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Brain Advance Access published online on October 18, 2008
Brain, doi:10.1093/brain/awn269
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© The Author (2008). Published by Oxford University Press on behalf of the Guarantors of Brain. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Visual system involvement in patients with Friedreich's ataxia

Filippo Fortuna1, Piero Barboni1,2, Rocco Liguori1, Maria Lucia Valentino1, Giacomo Savini3, Cinzia Gellera4, Caterina Mariotti4, Giovanni Rizzo1,5, Caterina Tonon5, David Manners5, Raffaele Lodi5, Alfredo A Sadun6 and Valerio Carelli1

1Department of Neurological Sciences, University of Bologna, Bologna, 2Studio oculistico d’Azeglio, Bologna, 3Fondazione G.B. Bietti-IRCCS, Rome, 4U.O. Biochemistry and Genetics, Fondazione IRCCS-Istituto Neurologico Nazionale ‘Carlo Besta’, Milan, 5Department of Clinical Medicine and Applied Biotechnology, University of Bologna, Bologna, Italy and 6Doheny Eye Institute, Keck School of Medicine, University of Southern California, USA

Correspondence to: Valerio Carelli, MD, PhD, Dipartimento di Scienze Neurologiche, Università di Bologna, Via Ugo Foscolo 7, 40123 Bologna, Italy E-mail: valerio.carelli{at}unibo.it


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Optic neuropathy is common in mitochondrial disorders, but poorly characterized in Friedreich's ataxia (FRDA), a recessive condition caused by lack of the mitochondrial protein frataxin. We investigated 26 molecularly confirmed FRDA patients by studying both anterior and posterior sections of the visual pathway using a new, integrated approach. This included visual field testing and optical coherence tomography (OCT), pattern visual evoked potentials (P-VEPs) and diffusion-weighted imaging. The latter was used to study optic radiation by calculating water apparent diffusion coefficients (ADC). All patients suffered optic nerve involvement with their disorder. Different patterns of visual field defects were observed and a variably reduced retinal nerve fiber layer thickness was seen by OCT in all cases. P-VEPs were abnormal in approximately half of the patients. Decreased visual acuity and temporal optic disc pallor were present in advanced stages of the disease, but only five patients were symptomatic. Two of these patients suffered a sudden loss of central vision, mimicking Leber's hereditary optic neuropathy (LHON), and of the other three symptomatic patients two were noted to be compound heterozygotes. ADC values of optic radiations in patients were significantly higher than controls (P < 0.01). Retinal nerve fiber layer thickness at OCT and P-VEPs correlated with age at onset and ICARS total score. ADC values correlated with age at onset, disease duration, GAA triplet expansion size, ICARS total score and P-VEPs. Visual pathway involvement is found consistently in FRDA, being previously underestimated, and we here document that it also involves the optic radiations. Occasional LHON-like cases may occur. However, optic neuropathy in FRDA substantially differs from classic mitochondrial optic neuropathies implying a different pathophysiology of visual system degeneration in this mitochondrial disease.

Key Words: optic neuropathy; Friedreich's Ataxia; mitochondria; OCT; frataxin

Abbreviations: ADC, apparent diffusion coefficients; DOA, dominant optic atrophy; DWI, diffusion-weighted imaging; EPI, echo planar imaging; F-ERG, flash-electroretinogram; FRDA, Friedreich's ataxia; ICARS, International Cooperative Ataxia Rating Scale; LHON, Leber's hereditary optic neuropathy; MERRF, myoclonic epilepsy, ragged-red-fibres; mtDNA, mitochondrial DNA; nt, nucleotide; OCT, optical coherence tomography; ONH, optic nerve head; PCR, polymerase chain reaction; P-VEPs, pattern visual evoked potentials; RGC, retinal ganglion cell; RNFL, retinal nerve fiber layer; ROI, regions of interest; RPE, retinal pigmented epithelium

Received May 30, 2008. Revised September 9, 2008. Accepted September 22, 2008.


