Neuropathological aspects in amyloidotic polyneuropathy

Introduction

Amyloidosis can be closely linked to multiple myeloma, a type of bone marrow cancer. In both diseases, abnormal antibody-producing plasma cells proliferate uncontrollably. Certain forms of amyloidosis can occur alongside or be associated with various neoplastic syndromes. In some specific types of amyloidosis, amyloid neuropathy is also present, which is a key focus of this discussion. These neuropathies occur due to the deposition of amyloid in nerve tissues, leading to symptoms affecting sensory, motor, and autonomic nerve fibers. Understanding the relationship between amyloidosis and these syndromes is crucial for diagnosis and management, particularly in cases where neoplasms are involved. In light-chain (AL) amyloidosis, these cells produce fragments of immunoglobulin light chains that misfold, forming amyloid fibrils which accumulate in tissues and organs. Similarly, identical clones of these plasma cells grow rapidly in multiple myeloma, leading to the overproduction of abnormal proteins that can cause organ damage and other complications. Despite this close relationship, AL amyloidosis is distinct from multiple myeloma, but often coexists or shares similar underlying mechanisms. The clinical manifestations of systemic amyloidosis are primarily influenced by the pathochemistry of the precursor protein and the affected organs. Commonly involved organs include the heart, kidneys, nervous system, liver, and gastrointestinal tract. However, the variability in symptoms and organ involvement complicates the diagnosis of these disorders. Peripheral and autonomic neuropathy are particularly common in both light-chain amyloidosis and hereditary transthyretin (ATTRv) amyloidosis.

Nevertheless, peripheral neuropathy can occur in other types of amyloidosis, most commonly gelsolin⁽¹⁾, and potentially in wild-type transthyretin (ATTRwt), apolipoprotein AI, and fibrinogen. AL amyloid is a clonal non-proliferative plasma cell disorder in which immunoglobulin light or heavy chain fragments are conveyed in tissues⁽²⁾. It is estimated to have a minimum incidence of 8 to 12 cases per 1 million people⁽³⁾. Approximately 3500-4500 new patients with AL amyloidosis are diagnosed in the USA each year^(4,5). The intermediate age at diagnosis is 76 years old. The most frequently involved organs are the heart (70-80%) and the kidneys (50-60%). The disease presents insidiously with constitutional symptoms, such as fatigue, weight loss, and anorexia. As the disease is heterogeneous, the supplementary presenting symptoms are directed by the involved organ. Peripheral and autonomic neuropathy occur in approximately one-quarter of AL patients^(6,7). The five-year overall survival improved from 15% between 1980 and 1989 to 48% between 2010 and 2019⁽⁸⁾. However, the one-year mortality rate stays high (12-30%) due to temporization in diagnosis, leading to irreversible organ failure⁽⁹⁾. Globally, ATTRv is the most frequent form of familial amyloidosis, with an estimated prevalence of 10,000 people⁽¹⁰⁾. The most common age of disease onset is 39 years old, differing from the second to ninth decades⁽¹¹⁾. Peripheral neuropathy is the global leading manifestation, occurring in approximately 80% of patients; however, the prevalence varies based on the specific mutation. Cardiac involvement is seen in 40% of ATTRv patients. Survival from symptom onset ranges from six to 12 years, with longer survival in those presenting before the age of 50 years old and in those with a Val30Met mutation⁽¹²⁾. Despite advances in treatment for AL and ATTRv, there continues to be a delay in the diagnosis of amyloid neuropathy, leading to increased morbidity and mortality.

Neurobiological pathology

Amyloidosis is a protein misfolding disease that arises when a protein fails to achieve its soluble functional fold and deposits in tissues in the form of amyloid fibrils^(13,14). The precursor proteins of amyloidosis, including immunoglobulin free light chain and transthyretin, are proteins that can adapt more than one conformation, making them prone to misfold and form into amyloid fibrils.

Immunoglobulin-free light chains are synthesized by a marrow plasma cell clone. Overproduction of a single clone of light chain results in permanently high concentrations of protein in serum promoting amyloidogenicity. The variable (V) domains of monoclonal light chains suffer somatic mutation during the immune response, which can affect critical structural sites, causing disconformity in the domain and favoring an aggregation-prone state^(15,16). This may explain why light-chain amyloidosis only occurs in a division of patients with a circulating monoclonal component. Lambda (ë) light chains are approximately two-fold more frequent than kappa (ê) light chains. Preferential expression of 2 Vë germline gene segments (3r and 6a) are responsible for 42% of total amyloidogenic ë chains and account for the ë chain predominance in AL amyloidosis⁽¹⁷⁾. High serum levels of free light chains can anticipate the appearance of AL amyloidosis for many years⁽¹⁸⁾.

