Understand what your diagnosis actually means.
161 terms · Searchable below · Free PDFUnderstanding what your diagnosis actually means is the first step to advocating for yourself. Our Medical Glossary defines more than 160 terms across syringomyelia, Chiari malformation, Ehlers-Danlos Syndrome, POTS, MCAS, and dysautonomia.
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Every term, tagged by condition.
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Terms are listed alphabetically. Each one carries a color-coded tag showing which condition it matters most for, so you can filter to just the vocabulary of your diagnosis or see how a single word connects several of them.
Many of these conditions share terminology because they are physiologically interconnected, so a term may appear under more than one tag. That overlap is the point: the same word can mean one thing to your neurosurgeon and another to your cardiologist, and knowing it lets you follow both.
SyrinxSyringomyelia / syrinx
ChiariChiari malformation
EDSEhlers-Danlos Syndrome
POTSPostural Orthostatic Tachycardia Syndrome
MCASMast Cell Activation Syndrome
DysautonomiaDysautonomia (general)
GeneralApplies across multiple conditions
Not medical advice. This glossary is provided for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your qualified healthcare provider regarding any medical condition, symptoms, or treatment decisions.
A to Z. 161 terms.
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Showing 161 of 161 terms
A
Acetylcholine
A neurotransmitter involved in muscle activation and autonomic nervous system signaling. Relevant to dysautonomia and MCAS because cholinergic pathways regulate mast cell activity and heart rate response.
Acquired Chiari Malformation
A Chiari malformation that develops after birth due to conditions such as tethered spinal cord, lumbar puncture, intracranial hypotension, or trauma — as opposed to a congenital structural defect.
Adrenaline (Epinephrine)
A hormone and neurotransmitter released during the stress response. In POTS, excessive or dysregulated adrenaline release contributes to tachycardia, anxiety, and tremors during positional changes.
Allodynia
Pain caused by a stimulus that does not normally provoke pain, such as light touch or clothing contact. Common in EDS neuropathic overlap and central sensitization conditions including syringomyelia.
Anaphylaxis
A severe, life-threatening allergic reaction involving multiple organ systems. In MCAS, anaphylaxis can occur without a clear trigger and may be refractory to standard epinephrine treatment in some patients.
Antidiuretic Hormone (ADH / Vasopressin)
A hormone that regulates water retention by the kidneys. Dysregulation in POTS can lead to excessive fluid loss, worsening orthostatic intolerance and blood volume depletion.
Arachnoid
The middle of three meningeal layers surrounding the brain and spinal cord. Arachnoid scarring (arachnoiditis) can obstruct CSF flow and contribute to syrinx formation.
Arachnoiditis
Inflammation and scarring of the arachnoid membrane. A known cause of secondary syringomyelia; may result from surgery, infection, trauma, or contrast agents used in spinal imaging.
Arnold-Chiari Malformation
An older term for Chiari malformation, particularly Type II, which is associated with myelomeningocele (spina bifida). The term 'Chiari malformation' is now preferred in clinical literature.
Atlantoaxial Instability (AAI)
Excessive movement between the first (atlas) and second (axis) cervical vertebrae. Occurs with higher frequency in hypermobile EDS and Chiari patients; may compress the brainstem or spinal cord.
Autonomic Nervous System (ANS)
The division of the nervous system that regulates involuntary functions including heart rate, blood pressure, digestion, temperature regulation, and glandular secretion. Dysfunction of the ANS underlies POTS, dysautonomia, and many MCAS triggers.
Autonomic Neuropathy
Damage to the nerves that control the autonomic nervous system. May manifest as POTS, orthostatic hypotension, gastroparesis, bladder dysfunction, and abnormal sweating.
B
Baroreflex
A feedback mechanism in which stretch receptors in large arteries detect blood pressure changes and signal the autonomic nervous system to compensate. Impaired baroreflex sensitivity is a hallmark of some dysautonomia subtypes.
Baseline Tryptase
A blood test measuring resting serum tryptase levels, a key marker of mast cell burden. Elevated baseline tryptase (>11.4 ng/mL) can support a diagnosis of systemic mastocytosis or clonal MCAS.
Benign Hypermobility
Hypermobility without significant pain, functional impairment, or connective tissue pathology. Distinguished from hypermobile EDS (hEDS) by the absence of musculoskeletal complications.
Biofeedback
A therapy technique that trains individuals to control physiological processes using real-time feedback. Used in dysautonomia and chronic pain management to improve heart rate variability and autonomic regulation.
