The main clinical and immunological characteristics of the six patients are summarized in Table 2. Three patients were male and the median age at the onset of symptoms was 53 years (range 48–77 years). The three patients diagnosed in life with positive IgLON5 antibodies were studied with video-polysomnography and presented a unique temporal sequence of sleep stages and behaviors, from very abnormal at the beginning of the night to close to normal by the end. The initiation and re-entering of sleep after awakening were characterized by undifferentiated non-rapid eye movement (NREM) sleep with frequent vocalizations, stereotyped movements, and finalistic behaviors (parasomnias). The REM sleep was present but only in the form of REM sleep behavior disorder. In addition, most patients had a sleep breathing disorder characterized by stridor and obstructive sleep apnea.
The clinical history of the three patients in whom IgLON5 antibodies were not tested was dominated by bulbar dysfunction and repetitive episodes of respiratory insufficiency that required tracheostomy or multiple admissions to ICU. Other symptoms included gait instability, frequent falls, dysphagia, gaze palsies, central hypoventilation, dysautonomia, and chorea. These symptoms can suggest other diagnoses such as progressive supranuclear palsy or multiple system atrophy, although no parkinsonian signs were present. In none of these patients was the sleep formally studied, but in two of the cases symptoms of excessive daytime sleepiness, stridor, sleep apnea or parasomnia were documented (Table 2).
Table 3 summarizes the neuropathological features of the six cases. The detailed neuropathological reports of the two cases not previously reported and the re-evaluation of the previously published UK case  are presented below.
Case 3 (Vienna–Innsbruck, Austria)
Detailed clinical information of the disease course has been previously reported  and is summarized in Table 2. Post-mortem examination of the cerebrum was performed after obtaining informed consent from the next of kin. Gross examination showed mild atrophy of superior cerebellar peduncles and brainstem tegmentum. Histology revealed a prominent tauopathy, characterized by numerous Gallyas-positive neurofibrillary tangles (NFT), diffuse granular cytoplasmic phospho-tau (pTau) immunoreacitivity (pretangles), and neuropil threads involving predominantly the hypothalamus, zona incerta, hippocampus, tegmentum of brainstem (mesencephalon, pons and medulla), and cervical spinal cord (Table 3; Fig. 1) in a symmetrical fashion. The hippocampus showed a high density of NFT and pretangles in the pyramidal cell layer (Fig. 2a), accentuated in the CA2 and CA4 sectors, and moderate amounts of pretangles in the dentate gyrus. Only occasional ghost tangles were detected. The pathology in the midbrain, pons, and medulla oblongata was mainly restricted to the tegmentum including periaqueductal gray matter, laterodorsal tegmental nucleus, pedunculopontine nuclei, median raphe nuclei, dorsal motor nucleus of the vagus nerve, nucleus ambiguus and neurons of the magnocellular nucleus (Fig. 2c). Substantia nigra was only mildly affected, and the subthalamic nucleus and basis pontis showed isolated threads and pretangles (Fig. 2d). The inferior olivary nucleus was well preserved but had frequent clusters of fine threads surrounding individual neurons (Fig. 2e). The cerebellar cortex was globally well preserved, but showed a fine granular synaptic-like pTau immunoreactivity pattern involving the glomerula of the granular cell layer of the vermis (Fig. 2f). In addition, a few Purkinje cells showed diffuse cytoplasmic pTau positivity. There was a prominent involvement of the cervical spinal cord with marked pTau pathology in the dorsal horns and lesser involvement of the anterior horns (Fig. 1j), with decreasing intensity further caudally in the lumbar spinal cord, where pathology became less evident but was still visible in the dorsal horn. pTau immunoreactivity was almost entirely confined to neurons and the tau-positive structures were also labeled with both, the 3R-tau (Fig. 2h) and 4R-tau (Fig. 2i) antibodies. All these areas showed moderate microglial activation. Neuronal loss correlated with the presence of NFT. The cranial nerves were not affected. Only very occasional coiled bodies and tau-positive granular fuzzy astrocytes were visible in the hypothalamus, substantia nigra, and anterior horn of the spinal cord. Other morphological alterations included a few ballooned neurons in the amygdala. Anti-LCA revealed only a few meningeal and perivascular leukocytes mainly composed of CD3 and CD8 positive T cells and a few CD20 and CD79A B cells in the hypothalamus, brainstem, and cerebellum without any parenchymal involvement that could suggest an inflammatory process. No obvious IgG4 deposits were detected, except for a faint staining of the cerebellar glomerula of the vermis (data not shown). Complement deposits were not visible.