   Introduction
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 Introduction
Methods
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 Discussion
 Funding
 References
 
Friedreich's; ataxia (FRDA) is the commonest form of hereditary ataxias with autosomal recessive transmission (Harding, 1993Go). Disease onset is usually before the age of 25 years. Clinical features include spinocerebellar and sensory ataxia with absence of deep tendon reflexes, dysarthria, hypertrophic cardiomyopathy and scoliosis (Harding, 1993Go). Additional clinical features may include diabetes mellitus, pes cavus, hypoacusia and optic atrophy (Harding, 1993Go).

The genetic defect has been located on chromosome 9q13-q21.1 and affected individuals present a GAA triplet expansion in the first intron of the FXN gene (Campuzano et al., 1996Go). This expansion may reach over 1000 repeats in both alleles, the normal range being 27–36 repeats. A minority of FRDA patients are compound heterozygotes for the GAA triplet expansion and a point mutation (Campuzano et al., 1996Go). The FRDA gene encodes a 210 amino-acid long protein called frataxin whose function is still not completely elucidated. Soon after its identification frataxin was recognized as a mitochondrial protein necessary to maintain respiratory function and mitochondrial DNA (mtDNA) in yeast (Koutnikova et al., 1997Go; Wilson et al., 1997Go). Frataxin is directed to the inner mitochondrial membrane and FRDA-associated mutations drastically reduce the protein amount (Campuzano et al., 1997Go). Further studies confirmed deficient mitochondrial respiration in the heart and muscle of FRDA patients, with a specific impairment of iron-sulphur containing proteins such as complexes I, II and III and aconitase (Rotig et al., 1997Go; Lodi et al., 1999Go). Frataxin has been implicated in iron homeostasis (Babcock et al., 1997Go) and hypersensitivity to oxidative stress (Wong et al., 1999Go). However, the role of oxidative stress in the disease pathophysiology remains unclear (Seznec et al., 2005Go). More recently, frataxin has been shown to be involved in the biosynthetic pathways related to Fe-S clusters containing enzymes (Tan et al., 2003Go; Shan et al., 2007Go).

Mitochondrial optic neuropathies are a relatively homogeneous group of genetic and acquired disorders characterized by the peculiar preferential involvement of the small axons serving central vision, colour vision and high spatial frequency contrast sensitivity; these fibers form the papillomacular bundle (Carelli et al., 2004Go). Non-syndromic mitochondrial optic neuropathies essentially include Leber's; hereditary optic neuropathy (LHON) and dominant optic atrophy (DOA), and present clinically as visual loss and optic nerve atrophy reflecting retinal ganglion cell (RGC) degeneration as the only or at least primary pathological feature (Carelli et al., 2004Go). Mitochondrial optic neuropathies may also occur in more complex syndromes such as encephalomyopathies due to mtDNA mutations, i.e. Leigh syndrome and myoclonic epilepsy, ragged-red-fibers (MERRF) (Carelli et al., 2006Go), or mitochondrial disorders due to nuclear DNA genetic defects, i.e. Charcot-Marie-Tooth 2A (mutations in mitofusin 2) (Zuchner et al., 2006Go), deafness-dystonia-optic atrophy (Mohr-Tranebjerg) syndrome (mutations in TIMM8A) (Jin et al., 1996Go), and complicated hereditary spastic paraplegia (mutations in paraplegin) (Casari et al., 1998Go). The occurrence of optic atrophy in FRDA belongs to this latter group of mitochondrial disorders and this feature has been recognized but poorly characterized for a long time (Newman, 2005Go). The last systematic study of optic atrophy in a series of FRDA patients was based on electrophysiology by Carroll et al. (1980Go), and more recently there have been only a few case reports published (Givre et al., 2000Go; Porter et al., 2007Go). However, the recent revolution in mitochondrial medicine had made the study of mitochondrial optic neuropathies a hot topic that is now being intensely investigated (Carelli et al., 2006Go), using a variety of new technical tools in ophthalmology such as optical coherence tomography (OCT) (Barboni et al., 2005Go; Savini et al., 2005Go).