Bladder Dysfunction (Neurogenic Bladder)
Impaired bladder control caused by nerve damage. Common in syringomyelia when the syrinx extends into thoracic or cervical spinal cord levels that control bladder innervation.
Blood Volume
The total amount of blood circulating in the body. Low blood volume (hypovolemia) is common in hyperadrenergic POTS and neuropathic POTS, contributing to orthostatic symptoms.
Bradycardia
A resting heart rate below 60 beats per minute. Can occur in vasovagal syncope and some forms of dysautonomia; distinct from the tachycardia pattern seen in POTS.
Brainstem Compression
Mechanical pressure on the brainstem, often at the foramen magnum. A serious complication of Chiari malformation that can cause cranial nerve dysfunction, central sleep apnea, and in severe cases, respiratory compromise.
C
Cape Distribution
A pattern of sensory loss or pain affecting the shoulders, upper arms, and back in a cape-like distribution, characteristic of central cord pathology such as syringomyelia.
Cardiac Dysautonomia
Autonomic dysfunction specifically affecting heart rate and rhythm regulation. Encompasses POTS, inappropriate sinus tachycardia (IST), neurocardiogenic syncope, and other heart rate variability disorders.
Central Pain Syndrome
Chronic pain arising from damage or dysfunction within the central nervous system. Syringomyelia is a recognized cause; pain is often described as burning, stabbing, or dysesthetic, and is frequently refractory to standard analgesics.
Central Sensitization
A state in which the central nervous system amplifies pain signals, causing heightened sensitivity throughout the body. Common in EDS, fibromyalgia, and conditions with chronic neuropathic pain overlap.
Cerebellar Tonsillar Ectopia
The descent of the cerebellar tonsils below the foramen magnum. Ectopia of 5mm or more is the diagnostic threshold for Chiari malformation Type I. Smaller degrees may still be symptomatic with obstructed CSF flow.
Cerebellar Tonsils
The paired, rounded lobes at the base of the cerebellum. In Chiari malformation, they herniate downward through the foramen magnum, potentially obstructing CSF flow and compressing the brainstem.
Cerebellomedullary Cistern (Cisterna Magna)
A CSF-filled space at the base of the skull between the cerebellum and brainstem. Crowding in this cistern is assessed on MRI to evaluate CSF flow obstruction in Chiari malformation.
Cerebrospinal Fluid (CSF)
The clear fluid that surrounds and cushions the brain and spinal cord. In syringomyelia, abnormal CSF flow dynamics are the primary mechanism of syrinx formation. In Chiari, tonsillar herniation obstructs normal CSF pulsation.
Cervical Instability
Excessive movement or laxity in the cervical spine joints, often caused by hypermobile connective tissue in EDS. Can worsen Chiari symptoms and contribute to cerebrospinal fluid obstruction or craniocervical junction compression.
Chiari Malformation Type I (CM-I)
Descent of the cerebellar tonsils ≥5mm below the foramen magnum without brain malformation. The most common type in adults; frequently associated with syringomyelia, headaches, and sensorimotor deficits.
Chiari Malformation Type II
A more complex malformation involving herniation of both the cerebellum and brainstem, typically associated with myelomeningocele (spina bifida). Usually diagnosed in childhood.
Chiari Malformation Type III
A rare, severe form in which the cerebellum and brainstem herniate through the back of the skull. Associated with significant neurological impairment.
Chiari Malformation Type IV
Cerebellar hypoplasia (underdevelopment) without herniation. The least common classification; sometimes considered a distinct condition from classic Chiari.
Cholinergic Urticaria
Hives triggered by heat, sweat, or exercise due to a hypersensitivity reaction to acetylcholine or sweat proteins. Overlaps with MCAS in patients with heat-triggered mast cell activation.
Chronic Fatigue
Persistent, debilitating exhaustion not relieved by rest. A central symptom across syringomyelia, EDS, POTS, MCAS, and dysautonomia, often driven by autonomic dysfunction, mitochondrial dysfunction, or inflammatory burden.
Clonal Mast Cell Disorder
A subtype of MCAS in which mast cells carry a genetic mutation (commonly KIT D816V) causing abnormal proliferation. Includes systemic mastocytosis and monoclonal mast cell activation syndrome (MMAS).
Collagen
The primary structural protein of connective tissue, providing tensile strength to skin, tendons, ligaments, blood vessels, and organs. Defective collagen production or structure underlies EDS and contributes to hypermobility.
