Magnetic Resonance Imaging Characteristics of Hereditary Polymyositis in the Dutch Kooiker Dog

Abstract

Background: Hereditary immune-mediated polymyositis has been reported in the Kooiker dog breed, associated with a 39 kb deletion and low penetrance. Approximately 10–20 percent of homozygous dogs and 0.5–2 percent of heterozygous dogs develop polymyositis. This study examines whether magnetic resonance imaging (MRI) can assist in diagnosing polymyositis in this breed. Methods: All dogs in this prospective case study were purebred Kooiker dogs referred for clinical examination to assess them for polymyositis. A dataset was compiled, including sex, neuter status, and, if applicable, age of onset, clinical signs, CK activity, electromyogram, and histopathological findings. MRI was performed using a 1.5 Tesla MRI scanner, with T1-weighted, T2-weighted, T2W fat-suppressed short tau inversion recovery (STIR), and T1-weighted post-contrast sequences. Results: Five Kooiker dogs were included in the study. Four dogs exhibited clinical signs compatible with polymyositis (one heterozygous and three homozygous for the 39 kb deletion), while one dog was homozygous for the 39 kb deletion but showed no clinical signs. The clinically affected dogs exhibited T2-weighted, STIR, and T1-weighted post-contrast muscular hyperintensity, and the diagnosis was confirmed with histopathology. The asymptomatic dog displayed no MRI abnormalities. Conclusions: MRI has proven to be a valuable tool in assisting with the diagnosis of Kooiker dogs carrying the 39 kb deletion. MRI can act as a screening tool for dogs with the 39 kb deletion, eliminating the need for an initial biopsy. A muscle biopsy, following a confirmatory MRI, is still the preferred method for diagnosing polymyositis.

1. Introduction

Hereditary immune-mediated polymyositis in the Dutch Kooiker dog (official name ‘Het Nederlandse Kooikerhondje’) is a debilitating disease with a poor outcome [1]. A recent study published the results of a genome-wide association study and next-generation sequencing, detailing the presence of a 39 kb DNA fragment deletion located 10 kb upstream of the interleukin gene pair IL21/IL2, which is associated with this disease [2,3]. This breed, which has a low population density, is currently affected by two other genetic diseases, hereditary myelopathy [4,5] and von Willebrand factor deficiency [6]. Polymyositis is now this breed’s third documented inherited disease [1,7]. Although a breeding programme, fit2breed, has been developed to reduce the frequency of the 39 kb deletion, it may take years before the frequency decreases [8]. The frequency of the deleted allele was calculated in 2024 to be 0.81 in affected cases and 0.25 in a random sample of Dutch Kooiker dogs. This indicates that the penetrance of the disease phenotype is estimated to be between 10–20% for homozygous dogs and 0.5–2% for those that are heterozygous for the 39 kb deletion [3]. The estimated number of carriers is approximately 4900 Kooiker dogs in the current population, with over 650 dogs being homozygous. Over the last four decades, approximately 180 affected dogs have been identified, which aligns with the low penetrance [2]. Diagnosis is made based on clinical signs, an elevated serum creatine kinase (CK) activity, and confirmatory histopathology of muscle biopsies [1]. Three clinical presentations have been identified: (1) locomotory, with signs such as inability to walk long distances, difficulty getting up, stiff gait, and walking on eggshells; (2) signs of dysphagia, such as drooling and difficulty eating and/or drinking; and (3) a combination of locomotory signs and dysphagia [1]. Serum CK activity is elevated in almost all cases, but it may be normal, especially in the early stages of the disease [1]. Muscle biopsies are the current diagnostic gold standard, but they are invasive [9,10,11]. Although a DNA test has been developed, the low penetrance of the mutation suggests that only a small proportion of heterozygous and homozygous Kooiker dogs will become clinically affected by polymyositis [3]. Monitoring all homozygous Kooiker dogs, for instance, with yearly measured CK activity can help identify Kooiker dogs developing polymyositis. However, it is possible for a Kooiker dog to develop polymyositis even if the CK activity has not yet increased. The next step is to take a muscle biopsy that requires surgery. Consequently, a safe and clinically relevant diagnostic screening test would be advantageous for these dogs prior to performing a muscle biopsy.

