue present in the epidural space of the spinal canal proliferates and compresses the surrounding nerve and vascular system (7). In humans, the prevalence of SEL is 1.1% to 2.5% (13,19), and it is extremely rare in veterinary medicine where only two cases have been reported (11,16). Clinical symptoms vary depending on the type, location, severity of nerves pressed by hyperplastic fat, and concurrent spinal canal stenosis, and non-specific symptoms such as back pain, lameness, and ataxia can occur due to SEL (21). In human medicine, long-term exogenous steroid therapy, Cushing syndrome, hypothyroidism, obesity, and surgery have been suggested as causes of SEL (21). In veterinary medicine, the case of SEL secondary to long-term steroid use has recently been reported (16). SEL can be diagnosed by confirming hyperplasia of fat in the spinal canal, and magnetic resonance imaging (MRI) is the most sensitive and specific diagnostic modality (19). Since the treatment of SEL is patient-specific and clinical symptoms can be improved by treatment of the underlying cause, conservative treatment is preferentially considered before surgical correction (21).

In this report, we describe a canine case of SEL secondary to hypothyroidism, treated by managing the underlying cause with medical treatment. To the best of our knowledge, this is the first case report of SEL secondary to hypothyroidism that was successfully treated with conservative therapy.

A 10-year-old neutered male Siberian Husky dog presented to the Veterinary Medical Teaching Hospital of Chungnam National University with severe lethargy and two months of paraparesis. When clinical symptoms first appeared, the patient had no history of steroid intake, was tentatively diagnosed with intervertebral disc disease (IVDD) at a local hospital, and was treated with conservative therapy including methylprednisolone sodium succinate (MPSS) pulse therapy; The patient’s symptoms temporarily improved and then worsened again. On physical examination, the patient was unable to bear weight and walk and exhibited significant muscle mass loss in both hindlimbs and generalized symmetrical truncal alopecia with a dull coat of hair. In addition, the patient’s lethargy was so severe that he fell asleep while eating and drinking. No specific findings were observed in complete blood counts and serum chemistry profiles. Sample were sent to commercial laboratories (IDEXX Laboratories, Inc, USA) for thyroid panel testing, and decreased total T4 (0.9 μg/dL, reference range, 1.0-4.0) and free T4 (0.5 ng/dL, reference range, 0.6-3.7) and increased TSH (0.58 ng/mL, reference range, 0.05-0.42) were identified.

On neurological examination, cranial lumbar vertebral pain, hind limb cross-extensor reflex, delayed hindlimb postural reaction, upper motor neuron bladder dysfunction, and total absence of cutaneous trunci reflex were identified. Based on the results of the neurological examination, a thoracolumbar spinal cord lesion was suspected. Computed tomography (CT) revealed diffuse idiopathic skeletal hyperostosis and spondylosis deformans in the cervical, thoracolumbar, and lumbosacral regions (Fig. 1A, B). In addition, the bone mineral density of the vertebra, sternum, and skull was decreased; the lumbar vertebrae density (the most severely reduced region) decreased to approximately 60 HU.

Figure 1. CT and MRI images of the thoracolumbar spine. (A, B) Sagittal CT images of the bone window. (C, D) Sagittal T2-weighted MRI images of thoracolumbar spinal cord. (A, B) There was severe bone proliferation, diagnosed as diffuse idiopathic skeletal hyperostosis and spondylosis deformans (arrowheads), and generalized decreased bone mineral density. (C, D) There was mild multifocal bulging of the intervertebral disc (<5%, arrows).

MRI showed hyperplasia of the epidural fat compressing the spinal cord from the dorsal to ventral thoracolumbar region. Epidural fat hyperplasia was the most severe in the L1-L2 vertebrae region, and the ratio of epidural fat diameter to spinal canal diameter was 62% (Fig. 2). Concurrent mild multiple IVDD was observed in the thoracolumbar region, which was mild enough to not cause clinical symptoms alone (Fig. 1C, D).

Figure 2. MRI images of the most severely affected area by SEL. (A) Sagittal T2-weighted image. (B) Transverse T2-weighted image at the L1-2 level. (A) There was prominent epidural fat tissue dorsally in the lumbar spine (arrowheads). (B) At the L1-L2 disc level, there was a markedly compressed spinal cord, measuring 4.3 mm (yellow two-sided arrow), the epidural fat measuring 7.1 mm (red two-sided arrow), and the spinal canal measuring 11.4 mm (green two-sided arrow). The spinal cord to epidural fat index was 0.6, and the epidural fat to spinal canal index was 62%, indicating grade 2 (moderate) SEL.

Cerebrospinal fluid (CSF) analysis revealed normal nucleated cell count (3 cells/μL, reference range, <5) and protein concentration (10 mg/dL, reference range, <25), and negative polymerase chain reaction results for infectious agents. Cytological examination of the CSF revealed a predominance of mononuclear cells.

Overall, the patient was tentatively diagnosed with SEL secondary to hypothyroidism. The patient was treated with levothyroxine (Synthyroid, Bukwang Pharm., Seoul, Korea; 0.34 mg/m², PO, q 12 h), firocoxib (Previcox, Merial, Ingelheim am Rhein, Germany; 1.14 mg/kg, PO, q 12 h), gabapentin (Gabapentin, Dong-A Pharm., Seoul, Korea; 10 mg/kg, PO, q 12 h), and esomeprazole (Nexium, AstraZeneca, Cambridge, UK; 1 mg/kg, PO, q 12 h). Rehabilitation therapy was initiated using passive range of motion and a sling. After the initiation of therapy, the patient’s lethargy markedly improved. Neurological signs gradually improved, and the patient showed normal ambulation 40 days after treatment initiation. To rule out Cushing’s syndrome, an ACTH stimulation test was conducted at a local hospital, and no specific finding was found. When visiting the hospital for a check-up after 200 days, there were no clinical signs related to hypothyroidism, and total T4 was also identified within the normal range (3.5 μg/dL, reference range, 1.0-4.0). On repeated neurological examinations, all abnormal findings observed before treatment disappeared, and there were no specific findings except for mild hyperesthesia in the thoracolumbar region. Unfortunately, CT and MRI could not be taken due to hypotension during anesthesia. There was no symptom recurrence during the 12-month follow-up period.