Connective Tissue
The tissue that provides structure, support, and cohesion to organs, joints, and vessels. In EDS, defective connective tissue causes joint hypermobility, skin fragility, and vascular vulnerability.
Corticotropin-Releasing Hormone (CRH)
A neuropeptide that activates the stress response and directly stimulates mast cells to degranulate. The CRH–mast cell axis may explain why stress reliably worsens MCAS symptoms.
Craniocervical Junction (CCJ)
The anatomical region where the skull meets the upper cervical spine, encompassing the foramen magnum, C1 (atlas), and C2 (axis). A critical area in Chiari malformation, syringomyelia, and EDS-related instability.
Craniotomy
Surgical removal of a portion of the skull to access the brain. In Chiari malformation, a suboccipital craniotomy is performed during posterior fossa decompression to enlarge the skull opening.
CSF Flow Study
A specialized MRI technique (phase-contrast MRI) that visualizes the movement of CSF through the brain and spinal cord. Used to confirm CSF obstruction at the foramen magnum in Chiari malformation and to assess flow restoration after surgery.
CSF Leak (Spontaneous Intracranial Hypotension)
Leakage of cerebrospinal fluid through a tear in the dural membrane, leading to low CSF pressure. Can cause Chiari-like symptoms and, paradoxically, may contribute to both tonsillar herniation and syrinx formation.
D
Degranulation
The process by which mast cells release preformed mediators — including histamine, tryptase, and heparin — from internal granules. The central mechanism of MCAS symptom flares.
Dermographism (Dermatographia)
A condition in which light pressure or stroking of the skin produces a raised, red weal within minutes. A common skin manifestation of mast cell hyperreactivity and MCAS.
Dura Mater
The outermost and toughest of the three meningeal layers surrounding the brain and spinal cord. In Chiari surgery, the dura is often opened and patched (duraplasty) to expand the posterior fossa space.
Dural Ectasia
Widening or ballooning of the dural sac (the protective covering around the spinal cord) due to weak connective tissue. A diagnostic criterion for vascular EDS and Marfan syndrome; sometimes seen in hypermobile EDS.
Duraplasty
A surgical technique in which the dura mater is opened and a patch (graft) is sewn in place to enlarge the posterior fossa space. Used in Chiari decompression surgery to improve CSF flow.
Dysautonomia
A broad term for disorders of the autonomic nervous system. Encompasses conditions including POTS, neurocardiogenic syncope, pure autonomic failure, and multiple system atrophy, as well as secondary autonomic dysfunction caused by EDS, MCAS, or syringomyelia.
Dysesthesia
Abnormal, often unpleasant sensations such as burning, tingling, or electric shocks. A hallmark neuropathic symptom of syringomyelia caused by damage to spinothalamic pathways within the spinal cord.
Dysmotility (Gastrointestinal)
Impaired movement of food and waste through the gastrointestinal tract. Caused by autonomic neuropathy affecting enteric nervous system regulation; common in POTS, dysautonomia, and EDS.
Dysreflexia (Autonomic)
An acute syndrome in patients with high-level spinal cord injury or syrinx, characterized by a sudden, dangerous rise in blood pressure in response to a stimulus below the level of injury.
E
Ehlers-Danlos Syndrome (EDS)
A group of inherited connective tissue disorders caused by abnormalities in collagen synthesis or processing. Characterized by joint hypermobility, skin extensibility, and tissue fragility. Thirteen subtypes are recognized, with hypermobile EDS (hEDS) being the most common.
Ehlers-Danlos Syndrome — Classical Type (cEDS)
An EDS subtype caused by mutations in COL5A1 or COL5A2 genes, characterized by significant skin hyperextensibility, abnormal wound healing, and joint hypermobility.
Ehlers-Danlos Syndrome — Vascular Type (vEDS)
The most life-threatening EDS subtype, caused by COL3A1 mutations affecting type III collagen. Carries risk of spontaneous arterial rupture, bowel perforation, and uterine rupture.
Electromyography (EMG)
A diagnostic test that measures the electrical activity of muscles. Used in syringomyelia and neuropathic conditions to assess muscle denervation and map the level of spinal cord involvement.
Ependymal Cells
Cells lining the central canal of the spinal cord and the ventricles of the brain. The central canal is the likely initial conduit for fluid in syrinx formation in some theories of syringomyelia pathogenesis.
Epinephrine (Adrenaline)
A catecholamine hormone used as the first-line treatment for anaphylaxis; also released endogenously during stress, potentially triggering mast cell degranulation in MCAS patients.