Case 5 (London, UK)
A 48-year-old man developed recurrent vertigo and marked involuntary fidgety movements. At the age of 53 he developed breathing and swallowing difficulties. Multiple episodes of aspiration pneumonia and respiratory arrests warranted four admissions to the intensive care unit and, on two occasions, intubation for ventilator support was required. Due to recurrent aspiration, a gastrostomy tube was inserted for feeding. One year later, he developed progressive difficulties with his balance and intermittent diplopia. He had lateral and upgaze nystagmus with limitation of vertical and abduction eye movements. There was cerebellar dysarthria, finger-nose dysmetria, and he was unable to tandem walk. Choreiform movements were present in all limbs. Deterioration in excessive daytime somnolence caused him to fall asleep even when he was standing. His wife reported that his breathing pattern changed as soon as he was asleep but no abnormal movements were noted. During a hospital admission to investigate his sleep attacks, he had a respiratory arrest and died at the age of 60.
Neuropathological examination was performed after appropriate written informed consent from the next of kin had been obtained. Consent for the use of this case for research purposes was also given. The brain was macroscopically unremarkable. Microscopic studies showed an increase in number of reactive astrocytes in the striatum and globus pallidus. There was some loss of neurons with accompanying astrogliosis in the dorsolateral aspect of the subthalamic nucleus while the substantia nigra was well populated with pigmented and non-pigmented neurons. There was some reduction of the height of the pontine tegmentum, which was marked in the medulla oblongata. Tau immunohistochemistry using the AT8 antibody demonstrated only occasional NFT and pretangles in neocortical areas. All hippocampal subregions were affected by neuronal tau pathology; there were moderate numbers of pretangles and NFT in the CA1 hippocampal subregion, which were sparse in the CA3 and CA4 subregions. There were frequent pretangles in CA2 and also in the granule cells of the dentate gyrus, which were ubiquitin and p62-negative. Fine neuropil threads were present in all hippocampal subregions. The tau pathology was similar, but mild in the subiculum while moderate numbers of Gallyas-positive NFT and neuropil threads were seen in the entorhinal cortex. There were scattered tau-positive neuropil threads, pretangles and occasional NFT in the striatum. Similar, but slightly more severe pTau pathology was seen in the globus pallidus. The subthalamic nucleus contained moderate numbers of pretangles, NFT and neuropil threads. A conspicuous feature was the severe tau deposition in the posterior hypothalamus and brainstem structures. In the midbrain a significant proportion of the substantia nigra pars compacta neurons showed granular or dot-like tau positivity, which was also seen in neuronal processes. Similar tau pathology was seen in the midbrain tegmentum, periaqueductal gray matter and tectum. The majority of the neurons of the otherwise well-populated locus coeruleus contained similar neuronal tau-positive inclusions and the pontine tegmentum and base were also extensively affected by similar tau pathology. The tau deposition was particularly severe in the tegmentum of the medulla including the dorsal motor nucleus of the vagus nerve, nucleus tractus solitarius, nucleus ambiguus and neurons of the magnocellular nucleus were severely affected. In the inferior olive neurons were often surrounded by a meshwork of fine tau-positive processes and dot-like tau positivity. There were scattered pretangles and NFT, neuropil threads and clusters of fine tau-positive dots in the cerebellar dentate nucleus. There was rather widespread granular tau positivity in the cerebellar cortex and an occasional Purkinje cell showed fine, dot-like tau positivity. Differential tau immunohistochemistry confirmed that the pTau deposits were composed of both 3R-tau and 4R-tau isoforms.