This study aimed to characterize the pathological features of visual pathway involvement in FRDA. Twenty-six molecularly confirmed FRDA patients were evaluated by applying an integrated approach able to explore the entire visual pathway by visual fields and pattern visual evoked potentials (P-VEPs). Additionally, the anterior visual pathways (retinal ganglion cell/optic nerve system) were directly assessed by OCT, and the posterior visual pathways (optic radiations) by magnetic resonance using the diffusion-weighted imaging (DWI) technique.


   Methods
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 Methods
Results
 Discussion
 Funding
 References
 
Patient sample
We recruited 26 patients (18 males, mean age 32 ± 8) from 21 Italian families with genetically confirmed FRDA diagnosis, by an advertisement on the website and the newspaper of the ‘Associazione Italiana per la lotta alle Sindromi Atassiche’ (AISA), the Italian association of patients with ataxia. Five pairs of siblings and 16 sporadic cases were included, the sample being composed of 18 males and eight females ranging from 15 to 45 years of age. The demographic and clinical features are summarized in Table 1. The quantification of disability was performed according to the ICARS (Trouillas et al., 1997Go), whose score progresses from 0 to 100 with the degree of disease severity. About one third of the patients were severely affected as they could not conduct activities of daily life independently, and about half were wheelchair bound. All patients had the typical features of spinocerebellar ataxia. Only one patient had diabetes and three suffered hearing loss, as inferred by previous medical reports. All participants gave their informed consent according to the Declaration of Helsinki and the study was approved by the internal review board of the Department of Neurological Sciences at the University of Bologna.


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  		Table 1 Demographic and clinical features of the 26 FRDA patients

 

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  		Table 2 Correlation of functional and anatomical ophthalmologic parameters with age of onset, disease duration, triplet expansion and ICARS score in the 21 FRDA patients, having excluded the two LHON-like and the two compound heterozygote cases

 
Neuro-ophthalmologic studies
Each patient had a comprehensive ophthalmologic examination, including best-corrected visual acuity measurement, slit lamp biomicroscopy, intraocular pressure measurement, and indirect ophthalmoscopy. Visual field testing by Humphrey Field Analyzer (HVF, Zeiss-Humphrey Systems, Dublin, CA, USA), retinal nerve fiber layer (RNFL) thickness measurement and optic nerve head (ONH) analysis by OCT (StratusOCT, software version 4.0.1; Carl Zeiss Meditec, Inc, Dublin, CA, USA) were carried out in all patients with a good fixation (23 out of 26).

The OCT acquisition protocols adopted were RNFL Thickness 3.4 and Fast Optic Disc, as previously reported (Barboni et al., 2005Go; Savini et al., 2005Go). Briefly, for each eye, we studied the mean RNFL thickness (360° measure), temporal quadrant thickness (316°–45° unit circle), superior quadrant thickness (46–135°), nasal quadrant thickness (136–225°), and inferior quadrant thickness (226–315°), all automatically calculated by OCT using the existing software. ONH evaluation consisted of six radial scans centred on the optic disc and spaced 30 degrees apart. The machine automatically defined the edge of the optic disc as the end of the retinal pigmented epithelium (RPE)/choriocapillaris and used smoothing with fit to circle to fill the gaps between scans. The examination was performed under mydriasis by an experienced operator (PB) who was masked regarding the clinical status of each subject. RNFL measurements in FRDA patients were compared to a control group matched for age and ONH size, since both factors have been found to influence RNFL thickness assessment by OCT (Barboni et al., 2005Go; Savini et al., 2005Go). The control group (n = 48, 28 males, mean age 32.9 ± 7.8) was composed of volunteers without evidence of either optic disc or retinal disease.

The neuro-physiological evaluation included P-VEPs and Flash-electroretinogram (F-ERG) performed in all patients. P-VEPs to stimulation with 31' checks and with 15' checks and F-ERG were recorded at least twice in order to ensure reproducibility using established methods (Ikeda, 1982Go).