Erythromelalgia
A condition characterized by burning pain, redness, and warmth in the extremities, typically triggered by heat or standing. Seen in MCAS and small fiber neuropathy overlapping with dysautonomia.
F
Fibromyalgia
A chronic condition characterized by widespread musculoskeletal pain, fatigue, and cognitive difficulties. Frequently co-occurs with EDS, POTS, and MCAS; may share pathophysiology involving central sensitization.
Fludrocortisone (Florinef)
A mineralocorticoid medication that promotes salt and water retention by the kidneys. A first-line treatment for POTS and orthostatic hypotension aimed at expanding blood volume.
Foramen Magnum
The large opening at the base of the skull through which the brainstem passes into the spinal canal. In Chiari malformation, the cerebellar tonsils herniate downward through this opening, obstructing CSF flow.
Functional Neurological Disorder (FND)
A condition in which neurological symptoms such as weakness, tremor, or sensory changes occur without structural damage. EDS, POTS, and MCAS patients are frequently misdiagnosed with FND before the underlying condition is identified.
G
Gait Ataxia
Impaired coordination of walking due to cerebellar or spinal cord dysfunction. A common symptom of Chiari malformation and advanced syringomyelia affecting the dorsal columns or cerebellar pathways.
Gastrointestinal Motility Disorder
Impaired movement of the gut, encompassing conditions from gastroparesis to slow transit constipation. Driven by autonomic neuropathy in dysautonomia and structural laxity in EDS.
Gastroparesis
Delayed gastric emptying in the absence of mechanical obstruction, caused by dysfunction of the vagus nerve. Common in POTS and dysautonomia; results in nausea, bloating, early satiety, and unpredictable blood sugar levels.
H
H1 Antihistamine
A class of medication that blocks histamine H1 receptors, reducing allergic symptoms such as itching, hives, and flushing. First-line treatment for MCAS; both sedating (first-generation) and non-sedating (second-generation) options are used.
H2 Antihistamine
A medication that blocks histamine H2 receptors in the stomach lining, reducing acid production. Used in MCAS to address gastrointestinal symptoms including acid reflux, nausea, and abdominal cramping driven by mast cell mediators.
Headache (Chiari)
Occipital (base of skull) or suboccipital headache worsened by Valsalva maneuvers such as coughing, straining, or bending forward. A cardinal symptom of Chiari malformation caused by pressure changes at the foramen magnum.
Heart Rate Variability (HRV)
A measure of the variation in time between consecutive heartbeats, reflecting autonomic nervous system tone. Reduced HRV is a marker of autonomic dysfunction in POTS and dysautonomia.
Heparin
An anticoagulant stored in mast cell granules and released during degranulation. Systemic heparin release in MCAS can contribute to easy bruising and bleeding symptoms.
Histamine
A chemical mediator stored in mast cell granules and released during degranulation. Responsible for itching, flushing, hives, nasal congestion, acid secretion, and vasodilation. Central to MCAS symptom production.
Histamine Intolerance
A condition in which the body cannot adequately break down histamine from food or endogenous sources, leading to MCAS-like symptoms. Caused by deficiency of diamine oxidase (DAO) enzyme; distinct from but often overlapping with true MCAS.
Hydromyelia
Dilation of the central canal of the spinal cord with cerebrospinal fluid. Often used interchangeably with syringomyelia, though technically distinct; hydromyelia is a true dilation of the ependymal-lined central canal.
Hyperadrenergic POTS
A subtype of POTS characterized by a marked increase in norepinephrine upon standing (>600 pg/mL), causing severe hypertension, palpitations, anxiety, and tremor on position change.
Hyperextensibility (Joint)
The ability to move a joint beyond its normal range of motion. A key diagnostic feature of EDS; assessed using the Beighton score and upper limb and lower limb hypermobility assessments.
Hypermobile EDS (hEDS)
The most prevalent EDS subtype, characterized by widespread joint hypermobility, joint pain, soft tissue injuries, and autonomic dysfunction. No confirmed genetic mutation has been identified; diagnosed clinically using 2017 criteria.
Hypersomnia
Excessive daytime sleepiness. Occurs in dysautonomia and Chiari malformation due to disrupted autonomic regulation of the sleep-wake cycle and central sleep apnea.
Hypotension (Orthostatic)
A drop in systolic blood pressure of ≥20 mmHg (or diastolic ≥10 mmHg) within three minutes of standing. Distinct from POTS, which features tachycardia without necessarily meeting blood pressure criteria.