Case 6 (London, UK)
For clinical history and the distribution of the NFT pathology see Lidov et al. . Tau immunohistochemistry with the AT8 antibody was carried out using tissue sections of paraffin blocks available in the archives of the Division of Neuropathology, UCL Institute of Neurology. The distribution of the pTau pathology, including involvement of the hippocampus was rather similar to that described in case 5. However, there were some differences, which included that in this case the striatum was virtually devoid of tau pathology and the involvement of the globus pallidus and subthalamic nucleus was considerably milder than that observed in case 5. The brainstem tegmentum showed severe deposition of disease-associated tau in a distribution as described above, but the substantia nigra and pontine base showed only sparse tau-positive pretangles, NFT and neuropil threads. There was tau pathology of moderate degree in the posterior horns of the cervical and thoracic cord (these were the levels that were available for review) and sparse neuropil threads and occasional NFT were also seen in the anterior horns. Occasional tau-positive granular deposits were found in the cerebellar cortex. The tau inclusions were both 3R-tau and 4R-tau immunoreactive.
Summary of neuropathological features and suggested neuropathological criteria
The neuropathological findings in the six patients showed remarkable similarity consistent with a neurodegenerative disease with neuronal loss and gliosis, without the presence of inflammatory infiltrates. The most relevant finding was a prominent neuronal accumulation of pTau, comprising both 3R-tau and 4R-tau isoforms, involving the hypothalamus and more severely the tegmental brainstem nuclei, with a cranio-caudal gradient of severity, reaching the upper cervical cord. The cerebral cortex, basal ganglia, thalamus and subthalamic nucleus were mostly unaffected or mildly affected. In five cases the entorhinal cortex and hippocampus were also affected including dentate gyrus and CA4 to CA1 sectors, with variable involvement of transentorhinal region. Interestingly, CA2 sector was consistently involved. There was a lack of glial tau pathology except for the presence of isolated coiled bodies and granular fuzzy astrocytes in the hypothalamus and amygdala in cases 3 and 4. A detailed semiquantitative assessment of pTau pathology in different brain areas of all cases is represented in Table 3 and Fig. 3 shows a schematic heat-map representation of the distribution and severity of pTau pathology throughout the brain for all cases.
There were no other abnormal deposits of proteins, or if present these were mild and localized in a few areas suggesting the co-existence of other neurodegenerative or age-related pathologies. Case 2 showed isolated Bunina bodies and TDP43 protein aggregates in few brainstem and motor neurons in the spinal cord. A few neuritic plaques were observed in cases 2, 4, and 6, along with cerebral amyloid angiopathy in cases 2 and 4. Case 4 additionally showed scattered alpha-synuclein positive Lewy bodies and Lewy neurites in the substantia nigra and dorsal nucleus of the vagus nerve and argyrophilic grain pathology, and case 5 showed sparse neuronal cytoplasmic TDP43 protein inclusions in the granule cells of the dentate gyrus.
Based on the similarities of these cases we propose research criteria for the neuropathological diagnosis of the tauopathy underlying the anti-IgLON5 syndrome (Table 4) and recommend a protocol of tissue sampling (Table 5). When a neuropathologist is confronted with a case with such a neuropathological phenotype in conjunction with positive IgLON5 antibodies in CSF or serum, the diagnosis of “definite anti-IgLON5-related tauopathy” should be considered. In contrast, when the information of the anti-IgLON5 antibodies status is missing but the clinical history is compatible or the patient harbored the HLA-DRB1*1001 and HLA-DQB1*0501 alleles, then the diagnostic category of “probable anti-IgLON5-related tauopathy” should be applied. The category of “possible anti-IgLON5-related tauopathy” should be reserved for those cases where clinical data are not available (e.g. due to the retrospective identification of archival cases or other reasons) and ancillary tests (anti-IgLON5 antibodies and/or HLA-DRB1*1001 and HLA-DQB1*0501 alleles) could not be performed. Note that both the HLA genotyping and the anti-IgLON5 antibody assay could be done post-mortem from DNA samples and serum or ventricular CSF, respectively. No exclusion criteria have been proposed as the whole spectrum of pathological changes and possible concomitant pathologies are yet unknown.