In human medicine, diagnostic imaging has become an essential non-invasive tool that aids in diagnosing, assessing disease extent, and monitoring progression or remission in inflammatory myopathies [12]. The imaging modalities that may offer the greatest clinical utility in myositis are magnetic resonance imaging (MRI), ultrasound, and positron emission tomography (PET) [12]. MRI is preferred as it can simultaneously identify the presence and extent of an inflammatory myopathy [12]. In human imaging, T2-weighted (T2W) and T2W fat-suppressed short tau inversion recovery (STIR) sequences are routinely used, as muscle oedema and fat replacement are both observed in inflammatory myopathy and can be visualised using these sequences [12,13,14]. Other modalities include Water T2 (T2H20) maps, which assess the status of residual skeletal muscle tissue independently from irreversible end-stage fatty replacement, and PET, which provides information on cellular muscle function and pathology, but they are not readily available in veterinary practice [12]. In veterinary imaging, T1-weighted (T1W), T2W, STIR, and T1W post-contrast sequences are available in most facilities equipped with a high-field scanner. A T1W sequence may exhibit hyperintensity in patients with muscular fat replacement, although this may be difficult to ascertain and observe. A STIR sequence identifies peri-fascial oedema, hypervascularization, and intramuscular oedema, which may be associated with inflammatory myopathies [12,13].

The use of MRI to diagnose inflammatory myopathies in dogs is not novel [15,16,17,18]. But MRI is not a substitute for a muscle biopsy; rather, it is a diagnostic tool that may aid in diagnosis. The discovery of the 39 kb deletion associated with polymyositis raises the question of whether MRI could serve as a supportive diagnostic tool for diagnosing or excluding polymyositis in Kooiker dogs, regardless of being homozygous or heterozygous for the 39 kb genetic deletion linked to this breed. This is particularly relevant given the low penetrance of the 39 kb deletion.

The objective of this study is twofold: (1) Can MRI be used to diagnose or rule out polymyositis in clinically affected Kooiker dogs; and (2) can it distinguish between affected homozygous or heterozygous dogs and unaffected homozygous dogs carrying the 39 kb deletion?

2. Materials and Methods

2.1. Dogs and Inclusion Criteria

A prospective case study design was selected. First, a Kooiker dog that showed no clinical signs but was homozygous for the 39 kb deletion was included; second, Kooiker dogs referred for examination exhibiting clinical signs suggestive of polymyositis (as described above) [1]. The dataset required for inclusion included the pedigree registration number, pedigree name, date of birth, sex, country of origin, a routine blood examination that assessed CK activity, and the results of the breed-specific DNA test for polymyositis. Owners and dogs could participate only if written informed consent was obtained. Before starting this study, approval was requested from the animal welfare body at Utrecht University. An approval was not required for clinically affected dogs referred for diagnostics and treatment. For the homozygous healthy dog, approval was obtained from the animal welfare body, with registration number AVD10800202216205.

2.2. Examination

Initially, the clinical signs were noted, the dataset was finalised, and a blood sample was taken to perform the routine blood examination, measure CK activity, Toxoplasma gondii and Neospora caninum serum titres, and conduct the DNA test. If the DNA test revealed either a homozygous or heterozygous result for the 39 kb deletion, an MRI was subsequently performed under general anaesthesia. Dogs were premedicated with dexmedetomidine hydrochloride 2 microgram/kg and methadon 0.2 mg/kg intravenously and inducted with propofol 2mg/kg. After intubation, dogs received inhalation anaesthesia with isoflurane MAC 1.1% and 100% oxygen. After the procedure, the dogs received antagonist with atipamezole 20 microgram/kg. Prior to this procedure, if the dog exhibited respiratory signs or dysphagia, thoracic radiographs were taken. If appropriate, an electromyogram (EMG) was included, and two muscle biopsies were collected for histopathological examination (see below). Histopathology would not be conducted if the clinically unaffected dog homozygous for the 39 kb deletion does not exhibit any abnormal MRI findings.

2.3. Laboratory Measurements

Haematology and serum biochemistry, particularly CK activity, were assessed using a freshly obtained heparin blood sample. All samples were analysed by the University Veterinary Diagnostic Laboratory (UVDL), Faculty of Veterinary Medicine, University Utrecht, The Netherlands. The laboratory had previously established that the normal reference value for CK activity was 10–200 U/L. Toxoplasma gondii and Neospora caninum serum titres were analysed using an immunofluorescence test (IFT) by the University Veterinary Microbiology Diagnostic Laboratory (VMDC), Faculty of Veterinary Medicine, University Utrecht, The Netherlands. The reference values for the titres are for Toxoplasma IgG 1 < 16, IgM 1 < 32, and for Neospora 1 < 32.