SEL was first described in human medicine by Lee et al. in 1975 (10). Since then, many studies have reported the clinical characteristics, prevalence, pathophysiology, and treatment of SEL (7,19,21). In veterinary medicine, there have been no related reports except for the studies on idiopathic SEL in a dachshund dog (11) and steroid-induced SEL in a Eurasian dog (16). Thus, little is known regarding this disease. Considering the previous case reports and the present case, many characteristics of SEL in dogs seem to be similar to those of human SEL. In a previous case report, cauda equina syndrome occurred because of the accumulation of epidural fat compressing the cauda equina in the lumbosacral spinal canal (11), and ambulatory paraparesis occurred by hypertrophic epidural fat in T8 to L3 vertebral canal segment (16). In this case, the patient's clinical symptoms were caused by hypertrophy of the epidural adipocytes pressing the spinal cord from dorsal to ventral and may be exaggerated by concurrent IVDD pressing the spinal cord from ventral to dorsal. Therefore, clinical symptoms of SEL in dogs can be caused by excessive epidural fat directly compressing the spinal cord and/or nerve root, and can be exacerbated by concurrent spinal diseases such as spinal canal stenosis and IVDD.

In human medicine, a clinical diagnosis of SEL can be made by confirming an increase in intraspinal epidural fat through MRI scans, excluding diseases that may be characterized or similar to the increase in fat, such as intradural lipoma and angiolipoma (5,18,19). Considering the definition of the disease, the diagnostic criteria from the human literature can be applied to veterinary patients. In human medicine, SEL is divided into grades 0 to 3 according to the degree of severity shown in MRI images, and through this, a diagnosis can be made more clearly (1). Although there is controversy over whether clinical symptoms worsen according to grading, a recent study diagnosed only grade 2 or higher SEL (19). Currently, there are no accurate standards in veterinary medicine. Therefore, for an accurate diagnosis, a study on the range of normal fat in the spine and the correlation between clinical symptoms and severity is necessary.

It is a well-known fact that the thyroid hormone is the main regulator of lipid metabolism, and it directly or indirectly acts on all processes such as the synthesis, mobilization and degradation of lipids (3,14). Thyroid hormones induce lipolysis by increasing the sensitivity of adipose tissue to lipolytic hormone (12). These hormones also increase non-esterified fatty acids oxidation and are involved in lipoprotein metabolism (14). Therefore, in hypothyroidism patients, the lipolytic sensitivity decreases and the synthesis increases compared to the degradation rate of fat, resulting in dyslipidemia and overgrowth of fat normally present in the body (3,9). Hypothyroidism can reduce metabolism and cause obesity, which is thought to cause SEL through chronic inflammation (4,15). However, there is controversy over whether hypothyroidism is a major risk factor for SEL or simply a surrogate for obesity (19), so further study is needed. In this case, the patient showed typical clinical signs of hypothyroidism including severe lethargy, symmetrical alopecia and dull hair. Although it may have been affected by the MPSS injection, two weeks have passed since the last injection, and considering that TSH was also elevated, the results of the thyroid panel showed that hypothyroidism was more suspected than drug interaction or Non-thyroidal illness (17). In addition, considering that the patient responded well to levothyroxine treatment, we diagnosed that the patient had hypothyroidism. Furthermore, there was no history of steroid intake when the patient’s symptoms first occurred, and even the symptoms were temporarily alleviated after steroid injection. In addition, there was also no evidence to support the diagnosis of Cushing’s syndrome. Considering that SEL is suspected to be associated with abnormal lipid metabolism (6), neurologic symptoms improved after treatment of hypothyroidism, and previous case reports of SEL related to hypothyroidism in humans (8,20), we surmised that the SEL of this patient was related to hypothyroidism.

In the present case, the patient was diagnosed with SEL concurrent with mild IVDD. It is uncommonly reported in humans, and it is known to be important since clinically insignificant concurrent lesions can exacerbate symptoms because SEL reduces the accommodating ability of the spinal cord, making it vulnerable to compression caused by concomitant diseases (2). In such cases, it is hard to discern whether the patient's major clinical symptoms are caused by, so it may be difficult to plan overall treatment strategies, such as surgery, especially in veterinary patients and evaluate treatment responses. This patient was specifically treated for hypothyroidism and conservatively treated for relieving pain from myelopathy at the same time; Unlike when only IVDD was treated at the local hospital, significantly favorable results were obtained after treatment for SEL. Moreover, the myelopathy-associated pain in this patient was also considered to be highly likely due to SEL based on the results of neurological examination and MRI results. Therefore, until further research is conducted, it is recommended that SEL and concurrent spinal cord diseases should be treated simultaneously regardless of their severity.

This report describes the clinical characteristics, diagnostic imaging findings, and therapeutic response to SEL in a Siberian Husky dog. We found that SEL can occur secondary to hypothyroidism in dogs and can be managed with medical treatment. Therefore, the underlying cause of SEL should be specifically investigated and treated. In addition, if SEL occurs concurrently with other spinal cord diseases, both diseases should be treated for better management.

This study received no external funding.

This research was supported by the National Research Foundation of Korea, funded by a grant from the Korean Government (NRF-2022R1G1A10036821131482092640101).

The authors have no conflicting interests.