TTR is a 55-kDa homotetrameric protein, synthesized predominantly in the liver, as well in the choroid plexus and retinal pigment epithelium. The tetramer has a central channel containing two thyroxine binding sites and functions as a transporter for thyroxine and retinol-binding protein identical to the vitamin A complex. In hereditary transthyretin amyloidosis, a point mutation replaces a single amino acid in the protein, conferring a higher probability of misfolding⁽¹⁹⁾. The morphology and biochemical aspects of amyloid fibrils can vary depending on the causal mutation and age of onset. Full-length (long and thick) amyloid fibrils with a strong affinity for Congo red were observed in patients with early-onset Val30Met, considered as a mixture of full-length and TTR nonfibrillar fragments (short and thin) that is weakly stainable by Congo red were seen in patients with late-onset Val30Met and other mutations⁽²⁰⁾. In ATTRwt, there is an increased intrinsic propensity of the protein to assume pathological conformation with the aging process. Age-related protein oxidative changes, failure in the proteostatic and repair mechanisms, and a decrease in TTR amyloid clearance may be involved⁽²¹⁾.

The pathophysiology of peripheral neuropathy in amyloidosis has been granted based on nerve biopsy findings and ultrastructural studies. Amyloid deposits are present in the endoneurium of peripheral nerves and are often extended in the dorsal root ganglia and sympathetic ganglia⁽²²⁾. The disposal of amyloid deposits corresponds with the extent of the degeneration of neurons⁽²³⁾. Destruction of dorsal root and sympathetic ganglion neurons, especially those of small size, is a main characteristic⁽²⁴⁾. Axonal degeneration is noticeable in the distal regions of nerves, whereas segmental demyelination and remyelination are observed in the proximal areas⁽²⁵⁾. Electron microscopic examination of sural nerve biopsy from AL amyloidosis patients shows atrophy of Schwann cells closeness to amyloid fibrils⁽²⁶⁾. The processes of these Schwann cells are entangled in amyloid fibrils, and their basement membranes are sometimes fused with the fibrillar masses⁽²⁷⁾. In ATTRv amyloidosis, similar disconformities in Schwann cell processes and bleared Schwann cell membrane contiguous with amyloid fibrils were remarked in early-onset cases. It is thought that the long and thick amyloid fibrils, characteristic in early-, but not late-onset cases, may cause tractions of neighboring membranes, resulting in the distortion of the Schwann cells⁽²⁸⁾. This pathological process seems more crucial to smaller-diameter nerve fibers corresponding to sensory and autonomic impairments observed at the early phase of neuropathy in these cases.

Neuromuscular involvement in amyloidosis

Both AL amyloidosis and ATTRv amyloidosis commonly affect the peripheral and autonomic nervous systems, though they rarely impact muscle tissue. While there are shared presentations and symptoms of neuropathy in both types, the prevalence of these symptoms varies between AL and ATTRv, as well as among different ATTRv mutations. Neuropathy can present as an initial symptom in both conditions, but it is more frequently seen in ATTRv than in AL amyloidosis. Additionally, coexisting autonomic symptoms are often present alongside peripheral neuropathy, and these autonomic issues can serve as crucial indicators of the disease⁽²⁹⁾. Demyelinating effects, such as conduction velocity slowing and conduction block, can occur in late-onset Val30Met and other mutations, potentially mimicking conditions like chronic inflammatory demyelinating polyradiculoneuropathy (CIDP). Diagnostic clues include a lack of response to typical intravenous immunoglobulin (IVIg) treatment and the presence of autonomic symptoms. Patients may present with both proximal and distal weakness, and the neuropathy can resemble Charcot-Marie-Tooth neuropathy, characterized by distal atrophy and pes cavus (high arches). Sensory and autonomic neuropathy symptoms are typically the first to manifest in carriers of these mutations.

In ATTRv, peripheral nerve involvement often begins with carpal tunnel syndrome, which can precede other symptoms by several years. This condition typically affects the non-dominant hand and can be bilateral, frequently presenting as severe, with significant decreases in median motor amplitude and denervation in the abductor pollicis brevis. The prevalence of carpal tunnel syndrome in ATTRv patients exceeds 70% and often appears more than 10 years before the diagnosis of ATTRv.