2.4. DNA Sample

For the DNA test, a minimum of 1 mL of EDTA blood sample is sent to the Expertise Centre of Genetics, Faculty of Veterinary Medicine, Utrecht University, with a request to perform the appropriate DNA test for polymyositis^. in the Kooiker dog as described earlier. The DNA test can be requested through the URL https://survey.uu.nl/jfe/form/SV_5bYsJ3YJIdgj4O2 (accessed on 1 May 2025) [3].

2.5. EMG and Histopathology

The EMG Medtronic^®, Dantec^® Keypoint EMG Unit (Appendix A), and muscle biopsies (Appendix B) were all performed under generalised anaesthesia. The EMG was performed on the appendicular, axial, and masticatory muscles. The muscle biopsies were taken from both the m. triceps brachii (TRI) and m. biceps femoris (BF), each measuring 1 cm by 1 cm by 0.5 cm. The tissues were placed in a tube containing 10% neutral-buffered formalin and sent to the Veterinary Pathology Diagnostic Centre at the Department of Biomedical Health Services, Faculty of Veterinary Medicine, Utrecht University in the Netherlands, where they were analysed as previously described [1].

2.6. MRI

MRI was performed using a high-field MRI (Canon Vantage Elan 1.5T at Arnhem Referral Hospital the Netherlands). The protocol included a T1W and T2W sagittal and transverse image, a STIR sagittal, dorsal and/or transverse image and a T1W (sagittal) after contrast injection with 0.15 mmol/kg bodyweight gadolinium. Dogs were positioned in dorsal recumbency with their hind limbs stretched backwards, using the body coil and adjusting the Field-of-View to include the thoracolumbar region. MRI sequences obtained included sagittal T2W images (TE 110 ms, TR 2600 ms, 2.5 mm slice thickness, 320 × 256 matrix), sagittal T1W images (TE 10 ms, TR 500 ms, 2.5 mm slice thickness, 320 × 256 matrix), transverse T2W images of the cervical spinal cord (TE 115 ms, TR 5100 ms, 3 mm slice thickness, 192 × 160 matrix), and transverse T1W images of the cervical spinal cord (TE 10 ms, TR 400 ms, 3 mm slice thickness, 192 × 160 matrix). Post-contrast images were acquired one to two minutes after the contrast injection. The area to be imaged was identified based on the individual dog’s clinical presentation and included a relevant section of the vertebral column, paravertebral muscles, and associated limbs.

3. Results

3.1. Dogs

Five Dutch Kooiker dogs (three males, one neutered, and two females, one spayed) were included; four of the dogs were referred because they exhibited clinical signs suggestive of muscle disease, while one Kooiker dog, owned by a breeder and board member of the Dutch Kooiker club, did not show clinical signs at the time of consultation. The physical stamina of these five Kooiker dogs was comparable. None of the five dogs had received any treatment at the time of inclusion.

The clinically normal Kooiker dog (case 1; 10.5 years old), homozygous for the 39 kb genetic deletion associated with polymyositis in this breed, displayed no abnormalities during the neurological examination. Haematology, clinical chemistry, and serology revealed no abnormalities. CK activity was below the reference range, and an MRI of the entire vertebral column—including the first cervical to the lumbosacral vertebrae, as well as at least the dorsal cervical, thoracic, and lumbar muscles—showed no abnormalities (Table 1).

Table 1. Findings in the five included Kooiker dogs. Two dogs presented as locomotory (L) and two with both locomotory and dysphagia signs (C). Polymyositis was confirmed through histopathology in these four dogs.

