Introduction

Kearns-Sayre Syndrome (KSS) is an extremely uncommon mitochondrial myopathy that mostly affects the heart, muscles, eyes and the central nervous system.^1,2,3,4,5 Deletions of mitochondrial DNA cause KSS, which progresses to neuromuscular failure. KSS was initially identified in 1958 and is classified as a mitochondrial condition as it impacts cellular energy generation.^4,6 KSS is one of several mitochondrial illnesses brought on by anomalies in the DNA of the mitochondria. These conditions affect the body's ability to produce energy and can affect several organ systems, especially those with high energy requirements, including the nervous system and muscles. KSS symptoms usually start to show up before the age of 20.^1,2,5 Typical symptoms include pigmentary retinopathy, which results in impaired vision, and progressive external ophthalmoplegia (PEO), which causes weakness or paralysis of the eye muscle. Additional symptoms may include low stature, sensorineural hearing loss, ataxia, cardiac conduction problems, and endocrine disorders like diabetes mellitus.

To manage KSS as well as possible, early diagnosis is essential.¹ Early identification enables preventive actions, such as heart monitoring and supportive care, to improve quality of life because the disease affects numerous organ systems.^1,6 To diagnose KSS and differentiate it from other neuromuscular illnesses, clinical features, muscle biopsy, and genetic analysis are essential.⁴ This article's goal is to review and evaluate the diagnostic procedures for KSS, paying special attention to muscle biopsies, genetic testing, and clinical criteria. Clinicians can improve diagnostic sensitivity and customise patient care by being aware of these techniques.

Clinical criteria for KSS diagnosis

Classical triad for KSS diagnosis

A definite diagnosis of Kearns-Sayre Syndrome requires the presence of three main clinical symptoms, which form the basis of the diagnostic criteria.^1,2,3

• Onset before age 20:Although usually before this age, research has shown that some patients may exhibit symptoms or be diagnosed beyond 20 years, perhaps as a result of an illness that develops slowly or manifests mildly^1,2,3,6
• Progressive external ophthalmoplegia (PEO): This is known as chronic progressive external ophthalmoplegia (CPEO), which causes drooping eyelids (ptosis) and reduced eye movement due to weakening or paralysis of the eye muscles.^1,2,3,4 ptosis frequently manifests in childhood or adolescence as one of the initial signs^3,4.6
• Pigmentary retinopathy: This condition is brought on by the degeneration of the retina, which is the tissue in the rear of the eye that senses light.³ It is characterised by an abnormal buildup of pigmented material on the nerve-rich membrane lining the eyes, which frequently takes the form of a distinctive "salt-and-pepper" pattern^2,5,6

Additional clinical findings

Affected people must exhibit at least one of the following disorders in addition to the traditional triad in order to be diagnosed with Kearns-Sayre Syndrome:

• Cardiac conduction defects
• Cerebellar ataxia
• Elevated cerebrospinal fluid (CSF) protein levels

Role of neurological and ophthalmologic exams

Because KSS is multisystemic, neurological and ophthalmological examinations are essential to the diagnosis process - see below.

Ophthalmological examinations 

As KSS affects the eyes, these are essential.⁶ They make direct assessments to determine whether external ophthalmoplegia and ptosis are present and how they are developing.^3,4.6 The distinctive "salt-and-pepper" appearance of pigmentary retinopathy can be detected with fundus photography.^1,2,6 In contrast to retinitis pigmentosa, KSS usually exhibits normal or slightly aberrant electroretinography (ERG) readings, which can help distinguish pigmentary retinopathy from other retinal illnesses. The examination also includes tests of visual acuity and evaluations for corneal clouding and nystagmus.

Neurological exams

These check for neurological symptoms such as weakness in the muscles of the face, throat, neck, shoulders, and limbs, as well as cerebellar ataxia (such as unstable walking or cerebellar tremor).^1,2,3,4 They also check for dementia, memory loss, and cognitive impairment.^3,4 To identify sensorineural hearing loss, which is prevalent in KSS, hearing tests (audiometry) are crucial. One important diagnostic procedure is a lumbar puncture, which measures the protein levels in cerebrospinal fluid. (CSF).^1,2,3,4 The diagnosis can be supported by abnormalities seen by brain MRI, such as symmetric high-T2 signals in the brainstem, thalamus, basal ganglia, and cerebral and cerebellar white matter.^1,2,3,4  

Genetic testing in KSS

Since Kearns-Sayre Syndrome (KSS) is mostly linked to extensive deletions in mitochondrial DNA (mtDNA), which impair the synthesis of cellular energy, genetic testing is essential to validating the diagnosis.^1,4

Mitochondrial DNA (mtDNA) deletions in KSS

Large-scale deletions in mitochondrial DNA are a defining feature of KSS.^1,2,4,6 A new, spontaneous (de novo) loss of a significant quantity (usually ~25%) of mtDNA causes about 90% of KSS cases to be sporadic.^2,3 The magnitude of these deletions might vary from 1.1 to 10 kilobases.^2,6 With about one-third of cases, the "common 4977 bp deletion" is the most commonly seen mtDNA deletion in KSS patients.²

Methods used

• Polymerase chain reaction (PCR) - a sensitive and quick way to detect deletions by amplifying particular mtDNA regions
• Southern blot analysis - uses tagged probes to hybridise DNA fragments in order to visualise large-scale deletions. Next-generation sequencing (NGS) - Offers a thorough examination of mtDNA, identifying degrees of heteroplasmy, mutations, and deletions