Hypovolemia
Abnormally low circulating blood volume. A contributing factor in POTS; worsened by inadequate salt and fluid intake, heat, deconditioning, and renin-angiotensin-aldosterone dysfunction.
I
Idiopathic
Of unknown cause. Many cases of syringomyelia, MCAS, and dysautonomia are initially labeled idiopathic before a structural or systemic cause is identified.
Immunoglobulin E (IgE)
An antibody produced by the immune system in response to allergens. In classic allergic disease, IgE triggers mast cell degranulation. In MCAS, many reactions are non-IgE-mediated, distinguishing it from standard allergy.
Inappropriate Sinus Tachycardia (IST)
A heart rhythm disorder characterized by a persistently elevated resting heart rate and exaggerated heart rate response to minimal exertion, without a discernible structural cause. Overlaps significantly with POTS.
Interstitial Cystitis (IC)
Chronic bladder pain and urinary urgency without infection. Mast cell infiltration of the bladder wall is a recognized feature; co-occurrence with MCAS is well documented.
Intracranial Hypertension (IH)
Elevated pressure within the skull. Can co-occur with Chiari malformation and syringomyelia; may mimic or worsen Chiari symptoms. Treated with acetazolamide or CSF diversion procedures.
Intracranial Hypotension
Abnormally low CSF pressure, typically due to a spinal CSF leak. May cause paradoxical cerebellar tonsillar descent, mimicking Chiari malformation on MRI.
Ivabradine (Corlanor)
A heart rate-lowering medication that specifically inhibits the sinus node's If current. Used off-label in POTS when beta-blockers are not tolerated or are contraindicated.
J
Joint Hypermobility Spectrum Disorder (HSD)
A broader classification of symptomatic joint hypermobility that does not fully meet the 2017 hEDS criteria. Causes pain, instability, and fatigue comparable to hEDS.
Joint Proprioception
The sense of joint position and movement provided by proprioceptive nerve fibers in the joint capsule and surrounding tissue. Impaired in EDS due to connective tissue laxity, increasing injury risk.
K
KIT (c-KIT / CD117)
A cell surface receptor expressed on mast cells. Mutations in the KIT gene — particularly D816V — cause constitutive mast cell activation and are the defining feature of systemic mastocytosis.
L
Laminectomy
Surgical removal of one or more vertebral laminae to decompress the spinal canal. Used in syringomyelia caused by spinal stenosis, tethered cord, or scar tissue compressing CSF pathways.
Leaky Gut Syndrome
Increased intestinal permeability allowing bacterial products and food antigens to enter the bloodstream. Theorized to contribute to mast cell activation and systemic inflammation in MCAS.
Low-Histamine Diet
A dietary approach reducing consumption of foods high in histamine or that trigger histamine release (fermented foods, alcohol, aged cheeses, processed meats). A common adjunct treatment strategy in MCAS.
Lumbar Puncture (Spinal Tap)
A procedure in which a needle is inserted into the lumbar spinal canal to obtain CSF samples or measure CSF pressure. May be therapeutic in intracranial hypertension co-occurring with Chiari.
M
Magnetic Resonance Imaging (MRI)
The primary imaging modality for diagnosing syringomyelia, Chiari malformation, and structural spinal cord pathology. T2-weighted sequences reveal the fluid-filled syrinx as a bright central cord signal. CSF flow studies assess obstruction dynamics.
Mast Cell
A type of immune cell found throughout connective tissue and mucosal surfaces. Mast cells store and release potent chemical mediators (histamine, tryptase, prostaglandins, cytokines) in response to physical, chemical, immunological, and emotional triggers.
Mast Cell Activation Syndrome (MCAS)
A condition in which mast cells inappropriately activate and release mediators, causing recurrent multi-system symptoms affecting the skin, gastrointestinal tract, cardiovascular system, respiratory tract, and nervous system. Diagnosed using consensus criteria requiring symptoms, elevated markers during reactions, and response to mast cell-directed therapy.
Mast Cell Stabilizer
A medication that prevents mast cells from degranulating. Cromolyn sodium (oral) and ketotifen are commonly used; provide a preventive rather than reactive approach to MCAS symptom management.
Mast Cell Tryptase
A protease enzyme released by mast cells during degranulation. Serum tryptase measured within 1–4 hours of a reaction that is ≥20% above baseline plus 2 ng/mL meets a key diagnostic criterion for MCAS.