Dog	DNA Test	Age at Scan in Months	Sex	Clinical Presentation	Time Sick at the Scan Date	CK in U/L	MRI Findings
1	Del/Del	126	F	Normal	-	186	Normal
2	Del/WT	57	MC	Polymyositis (L)	9 months	764	T1W normal findings, T2W and STIR multifocal hyperintensity (Figure 1)
3	Del/Del	35	FC	Polymyositis (C)	5 months	2170	T1W marginal, T2W, STIR and T1W post contrast multifocal hyperintensity (Figure 2)
4	Del/Del	31	M	Polymyositis (C)	1 week	1337	T2W, STIR and T1W post contrast multifocal hyperintensity (Figure 3)
5	Del/Del	40	M	Polymyositis (L)	2 weeks	1000	T2W and STIR multifocal hyperintensity (Figure 3)

Table 1. Findings in the five included Kooiker dogs. Two dogs presented as locomotory (L) and two with both locomotory and dysphagia signs (C). Polymyositis was confirmed through histopathology in these four dogs.

Dog	DNA Test	Age at Scan in Months	Sex	Clinical Presentation	Time Sick at the Scan Date	CK in U/L	MRI Findings
1	Del/Del	126	F	Normal	-	186	Normal
2	Del/WT	57	MC	Polymyositis (L)	9 months	764	T1W normal findings, T2W and STIR multifocal hyperintensity (Figure 1)
3	Del/Del	35	FC	Polymyositis (C)	5 months	2170	T1W marginal, T2W, STIR and T1W post contrast multifocal hyperintensity (Figure 2)
4	Del/Del	31	M	Polymyositis (C)	1 week	1337	T2W, STIR and T1W post contrast multifocal hyperintensity (Figure 3)
5	Del/Del	40	M	Polymyositis (L)	2 weeks	1000	T2W and STIR multifocal hyperintensity (Figure 3)

The remaining four Kooiker dogs (cases 2 to 5) included in the study exhibited clinical signs consistent with polymyositis—difficulty getting up from a recumbent position, inability to walk more than 100 metres, and a stiff gait as if walking on eggshells (see Video S1, case 2 and Video S2, case 4). Although none of the four dogs showed clear muscle atrophy, they did exhibit signs of pain upon palpation of the appendicular muscles. Two of these dogs presented with ptyalism and dysphagia (see Video S2, case 4). Of the four dogs, two exhibited clinical signs for two weeks, while the other two had been affected for several months; however, the diagnosis was confirmed only at the time of referral (Table 1). None of these dogs had received treatment before inclusion. All four affected Kooiker dogs exhibited distinctly elevated CK activities (Table 1). However, a complete blood count (haematology) and serum chemistry were conducted, along with serum titres for Neospora caninum and Toxoplasmosis gondii, which were found to be negative. Three dogs showed mild leucocytosis. Radiographs of the thorax were taken of two dogs that exhibited signs of dysphagia, yet they appeared normal. All histological findings confirmed polymyositis, revealing moderate-to-marked, chronic-active, diffuse, interstitial, and myofiber-directed lymphohistiocytic myositis [1,7].

3.2. DNA Test

Three out of the four clinically affected dogs were homozygous (cases 3—5), while one was heterozygous (case 2) for the genetic 39 kb deletion linked to polymyositis in this breed. There was no correlation between serum CK activity and the duration of clinical disease in these four dogs (Table 1).

3.3. MRI Findings in the Clinically Affected Dogs

The MRI results for the heterozygous dog, affected for nine months (case 2), showed that the lesions were isointense in T1W images. Both T2W and STIR sequences revealed a multifocal, sometimes diffuse, poorly marginated, generalised intramuscular hyperintensity in the sagittal and transverse planes of both the cervical and thoracolumbar muscles Similar findings were noted in both dogs’ thoracic and pelvic limb muscles (Figure 1).

In one homozygous Kooiker dog (case 3), mild diffuse hyperintensity was observed on the T1W images, along with marked, widespread, and generalised intramuscular hyperintensity evident in the T2W and STIR sequences. Following the administration of contrast medium, the lesions exhibited contrast enhancement in all dogs (Figure 3). Overall, the hyperintensity on the STIR was more obvious than the T2W and T1W post-contrast sequences. The two dogs (cases 2 and 3), which had been affected for several months, exhibited similar MRI findings when compared to the two dogs (cases 4 and 5), which had been affected for one and two weeks, respectively (Table 1).

