Epilepsy is one of the cardinal symptoms of NCLs (5). The age at epilepsy onset may differ according to the NCLs variant, starting from the first months of life to adulthood; both focal and generalized seizures can be seen at onset (5). Epilepsy may represent the presenting symptom, as it happens in CLN2 and CLN6 diseases, or appear after visual loss or cognitive and motor decline, as in CLN3 and CLN5 diseases (20).

In the classic form of CLN1 disease, symptoms begin during infancy, and seizure onset is relatively early, typically between the age of 14 and 36 months. Additional phenotypes have been observed with late infantile, juvenile, and adult onset. In the latter cases, epilepsy typically begins several years after the initial symptom of visual impairment, following developmental regression and behavioral changes (20). Few data are available on EEG features in CLN1 disease (21, 22). It has been reported that a slowing of background activity and loss of sleep spindles is associated with high-voltage slow waves and spike and waves abnormalities (22). Between 5 and 12 years of age, a progressive flattening of cerebral activity consistent with the marked cortical atrophy due to massive neuronal death is evident (21). In summary, in CLN1 disease, changes of background activity have been distinguished in three stages: (1) decreasing of reactivity of the posterior rhythm to eye opening and closing; (2) decreasing of sleep spindles and subsequent disappearance; and (3) slowing and/or attenuation of EEG to inactivity, the so-called vanishing EEG pattern (22, 23).

In CLN2 disease, a late-infantile neuronal ceroid lipofuscinosis (LINCL), seizures often appear in an explosive fashion (24). Seizures are resistant to common anti-seizure medications (ASMs), and sodium channel blockers, such as carbamazepine, can even worsen seizure frequency and may increase myoclonus and ataxia (24).

The first EEG is usually performed within the fourth year of life and shows a normal background activity, associated with focal or diffuse abnormalities in 75% of the patients. High-voltage focal slow waves are prevalent over the temporal and occipital regions (25–28). Photoparoxysmal response (PPR) to intermittent photic stimulation (IPS) delivered at a low frequency of stimulation is typically seen in patients with CLN2 during the first stage of the disease (27). With disease progression, the background activity becomes slow and without reactivity to eye opening (29, 30). No spindles are present during sleep recordings. Generalized, focal and multifocal epileptiform discharges characterized by irregular spikes and poly-spikes and waves become common; however, a posterior predominance persists (24) (Figures 1A–E).

In CLN3 disease, the juvenile neuronal ceroid lipofuscinosis (Spielmeyer–Vogt disease or Batten disease), the presenting manifestation is a progressive visual loss starting around the age of 6–8 years. Epilepsy usually starts at the age of 10 years. Seizures at onset are usually bilateral tonic–clonic, whereas focal seizures increase during adolescence, and they are mostly characterized by clonic manifestations. Myoclonic seizures are rarely reported in CLN3 disease. EEG, in the early stages of the disease, shows focal epileptiform abnormalities (before 10 years of age), whereas bilateral and multifocal epileptiform discharges are significantly more prevalent in the later stages, along with a progressive slowing of the background activity (31). PPR has not been reported as a prominent feature in CLN3 disease (31).

The CLN6 disease may start both during infancy and adulthood. In both cases, epilepsy is one of the presenting symptoms (32). Background activity is poorly organized during awake and sleep since the onset, associated with irregular slow spike and waves discharges (at about 2.5 Hz). PPR to lower frequency is an early neurophysiological finding. During the advanced stage of disease, the cerebral activity became extremely slow, and low-voltage and spike and waves discharge became rare, often replaced by single spikes with multifocal distribution (32).

In adult patients affected by CLN6, the background activity is almost preserved in the early stage of the disease. Epileptiform abnormalities are the most prevalent over the posterior regions. Myoclonus is typically induced by active movements or provoked by IPS at a low frequency of stimulus. Abnormal PPR persists until the advanced stages of the disease (32).

The CLN5 disease, also known as the Finnish variant, starts between the age of 2 and 6 years with clumsiness and mental decline (33, 34). Seizure occurs relatively late (median age of 8 years), and epilepsy has its major expression with myoclonic seizures between the age of 7 and 11 years. In addition, for this LINCL, it has been reported that a progressive slowing of background activity is associated with multifocal epileptiform discharges (spikes, multiple spikes, and spike and wave complexes) (34) and the presence of posterior spikes triggered by low-frequency IPS (33, 34).

Regarding CLN7 disease, in few published patients, it has been reported that a progressive generalized slowing of background activity is associated with diffuse or multifocal abnormalities with occipital prevalence (Figures 2A–C). PPR has not been reported (35, 36).

In patients with CLN8, it has been described as a progressively slowing of background activity with focal and/or generalized abnormalities since the onset. PPR has rarely been reported (37).

Few cases of CLN11 have been reported (38, 39). EEG is characterized by a quite preserved background activity associated with rare paroxysms of diffuse spikes and slow waves, prevalent over bilateral parietal regions. PPR has not been detected in the three cases reported so far (38, 39).

Limited data have been reported on EEG features for the other NCL variants, from CLN9 to CLN14, which are the more recently identified forms. More cases need to be reported to understand their typical neurophysiological findings.