DWI studies
We used a 1.5-T General Electrics Medical Systems (Milwaukee, Wisconsin) Signa Horizon LX whole-body scanner to study a subset of 13 FRDA patients (11 males, mean age 31 ± 8) and 18 healthy age-matched volunteers (13 males, mean age 28 ± 8). Axial DW images were obtained (slice thickness = 5 mm, inter-slice gap = 1 mm) using a single-shot echo planar imaging (EPI) sequence (matrix size = 192 x 192 mm) as previously reported (Nicoletti et al., 2006Go). Orthogonal x, y and z diffusion encoding gradients were applied with gradient strengths corresponding to b-values of 900 s/mm2. In addition, images without diffusion weighting were acquired, corresponding to b = 0 s/mm2 and exhibiting T2-contrast. The apparent diffusion coefficient (ADC) in each direction was determined pixel-wise using a least-squares fit, assuming a signal attenuation depending mono-exponentially on b-value. By calculating the mean of three directions, the ADC map was generated. Regions of interest (ROI) (Fig. 1A) were determined by segmentation of the left and right optic radiations (Yamamoto et al., 2005Go) on three slice using the EPI images with b = 0 s/mm2 and were copied on the ADC maps to obtain the mean ADC values.


Figure 1
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  		Fig. 1 (A) ROIs including optic radiations. (B) Scatter plot of optic radiations mean ADC values of FRDA patients and controls. (Each box shows the median, quartiles and extreme values; **P ≤ 0.01.)

 
Polymerase chain reaction amplification and sequence analysis of the FXN gene
Genomic DNA was prepared from peripheral-blood lymphocytes using standard procedures. The detection and quantification of the (GAA)n repeat size was achieved by polymerase chain reaction (PCR) amplification performed in three independent reaction mix, using different forward primers (Gellera et al., 2007Go). Amplifications of exons 1–5A and 5B of the FXN gene were performed as previously described (Campuzano et al., 1997Go). Direct sequence analysis of PCR products from patients and controls was performed by automated sequencing on an ABI PRISM 3100 Genetic Analyzer (Applera, Foster City, CA) using the BigDyeDeoxyTM Terminator Cycle Sequencing Kit (Applera) according to the manufacturer's recommendations. Nucleotides were numbered so that the first nucleotide (nt) of the first in-frame ATG codon was nucleotide +1 [GenBank cDNA reference sequence: NM_000144 [GenBank] .3 (GI:31742514)].

Statistical analysis
For the ophthalmologic studies, one eye was chosen randomly to be considered for each patient and for each control subject. Statistical analyses were performed with SPSS 12.0. The Mann–Whitney U-test was used to evaluate differences in demographic, clinical, electrophysiological and imaging data between FRDA patients and healthy controls. Possible relations between the structural and functional ophthalmologic parameters and the onset and duration of the disease, triplet expansion of the shorter expanded allele and ICARS score were analysed using the Spearman rank test for non-parametric data. Moreover, data for correlation between the structural and functional ophthalmologic parameters were also analysed. For all analyses, only P < 0.05 was accepted as statistically significant.


   Results
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 Introduction
 Methods
 Results
Discussion
 Funding
 References
 
Molecular investigation
All patients recruited in this study underwent genetic analysis of the FXN gene and the triplet expansion of the minor affected allele ranged from 200 to 1100 repetitions (mean = 435 GAA). Two patients were compound heterozygotes for the expansion and a point mutation. The first patient carried a GAA expansion of 900 triplets and a nucleotide deletion (157delC) producing a frameshift at codon 53 and the premature truncation of the protein at codon 75. The second carried a GAA expansion of 470 triplets and a 13-nt deletion at exon 3-intron three boundaries (c.381_384delTGGG+g.IVS3+1_+9delGTACCTCTT). This latter deletion eliminates the last four nucleotides of exon 3 and the first nine nucleotides of intron 3, including the splice donor site, possibility leading to an aberrantly spliced mRNA that could be unstable and rapidly degraded (Gellera et al., 2007Go).