4. Discussion

This study describes the MRI characteristics of immune-mediated polymyositis with a genetic basis in the Dutch Kooiker dog. The muscular MRI lesions appeared hyperintense on the T2W, STIR, and T1W post-contrast images. The gold standard for diagnosing a myopathy in dogs involves a combination of clinical signs, elevated serum CK levels, abnormal EMG, and confirmatory histopathology [11]. In the 4 Kooiker dogs that presented with clinical signs suggestive of polymyositis, histopathology confirmed the findings consistent with previously published results [1]. A first step in screening a Kooiker dog with clinical signs suggestive of myopathy is measuring CK activity. In an earlier study, CK activity was elevated in 99% of all Kooiker dogs with polymyositis [1]. However, serum CK activity is not always elevated in canine inflammatory myopathies [15]. Similar findings have been reported in humans [19]. Possible explanations could be the fact that not all muscles are equally affected, and if the muscle damage is mild or localised, there may be insufficient CK leakage into the bloodstream. Furthermore, the half-life of CK is only 36 h [20] which means that a mildly affected dog can be overlooked if the CK activity returns to normal again. In dogs with severe muscle atrophy, where most muscle tissue is dystrophic and fatty infiltration has occurred, CK levels will also decrease. Naturally, dogs that are severely affected will exhibit clear clinical signs. A second screening step could be an EMG. However, only one-third of the 90 Kooiker dogs diagnosed with polymyositis exhibited clearly abnormal EMG findings, such as myotonic discharge, fibrillation potentials, positive sharp waves, and prolonged insertion activity [1]. A third possible step is the available breed-specific DNA test; however, the deletion has low penetrance. Only 10–20% of the homozygous and 0.5–2% of the heterozygous Kooiker dogs with the 39 kb deletion develop polymyositis [2,3]. At present, the estimate suggests that there are 650 homozygous and 4900 heterozygous Kooiker dogs. Owners of a homozygous or heterozygous dog are advised to monitor their Kooiker dog with annual measurements of CK activity. If this becomes elevated, or if the dog shows clinical signs suggestive of polymyositis, performing an MRI is a less invasive technique to screen their Kooiker dog. Muscle biopsies and histopathological analyses are the most valuable tests for diagnosing polymyositis [1,7]; however, a muscle biopsy is a very invasive screening method that causes tissue damage and delays the patient’s mobilisation [13]. For these reasons, MRI was evaluated as a screening method. Although MRI requires anaesthesia, it is otherwise a non-invasive and proven imaging technique for diagnosing inflammatory myopathies in both dogs and humans, typically revealing minimal hyperintensity on T1W, and hyperintensity on T2W, STIR, and T1W post-contrast sequences [12,13,15,16]. The change in water content and structure affects the proton relaxation times of the muscle tissue involved, leading to variations in signal intensity [15,21]. If the relaxation time of the affected muscles increases due to inflammation, this will result in a rise in signal intensity on T2W images and a decrease in signal on T1W images. The changes are more pronounced on the T2W images [15]. Using T1W and T2W image sequences, it is possible to differentiate between fat and oedema [15]. The signal will be high in T2W images and reduced in T1W images [15]. Since it is impossible to detect oedema superimposed on fat, either a fat suppression technique such as STIR may be employed or post-contrast series [15]. Both STIR and T1W post-contrast scans enhance the visibility of muscle inflammation by highlighting the high signal intensity associated with inflammation and reducing the fat signal. Moreover, post-contrast series are advantageous for dogs treated with glucocorticosteroids, as STIR images might present as negative [15]. Besides immune-mediated polymyositis, the differential diagnosis of these findings includes protozoal, bacterial, parasitic, and rarely rickettsial infection [11]. The usefulness of MRI in veterinary medicine to diagnose an inflammatory myopathy was recently also demonstrated in a Poodle [22]. The authors used their MRI findings to locate an optimal spot for a biopsy to get a confirmatory diagnosis [18].

The first research question was whether MRI can be used to diagnose or exclude polymyositis in clinically affected Kooiker dogs. Four clinically affected Dutch Kooiker dogs were examined, and the MRI was abnormal in all cases, displaying marked multifocal, diffuse muscular hyperintensity on T2W and STIR sequences. In the heterozygous dog, the T1W sequence was considered normal. Only mild muscular hyperintensity was observed on the pre-contrast T1W sequence in one of the three homozygous dogs, suggesting that this sequence is less sensitive for detecting muscle pathology compared to the T2W sequence. The presence of a diffuse mild T1W hyperintensity could indicate a low-level alteration in muscle composition and inflammatory debris, although this has not been clearly identified by histopathology in the previous studies [1,7]. End-stage diseased Kooiker dogs have been described with muscular atrophy [1]. It is possible that the MRI findings may differ in these dogs.