While carpal tunnel syndrome can occur in up to 30% of AL amyloidosis cases, it is less commonly the presenting symptom. Other mononeuropathies reported in AL include ulnar and radial mononeuropathies, which are more frequent than lower extremity mononeuropathies⁽³⁰⁾.

Autonomic symptoms are prevalent in both AL and especially ATTRv amyloidosis, affecting most patients with the Val30Met mutation, even in less neuropathic mutations.
 

Clinical presentation

Gastrointestinal symptoms. Often, these are the earliest signs, with diarrhea or alternating between diarrhea and constipation, though some patients may only experience constipation. Diarrhea is particularly important for distinguishing amyloid-related autonomic failure from other conditions, such as alpha-synuclein-related disorders (e.g., pure autonomic failure and Parkinson’s disease), which typically present with constipation. Other gastrointestinal issues include nausea, vomiting, abdominal pain, bloating, and early satiety, which can lead to weight loss.

Heat sensitivity. Many patients report heat hypersensitivity and poor sweating.

Orthostatic hypotension. Common in many patients, orthostatic hypotension can complicate the treatment of heart failure and may vary significantly based on genetic factors.

Diagnostic considerations

Autonomic function testing. This testing frequen­tly shows abnormalities in both AL and ATTRv amyloidosis when conducted, highlighting the extent of autonomic involvement in these conditions⁽³¹⁾. Neurogenic orthostatic hypotension is found in many patients and is sporadically the presenting symptom of AL amyloidosis⁽³²⁾. Between 50% and 96% of patients with AL may have abnormalities in autonomic function testing when performed^(33,34). Neurogenic orthostatic hypotension is found in 40% to 60% of ATTRv patients. Autonomic function testing is abnormal in 74% of ATTRv patients and in 28% of asymptomatic carriers. The Valsalva ratio and parasympathetic testing are commonly abnormal, suggesting early vagal involvement^(35,36). In addition to affecting peripheral nerves, circulating amyloid fibrils can deposit in various other tissues, resulting in multiorgan dysfunction. This disease often presents with a diverse range of clinical signs and symptoms, which can be attributed to the involvement of amyloid deposition (Figure 2).
 

Ocular: patients may experience vision problems or other eye-related issues.

Renal system: amyloid deposits in the kidneys can lead to kidney dysfunction, proteinuria, and other renal complications.

Central nervous system: neurological symptoms may arise due to amyloid infiltration in the brain and spinal cord.

Gastrointestinal tract: patients often present with gastrointestinal symptoms, including nausea, vomiting, diarrhea, or constipation.

The widespread deposition of amyloid fibrils contributes to the complexity and variability of clinical presentations in amyloidosis, complicating diagnosis and management.

Diagnosis and diagnostic pitfalls

Neuropathy Impairment Score (NIS) comprises quantitative sensory and autonomic items and nerve conduction studies, achieving a higher detection of disease progression. Neurological involvement usually starts with sensory loss in the extremities and progresses with motor neuropathy.

Amyloid light chain amyloidosis (AL amyloidosis) should be suspected in patients with nephrotic range proteinuria, unexplained cardiomyopathy, unexplained weight loss, diarrhea, and other red flag symptoms. All patients with polyneuropathy should be monitored with serum protein electrophoresis and immunofixation, which will detect monoclonal proteins 85% of the time. Abnormal light chains reveal a peak in the ɣ range, producing an “M spike,” which can be valid to be immunoglobulin by immunofixation. In addition, a serum-free light chain assay for ê and ë immunoglobulin light chains can increase the detection of abnormal protein⁽³⁷⁾. Urinary light chains with immunofixation can find free light chains, not any other way detected, increasing the sensitivity to 98%⁽³⁸⁾.

Hereditary transthyretin amyloidosis (ATTRv amyloidosis). When there is suspicion of amyloidosis, TTR gene sequencing should be performed. Identification of TTR variants can help differentiate ATTRv from ATTRwt (wild-type transthyretin). Molecular genetic testing can be done either by single-gene or multigene panel approach. Sequence analysis of TTR detects missense, nonsense, and splice site variants; almost all probands with a pathogenic variant were detected by this method. An online registry (www. amyloidosismutations.com) provides a readily accessible repository of mutations and phenotypes in hereditary amyloidosis. It is important to note that two different nomenclatures are used in the literature to describe genetic variants⁽³⁹⁾. However, the genetic screening seems to be of lower yield among patients with idiopathic small fiber neuropathy without coexistent carpal tunnel syndrome.