Neuro-ophthalmologic investigation
Two patients presented with sudden bilateral loss of central vision at 25 and 29 years of age respectively, mimicking the clinical course of LHON. They had 1100 and 850 triplets in the shorter expanded allele, respectively. One of these patients reported visual loss immediately after vigabatrin administration as antispastic; this is a drug with recognized optic nerve toxicity (Frisen et al., 2003Go). At the time of examination, 2 and 8 years respectively after onset, the patients presented a residual visual acuity of counting fingers and fundus examination demonstrated diffuse optic disc atrophy (Fig. 2A). Furthermore, P-VEPs were absent and RNFL thickness at OCT was severely reduced compared to controls (average 49 and 33 versus 100 ± 8.9).


Figure 2
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  		Fig. 2 Colour fundus pictures of three FRDA patients with different levels of severity of optic neuropathy. (A) Diffuse optic disc atrophy in a patient with a Leber-like acute and severe loss of visual acuity. (B) Diffuse pallor of the optic disc in a patient with intermediate pattern of RNFL thickness reduction and normal visual acuity. (C) Normal appearance of the optic disc in a patient with mild pattern of RNFL thickness reduction and normal visual acuity.

 
Concerning the remaining 24 patients, three were symptomatic with reduced visual acuity, and 21 were completely asymptomatic for visual disturbances. The three symptomatic individuals included two compound heterozygotes (900 and 470 GAA triplet expansion) and one homozygous patient with 600 GAA triplet expansion in the shorter allele and a disease duration of 24 years. This latter patient was only partially investigated because of severe nystagmus. The two compound heterozygote cases stand out from the rest of our case series and had severe ophthalmologic features, close to those suffering the LHON-like visual loss. In fact, they had consistent reduction of RNFL thickness compared to controls (average 53 and 42 versus 100 ± 8.9), bilateral involvement of central visual field with reduction of visual acuity (20/200 and 20/25 respectively) and absent P-VEPs response.

In the 21 asymptomatic patients the ophthalmoscopic investigation showed a variable optic disc appearance (Fig. 2B and C), ranging from diffuse optic disc pallor to essentially normal fundus appearance. Visual field examination showed variable patterns of impairment as reported in Fig. 3. All examined patients demonstrated one of three different common patterns of visual field defect. The first pattern represented severe visual field impairment with general and concentric reduction of sensitivity (Fig. 3A). A second pattern was characterized by a mild reduction of sensitivity and a concentric superior or inferior arcuate defect (Fig. 3B). Finally, the third pattern showed very little depression and only an isolated small paracentral area of reduced sensitivity (Fig. 3C).


Figure 3
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  		Fig. 3 Full-threshold, 24° and 30° visual field of the right eye (OD), left eye (OS) and the corresponding pattern of RNFL thickness in each quadrant at OCT. (A) Severe visual field impairment with general and concentric reduction of sensitivity (Type 1 in D). (B) Mild reduction of sensitivity and concentric inferior arcuate type defect (Type 2 in D). (C) Isolated area of reduced sensitivity (Type 3 in D). (D) The increasing reduction of RNFL thickness at OCT was graded to three types that corresponded to the visual field patterns shown in A–C. Type 1 shows diffuse and severe reduction of RNFL thickness in all quadrants. Type 2 shows diffuse RNFL reduction more marked in the superior quadrant corresponding to the inferior visual field defect. Type 3 shows diffuse but mild reduction of RNFL thickness in all quadrants.