We expected to find a difference in muscular volume among the various dogs with differing disease durations, but we were unable to detect this difference with physical examination. The four Kooiker dogs displayed comparable levels of physical stamina at the time of the MRI examination was comparable, despite variations in disease duration. The T1W post-contrast administration revealed intramuscular hyperintensity, which is expected in polymyositis [15]. The STIR sequence exhibited a greater subjective intramuscular hyperintensity compared to the T2W and T1W post-contrast sequences, indicating a higher contrast between unaffected and affected muscle tissues. Due to muscle oedema and fat replacement, inflammatory response hyperintensity was observed on T2W and STIR sequences [12,13,14]. Histopathology was performed on all four affected Kooiker dogs, providing definitive confirmation. If the MRI findings are abnormal, the recommendation is to take muscle biopsies. The findings were also abnormal in two dogs that exhibited clinical signs for only a brief period, suggesting that MRI is a valuable diagnostic test for Kooiker dogs presented during the early stages of the disease.

The second research question was whether it is possible to differentiate between affected homozygous or heterozygous Kooiker dogs and homozygous unaffected Kooiker dogs with the 39 kb deletion using MRI. The asymptomatic Kooiker dog, homozygous for the deletion, showed a normal MRI, while the others displayed abnormal images.

This study has limitations. Ideally, a larger number of affected Kooiker dogs would have been examined, along with more clinically normal Kooiker dogs either heterozygous or homozygous for the 39 kb deletion. Owners may feel reluctant to perform an MRI on an otherwise healthy dog solely for scientific reasons. This is likely the reason only one healthy Kooiker dog was included. Furthermore, it would have been ideal to image the pharyngeal muscles in dogs with dysphagia. This has not been done, but it is to be expected that these muscles will be equally affected [15].

This study is of interest for future studies. Currently, several treatment protocols have been used in these dogs. A recent publication [23] described the use of three treatments: (1) glucocorticoid monotherapy, (2) glucocorticoids combined with supplements (vitamin B, L-carnitine, and coenzyme Q10), and (3) glucocorticoids combined with supplements and oclacitinib. Oclacitinib is a Janus Kinase (JAK) inhibitor and was added to the treatment of affected Kooiker dogs in 2015 following the discovery of the 39 kb deletion [3]. Oclacitinib inhibits IL2 signalling [24] and depletes CD8 + T cells [25]. However, in this study, none of the treated Kooiker dogs were evaluated using MRI. Repeated MRI scans, as performed in humans, may serve as a valuable monitoring test to evaluate the impact of these treatment protocols in Kooiker dogs. In this study, none of the Kooiker dogs examined had received any prior treatment. This implies that the findings of this study can be utilised, as in humans, as a baseline [22]. Previous research has revealed that EMG had low sensitivity in Kooiker dogs with polymyositis [1]. It would be interesting to see if an MRI-guided EMG (and biopsy) site improves the usefulness of this technique, as was previously done in humans [26]. In this study, we only focused on the use of MRI. Ultrasound is even less invasive than MRI. Ultrasound can be useful in these dogs as it effectively detects fat replacement and fibrosis [12], which is expected in this polymyositis variant. Ideally, this will be explored in future studies as well.

5. Conclusions

MRI is a valuable imaging technique for diagnosing and assessing the extent of polymyositis in Kooiker dogs. The optimal sequences identified were STIR and T1W post-contrast, considering the inclusion of fat suppression techniques (such as proton density) for future studies. Clinically affected Kooiker dogs, whether heterozygous or homozygous for the 39 kb deletion, can be evaluated using MRI. Although the genetic mutations associated with polymyositis in Kooiker dogs show low penetrance, this study suggests that combining genetic testing with MRI could facilitate non-invasive screening. Kooiker dogs, homozygous or heterozygous for the 39 kb deletion, can be screened using MRI if the CK activity rises or they develop clinical signs. This thereby eliminates the need for an initial biopsy. However, muscle biopsy remains the preferred method for diagnosing polymyositis. Further research is required to assess its impact on treatment efficacy.