Photoparoxysmal response in NCLs was firstly described in 1970 when it was reported in about 45% of patients (29). Later, other studies confirmed the presence of this peculiar neurophysiological finding (27, 32, 40, 41).

The susceptibility to PPR varies accordingly to the different diseases which are more prominent in the LINCLs, such as CLN2, CLN5, and CLN6 (Figures 1F–H). In CLN2 disease, PPR has been reported from 27 to 93% of patients (25, 27, 40, 41). This wide range may be explained by the lack of a universal approach of low frequency IPS in routine pediatric EEG recordings (24). In fact, in NCLs, as well as in other progressive myoclonic epilepsies, PPR is one of the earliest pieces of evidence of a neurodegenerative disorder, even before the onset of cognitive and motor regression (42). PPR to low-frequency stimulation has also been reported to occur in Lafora disease and mitochondrial diseases (43, 44).

In a recently reported series of 14 cases of CLN2, serial EEGs revealed a PPR in 93% of patients (27). PPR was evident since the first EEG, which was performed at 3.6 (3.1–4.0) years, in 43% of patients; it was documented at low (1–3 Hz) frequencies of stimulation in 69% of patients and has acquired the form of a flash-per-flash response in 69% of patients (27). Moreover, in the advanced stages of the disease, PPR was associated with massive myoclonic jerks (27). In about half of patients (54%), PPR disappears over time (25). PPR changes over time reflect the gray matter changes due to the progression of neurodegenerative disease (27).

The PPR seen in the NCLs is characterized by an occipital spike and waves response to the photic stimuli. However, it should also be highlighted that not in all NCL variants do patients have the same susceptibility to PPR; it has been reported more frequently in CLN2, CLN5, and CLN6 diseases (24, 27, 32, 33) and seems to not be a prominent feature in CLN1, CLN3, CLN8, and CLN11 diseases (21, 31, 37, 38). For the remaining NCLs, we do not have sufficient data about PPR to find out conclusive data.

On the other hand, the lack of the characteristic IPS response cannot rule out an NCL disease (45). The implementation of IPS, including IPS at low frequencies of stimulation as part of a standard EEG, may be useful as an early disease marker if associated with other clinical findings (24, 45, 46).

Few data are available in the literature regarding visual evoked potentials (VEPs) changes during the disease course in NCLs. What is known is that with disease progression, the so-called giant VEPs appear (Figures 3D,E). They are abnormally broad and of high amplitude, and their presence is a marker of cortical hyperexcitability, similar to PPR (32).

Abnormal VEPs have been found in about 75% of patients with CLN2 at a median age of 4.5 years (25). Furthermore, a more recent study on patients with CLN2 evidenced that most patients (89%) had an early and high-amplitude pattern reversal in VEPs, whereas few patients showed a bifid waveform, which is associated with a central scotoma, indicative of maculopathy or macular pathway dysfunction such as optic atrophy (47).

With further disease progression, the amplitude of VEP decreases, as it happens in other neurodegenerative disorders (48, 49).

The pattern-reversal VEP waveform is preferred to the flash VEP waveform, which shows a wide inter-individual variability. On the contrary, the pattern-reversal VEP has a relatively constant single positive peak throughout life and is a strong index of macular pathway function (47).

Variability of results may be due to the retrospective nature of the studies and the different ages and phases of the disease in which visual tests have been performed (25, 41, 47).

Vision-related problems are one of the cardinal signs of NCLs and, as in CLN3, are often an early sign, appearing prior to motor and mental deterioration (5). This is due to the accumulation of storage material into the retina, leading to its degeneration (50, 51).

Electroretinogram (ERG) represents the tool that allows to monitor the involvement of retina, and its use progressively disappears with disease progression (Figures 3A–C) at different ages according to the NCL variants (50). ERG is currently used to characterize the physiological changes in the degenerating retina in patients affected by NCLs. It allows to identify the retinopathy, which consists of symmetrical cone-rod dystrophy (47, 50).

In patients with CLN2, it has been demonstrated that changes in ERG appear even before the macular disruption on optical coherence tomography (OCT) from the age of 4 years and 10 months (47). Abnormal ERG has been reported in 66% of patients at a median age of 4.5 years (25). However, it is important to underlie that the flattening of ERG does not necessarily imply the total loss of retinal function, if some cortical VEP functions persist (29).

In CLN5 disease, retinal degeneration has been confirmed between the age of 6 and 10 years, when ERG has been found abolished in most of the patients (33, 34).

The use of ERG might be implemented in the future to evaluate the efficacy of experimental treatments with intravitreal therapies.

Somatosensory evoked potentials (SEPs) have been poorly investigated in NCL diseases. There are sporadic cases, across different forms of NCLs (21, 32, 33, 52), where it is highlighted that the presence of high-amplitude evoked potential, giant SEPs, which are the expressions of cortical hyperexcitability due to neuronal degeneration (32).

Giant SEPs are also a typical and specific marker of patients with cortical myoclonus, which is one of the main clinical features of NCLs (38).