Immunohistochemistry

The gold standard method for confirming amyloid deposition in tissue biopsies involves Congo red staining. When viewed under polarized light, the amyloid deposits show apple-green birefringence. Additionally, amyloid can be verified by its ultrastructural appearance, which consists of rigid, non-branching fibrils approximately 10 nm in diameter⁽⁴⁰⁾.

To determine the type of amyloidosis and guide treatment, the protein-forming amyloid fibrils must be identified. The new gold standard for fibril typing is proteomics using liquid chromatography-tandem mass spectrometry (LC-MS/MS), which directly analyzes proteins in amyloid deposits from paraffin-embedded tissue samples. This method is superior to antibody-based techniques like immunohistochemistry, immunofluorescence and immune-gold, as LC-MS/MS can identify all amyloid types in one assay, whereas antibodies target only a single type. In rare cases, two different types of amyloidosis can occur in the same patient, either in different anatomical sites or within the same tissue sample. When the clinical presentation is unclear, identifying the amyloid subtype in all affected tissues can have significant therapeutic implications⁽⁴¹⁾. A biopsy can be performed on the affected organ or a surrogate site (such as abdominal fat or minor salivary gland). While a biopsy of the clinically affected organ is more sensitive and can detect additional pathologies, a biopsy of alternate sites is often less invasive.

Imagistic studies

Cardiac imaging is mandatory in diagnosing and managing amyloidosis, with echocardiography being the most commonly used first-line modality. In patients with suspected cardiac amyloidosis (AL or ATTRv types), typical echocardiographic findings include:

Left ventricular (LV) hypertrophy (>1.2 cm).

Small LV cavity size.

Right ventricular hypertrophy.

Reduced LV global longitudinal strain, with preserved strain at the apex compared to basal segments.

Diastolic dysfunction.

Biatrial dilation.

Discordance between LV hypertrophy and low voltage on an electrocardiogram is another indicator of cardiac amyloidosis. In the early stages, LV ejection fraction is usually preserved but can deteriorate as the disease progresses⁽⁴²⁾.

Other important aspects

The symptomatic treatment of neuropathic and autonomic symptoms in amyloidosis follows similar protocols to other peripheral and autonomic neuropathies, with key considerations for potential side effects.

For neuropathic pain:

Tricyclic antidepressants and serotonin and norepinephrine reuptake inhibitors (SNRIs) may worsen autonomic symptoms and should be used cautiously.

Gabapentin and pregabalin may have limited use due to renal involvement and side effects like peripheral edema, particularly in patients with pedal edema.

Duloxetine and SNRIs might elevate blood pressure, requiring consultation with a cardiologist for patients with congestive heart failure.

For neurogenic orthostatic hypotension (OH), common in autonomic neuropathy, special care is needed when cardiomyopathy and congestive heart failure are present:

Volume-expanding treatments like fludrocortisone and additional salt should be minimized.

Higher dosages of pressor agents like droxidopa (600 mg three times daily) are often required, compared to typical lower doses.

Midodrine is also used for orthostatic hypotension and has efficacy even in patients with renal failure requiring dialysis.

A reasonable approach is allowing patients to maintain their fluid volume 1 kg above their dry weight.

Autonomic involvement, especially with OH, may disqualify patients with AL or ATTRv amyloidosis from transplant eligibility at many centers⁽⁴³⁾.

Conclusions

Amyloidosis is challenging to diagnose because of its various organ involvement and clinical presentations, but recent advancements in diagnostic techniques have improved the recognition of these disorders. Additionally, new treatments are emerging for both AL and ATTRv amyloidosis, offering hope for more effective management of the disease. Clinical electrophysiology studies (electromyography) combined with anatomical-pathological and imaging studies come to complete the scheme of investigations, contributing to good choice, dosage, and monitoring of therapy. 

Autori pentru corespondenţă: Bogdan-Marius Istrate E-mail: istratem.bogdan@yahoo.com

CONFLICT OF INTEREST: none declared.

FINANCIAL SUPPORT: none declared.

This work is permanently accessible online free of charge and published under the CC-BY.