 
OCT measurements of RNFL thicknesses showed a statistically significant reduction in the temporal (61 ± 9.6 versus 71 ± 14.4; P = 0.006), superior (87 ± 21.4 versus 125 ± 14.0; P < 0.0001), nasal (58 ± 20.9 versus 78 ± 15.6; P = 0.0005), inferior (98 ± 20.1 versus 126 ± 14.4; P < 0.0001) quadrants and the 360° average measurement (76 ± 12.0 versus 100 ± 8.9; P < 0.0001) in patients compared to controls (Fig. 4). Thus, all patients had a reduced number of axons, which were graduated in three categories, progressively more severe, that corresponded with the previously defined patterns of visual field defects (Figs 3D and 4).


Figure 4
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  		Fig. 4 RNFL thickness (average and each quadrant) of FRDA patients, having excluded the two LHON-like and the two compound heterozygote patients, compared to controls. (Each box shows the median, quartiles and extreme values; **P ≤ 0.01).

 
P-VEPs 31' were abnormal in 27 (61%) out of 44 eyes investigated (14 out of 21 patients), whereas P-VEPs 15' were abnormal in 29 (66%) out of 44 eyes (15 out of 21 patients). Absent response was detected in four eyes at 31' (9%) and 10 at 15' (23%) out of the 44 eyes investigated. Eight eyes (18%) showed only decreased amplitude at 31', whereas at 15' showed also increased latency. F-ERG was normal in all patients. In order to include the patients with absent VEP response in the correlation analysis, we used a semi-quantitative score for latency abnormalities (normal = 0; 1–25% increase= 1; 26–50% = 2; 51–75% = 3; 76–100% = 4; >100% = 5; absent = 6).

We then correlated functional (visual field mean deviation, P-VEPs 31' and 15') and anatomical (average RNFL thickness at OCT) parameters with age of onset, disease duration, GAA triplet expansion, and ICARS total score. Table 2 shows that P-VEPs and RNFL thickness were significantly correlated with age of onset and ICARS total score. None of these parameters was correlated with disease duration and the GAA triplet expansion in the 21 patients considered, having excluded the two LHON-like and the two compound heterozygote patients (Table 2). This discrepancy was possibly due to three outlier cases, two of whom carried high triplet expansion (768 and 1000 GAA) and did not present a severe ophthalmologic pathology. The opposite was observed in one case with low number of repeats (200 GAA). The correlation between the functional (visual field mean deviation, P-VEPs 31' and 15') and anatomical (average RNFL thickness at OCT) parameters are shown in Table 3. A significant correlation was detected between visual fields and P-VEPs, both parameters exploring the entire visual pathway.


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  		Table 3 Spearman correlation (r) between functional and anatomical ophthalmologic parameters in 21 FRDA having excluded the two LHON-like patients and the two compound heterozygote patients

 
DWI investigation
A subset of 13 patients (age 31 ± 8 years, mean ± SD) and 18 matched healthy volunteers (28 ± 8; P = 0.3) were studied by DWI. Water ADC values were not statistically different in corresponding right and left hemisphere ROIs in either normal subjects or patients and hence are reported as mean values. The FRDA patients had significantly higher ADC values in the optic radiations than healthy subjects (P < 0.01) (Table 4A and Fig. 1B). ADC values correlated with age of disease onset (r = –0.66, P < 0.01), disease duration (r = 0.65, P < 0.05), GAA triplet expansion (r = 0.71, P < 0.01), total ICARS score (r = 0.69, P < 0.01) and all neurophysiologic parameters, particularly P-VEPs-15' latencies (r = 0.80, P = 0.001) (Table 4B). We failed to find a correlation between ADC values and average RNFL thickness or visual field from the same patients.