Importance of testing multiple tissues 

The selection of tissue for genetic testing is crucial because of the nature of mitochondrial disorders, which can cause both normal and mutated mtDNA to coexist in the same cell (a condition known as heteroplasmy).³ One popular technique for verifying the diagnosis is a muscle biopsy.^1,3,6 Muscle biopsy tissue samples can be examined under a microscope to identify histopathological characteristics of mitochondrial illness, such as "ragged-red fibres" (RRFs), "ragged blue fibres" (RBFs), and cytochrome oxidase (COX) - negative fibres.^1,2,6 Deleted mtDNA, which might not be found in blood samples, can also be found by muscle biopsy.³

The first stage in genetic testing is frequently blood testing.^2,4 Nonetheless, it is essential to sequence the muscle mtDNA of individuals with a high clinical suspicion if blood testing for KSS comes back negative.^2,3 This is because low blood heteroplasmy levels may result in false-negative results and miss tissue-specific alterations. Different organs may be impacted, resulting in a range of symptoms and severity within a family, which can also be explained by the unequal distribution of normal and mutated mtDNA in various tissues.³

Genetic counselling implications

Because KSS is sporadic, mtDNA deletions occur de novo during development and are not inherited from either parent, genetic counselling is advised for those who have it and their families.^3,4 Rarely, a mother's mtDNA deletions may be inherited, enabling impacted mothers to pass on mutations to their offspring.^3,4 Finding out if a mutation happened de novo or was inherited can be aided by genetic testing.⁴ The reason why symptoms could not show up until a sizable percentage of mitochondria are impacted by the deletion is explained by heteroplasmy and varied expression.³ Families can better comprehend this diversity and its effects on future generations with the use of genetic counselling.

Muscle biopsy in diagnosis

A crucial diagnostic procedure for Kearns-Sayre Syndrome (KSS) is muscle biopsy, which is frequently carried out for this reason and validated in patient cohort studies.^1,6 It is essential for distinguishing KSS from other conditions that manifest as ophthalmoplegia.¹

Histological and histochemical findings

Muscle biopsy tissue samples exhibit distinctive abnormalities that are thought to be histological hallmarks of mitochondrial illness when examined under a microscope.¹ These consist of:

• Ragged red fibres (RRF) - These are aberrant muscle fibres that show up as "ragged-red" when stained with modified Gomori trichrome (MGT)^1,2,3,6
• Cytochrome c oxidase (COX)-negative fibres - When using succinate dehydrogenase (SDH) staining, they are also visible.^1,6 The proportion of RBFs in biopsied individuals varied from 0.3% to 16.0% in one research¹

Role of electron microscopy

Numerous aberrant mitochondria in skeletal and ocular muscle tissue can be seen by electron microscopy on biopsy tissue samples.³

Muscle biopsy benefits and drawbacks in mitochondrial dysfunction

Benefits include:

• Confirmatory Diagnosis: According to clinical criteria, muscle biopsy is frequently used in conjunction with a mitochondrial DNA (mtDNA) study to confirm a KSS diagnosis^1,6
• Identification of Tissue-Specific Mutations: Deleted mtDNA, which blood samples might not identify, can be found by muscle biopsy.^2,3 - This is especially crucial since tissue-specific mutations or low blood levels of heteroplasmy might cause blood tests to come back falsely negative^2,3
• Histopathological Hallmarks: These prominent markers of mitochondrial myopathy include the distinctive ragged-red, ragged-blue, and COX-negative fibers^1,2

Drawbacks include:

• Invasiveness: Muscle biopsies are invasive operations¹
• Historical Availability: Because muscle biopsies may not have been common or generally accessible at the time of diagnosis, not all patients in older case series may have had them⁶
• Findings Variability: Although typical, the percentage of ragged blue fibres (RBFs) has been found to fluctuate and, in one investigation, did not correspond with the age of onset or the magnitude of mtDNA deletions.¹ There is also no correlation between the size of mtDNA deletions and the degree of muscle pathogenic alterations¹

Summary

Kearns-Sayre Syndrome (KSS) is a multisystem, progressive condition with aspects of progressive external ophthalmoplegia, pigmentary retinopathy, heart block, cerebellar ataxia, or elevated cerebrospinal fluid protein. Genetic testing, muscle biopsy, heart evaluation, brain MRI, and CSF analysis are all used in the multimodal diagnosis process. Genetic testing in the guise of next-generation sequencing (NGS) is the imaging modality of choice for extensive mitochondrial DNA (mtDNA) deletions. The distinctive ragged-red, ragged-blue, and COX-negative fibres found in muscle biopsies are diagnostic.

Ancillary tests include cardiac assessment, brain MRI, and CSF examination for elevated protein and cerebral folate deficiency. Prompt diagnosis is necessary for the management of complications, particularly cardiac complications, because heart conduction blocks can go on to total atrioventricular block rapidly, with serious risk of sudden cardiac death. Future molecular diagnosis includes more accurate evaluation of mtDNA deletion heteroplasmy in individual tissues, personalised medicine with targeted genetic therapy, and metabolic supplementation with antioxidants. Growth hormone therapy and others are still controversial, with potential risks.