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  		Table 4 Panel A Comparison of mean ADC values in optic radiations of FRDA patients and controls. Panel B Correlations between mean ADC values of optic radiations in FRDA patients and age at onset, disease duration, triplet expansion, ICARS score, P-VEP values, average RFNL thickness and visual field defect

 

   Discussion
Top
 Summary
 Introduction
 Methods
 Results
 Discussion
Funding
 References
 
Our study fills a historical gap in the clinical description of FRDA, the best studied form of hereditary ataxia, and provides novel details on visual pathway involvement and pathophysiology due to mitochondrial dysfunction. We systematically studied the visual pathway by the integrated use of different techniques, exploring the anterior visual pathways (RGCs and their axons forming the optic nerve) by OCT, the posterior pathways (optic radiation) by DWI, and both by visual fields and P-VEPs. We showed that all FRDA patients investigated presented some degree of visual pathway involvement, notwithstanding a wide range of variability in multi-systemic clinical expression. However, only in a small subset was this apparent clinically, explaining why visual impairment is underestimated in FRDA. Loss of central vision associated with poor visual acuity appears to occur late in the course of FRDA, with predilection for patients compound heterozygotes, as previously reported (Cossee et al., 1999Go). Our findings suggest that in FRDA there exists a slowly progressive degenerative process involving both the optic nerve and the optic radiations. In a small number of cases, all with severe diseases and large triplet expansion, a subacute/acute visual failure mimicking the mitochondrial disease LHON may overlap the slow progression of FRDA optic neuropathy.

The integrated findings with visual fields and OCT, link a functional evaluation with a technique exploring objectively the structural changes of fiber loss in the optic nerve. The present study showed a diffuse and progressive pattern of fiber loss, which initially did not result in a visual field defect (Fig. 3). The appearance of such defects, starting from the periphery and becoming increasingly severe concentrically, relates to the crossing of a threshold in terms of fiber loss. This loss, based on the defect observed, is most probably uniformly scattered over the entire RGCs pool, without the typical selectivity shown by other mitochondrial optic neuropathies, such as LHON or OPA1-related DOA where the papillomacular bundle is preferentially affected (Carelli et al., 2004Go). The visual field and OCT data were corroborated by electrophysiology, which investigates the integrity of the entire visual pathway, from the eye to the visual cortex.

In contrast, we observed in two cases a rapid, subacute loss of vision that clearly resembled the typical pattern of LHON, with a central vision defect. Prior to their subacute loss of vision, these two patients did not complain of visual defects, though often peripheral field losses are not subjectively evident. These two cases were peculiar for having a large GAA triplet expansion (1100 and 850 repeats in the shorter allele, respectively), and in one patient a possible exogenous trigger (vigabatrin) may have played a role. The ophthalmologic features of these two acute cases were similar to other two recently reported, one of which was a compound heterozygote (Givre et al., 2000Go; Porter et al., 2007Go). In our study too, despite the small number of cases, the only two compound heterozygote patients were in fact most severely affected with symptomatic visual loss. This confirms the findings of Cossee et al. (1999Go) that showed the high recurrence of pale optic discs in FRDA compound heterozygote patients.

We also explored the optic radiations by DWI, evaluating the post-geniculate visual pathway separately from the pre-synaptic anterior visual pathway that was explored by OCT. DWI allows the assessment of water ADC, a measure of tissue water diffusivity. Typically, pathological processes that modify tissue integrity, as in neurodegenerative disorders, result in an increased ADC (Nicoletti et al., 2006Go). This study demonstrates that a pathological process affects also the post-geniculate posterior visual pathway (Fig. 1). The changes revealed by DWI may relate to a degenerative process of the posterior visual pathways that is independent from the anterior visual pathways (optic nerves). The alternative explanation would entail a trans-synaptic mechanism involving first the optic nerve and only secondarily the post-geniculate radiations. However, we failed to find a correlation between ADC values and RNFL thickness, as evaluated by OCT (Table 4B), suggesting that involvement of the anterior and posterior visual pathways may be independent and asynchronous. This is further supported by the correlation found between the functional parameters visual field and P-VEPs (Table 3), both involving the evaluation of the entire visual pathway, but not with the RNFL thickness (anterior pathway).

Our current results provide new details on visual pathway involvement in FRDA, and also provide evidence that in this mitochondrial multi-systemic disorder the pattern of visual loss and the underlying pathological mechanism are clearly different from the classical clinical features that characterize the so-called non-syndromic mitochondrial optic neuropathies, such as LHON and OPA1-related DOA (Carelli et al., 2004Go, 2006Go). In particular, in FRDA the papillomacular axonal system is not preferentially involved. However, under special conditions a different, rapid acute/subacute loss of vision mimicking LHON may occur, as seen in two patients from our case series, and as previously reported (Givre et al., 2000Go; Porter et al., 2007Go). The selectivity for the papillomacular bundle in LHON and DOA has been related to the general feature of all RGC axons, which are non-myelinated in their initial portion and myelinated after the emergence from the lamina cribrosa at the posterior pole of the ocular globe (Carelli et al., 2004Go) and to special features of the papillomacular bundle fibers that consist of the smallest and least myelinated axons. A combination of energy failure, oxidative stress, predisposition to apoptosis and mitochondrial distribution within this axonal system is considered the pathological basis for RGC degeneration in both LHON and DOA (Carelli et al., 2007Go). In the case of FRDA, the progressive scattered loss of RGCs strongly indicates that other pathological mechanisms and timing are at work.

The currently favoured hypothesis for the main functional role of frataxin is its involvement in the biogenesis and the chaperoning of the Fe-S cluster insertion in various enzymes (Tan et al., 2003Go; Shan et al., 2007Go). Alterations in frataxin may have consequences in the respiratory chain leading to bioenergetic impairment (Rötig et al., 1997Go; Lodi et al., 1999Go) and increased oxidative stress (Wong et al., 1999Go), accumulation of iron (Babcock et al., 1997Go) and thus increased sensitivity of cells to undergo apoptosis (Tan et al., 2003Go). Most of these features are common to the non-syndromic optic neuropathies LHON and DOA, which however seem more specifically related to a defect of complex I (Carelli et al., 2004Go; Zanna et al., 2008Go) and involve a more specific subset of visual fibers. In FRDA complexes I, II and III are all involved, thus limiting the compensatory mechanisms, which may operate in LHON and DOA. In fact, the clinical expression is a much more severe and widespread disorder, involving the central nervous system and heart as main targets. The issue of mitochondrial transport down the long axons of spinocerebellar tracts, or along the vulnerable fibers of the optic nerve associates neurodegeneration in FRDA with other multisystemic mitochondrial disorders that involve the optic nerve such as, for example, complicated spastic paraplegia (Casari et al., 1998Go). However, it is remarkable that despite the different pathological mechanisms that apparently involve the visual pathways in FRDA, LHON-like cases may also occur in the presence of high GAA triplet expansion (our cases) or compound heterozygosis and severe clinical expression (Givre et al., 2000Go; Porter et al., 2007Go). Thus, even in FRDA we have a spectrum of clinical expression that goes from the slowly progressive pattern of RGC degeneration to the rapid, more catastrophic pattern of papillomoacular RGC loss, peculiar to the non-syndromic mitochondrial optic neuropathies (Carelli et al., 2004Go).

In conclusion, we have provided a detailed characterization of visual pathway involvement in FRDA, showing an asynchronous, slowly progressive degeneration of both anterior and posterior visual pathways. In our series of patients we confirmed the possible occurrence of acute/subacute cases mimicking LHON. Clinically significant visual impairment is late in FRDA patients, but degeneration of these neurons must be not underestimated and should be considered in therapeutic decision making. Furthermore, the correlation of both RNFL thickness and optic radiation ADC values with ICARS suggests that OCT and DWI may become tools for evaluating disease activity and progression, possibly useful for future clinical trials in FRDA.


   Funding
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 Summary
 Introduction
 Methods
 Results
 Discussion
 Funding
References
 
Fondazione Gino Galletti (grant to V.C.); the Ministry of Health (Ricerca Finalizzata 2006 to V.C.).


   Acknowledgements
 
We are deeply indebted to all patients and their families for participating to this project. We also thank Ms Monica Lucchi for her technical help.


   References
Top
 Summary
 Introduction
 Methods
 Results
 Discussion
 Funding
 References
 
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