Meninges
The three protective membranes (dura mater, arachnoid, pia mater) surrounding the brain and spinal cord. Meningeal pathology — including arachnoiditis and dural scarring — can obstruct CSF flow and cause secondary syringomyelia.
Methylcellulose / Viscous Lidocaine
Topical agents used to manage oral mucositis or esophageal symptoms in MCAS patients experiencing mucosal inflammation as part of their mast cell activation spectrum.
Micturition Dysfunction
Impaired urination, including urinary urgency, frequency, retention, or incontinence. Associated with syringomyelia due to cord compression affecting sacral micturition centers.
Midodrine
An alpha-1 adrenergic agonist that causes vasoconstriction, raising blood pressure. Used in POTS and orthostatic hypotension to prevent blood pooling in the lower extremities upon standing.
Mononeuropathy
Damage to a single peripheral nerve. May co-occur with EDS due to joint instability causing nerve entrapment.
Motor Deficit
Loss of voluntary muscle strength or coordination. In syringomyelia, motor deficits typically appear in the hands and arms first as the syrinx disrupts anterior horn cells.
Multiple System Atrophy (MSA)
A rare, progressive neurodegenerative disorder causing autonomic failure alongside parkinsonism or cerebellar ataxia. Represents severe, non-POTS dysautonomia.
Myelopathy
Injury or dysfunction of the spinal cord itself, as opposed to the nerve roots. Syringomyelia produces myelopathy through direct pressure and ischemia of the central cord.
N
Neurogenic Orthostatic Hypotension (nOH)
Orthostatic hypotension caused by failure of the autonomic nervous system to trigger vasoconstriction upon standing. Distinct from POTS; common in pure autonomic failure and multiple system atrophy.
Neuropathic Itch (Neurogenic Pruritus)
Chronic itch arising from dysfunction in the central or peripheral somatosensory nervous system rather than skin pathology. Occurs in syringomyelia due to spinothalamic tract disruption; also reported in MCAS and small fiber neuropathy.
Neuropathic Pain
Pain arising from damage or dysfunction of the somatosensory nervous system. Described as burning, electric, shooting, or squeezing; characteristic of syringomyelia, EDS peripheral nerve involvement, and small fiber neuropathy in dysautonomia.
Norepinephrine (Noradrenaline)
A neurotransmitter and hormone released by sympathetic nerve terminals. Plays a central role in POTS pathophysiology, particularly in the hyperadrenergic subtype where standing causes disproportionate norepinephrine surges.
Nystagmus
Involuntary, rhythmic eye movement. Occurs in Chiari malformation due to cerebellar and brainstem dysfunction affecting ocular motor pathways.
O
Occipital Neuralgia
Pain arising from the greater and lesser occipital nerves at the base of the skull. Common in Chiari malformation and upper cervical instability in EDS, often confused with migraine.
Odontoid Process (Dens)
A tooth-like bony projection from the second cervical vertebra (C2/axis) that fits into the atlas (C1). Abnormalities of the odontoid — including basilar invagination — can compress the brainstem in Chiari and EDS patients.
Orthostatic Hypotension (OH)
A clinically significant drop in blood pressure within three minutes of standing. Different from POTS, which is defined by heart rate increase; OH involves blood pressure decrease, though both can co-occur.
Orthostatic Intolerance (OI)
A broad term for symptoms triggered or worsened by upright posture, including lightheadedness, palpitations, fatigue, and cognitive impairment. The umbrella category under which POTS, orthostatic hypotension, and related conditions fall.
Osteoarthritis (Secondary)
Joint degeneration caused by chronic instability and abnormal joint loading. EDS patients are at elevated risk due to repeated subluxations, dislocations, and inadequate joint stabilization over time.
P
Parasympathetic Nervous System
The division of the autonomic nervous system responsible for 'rest and digest' functions: slowing heart rate, stimulating digestion, and promoting recovery. Impaired in various dysautonomia subtypes.
Paresthesia
Abnormal sensations including tingling, numbness, 'pins and needles,' or crawling feelings. Common in syringomyelia (due to spinothalamic tract compression), EDS (peripheral nerve entrapment), and POTS (small fiber neuropathy).
Pelvic Floor Dysfunction
Impaired function of the pelvic floor muscles and nerves. Occurs in EDS due to connective tissue laxity and in syringomyelia due to sacral cord compression.
Posterior Fossa
The lower, posterior compartment of the skull housing the cerebellum and brainstem. In Chiari malformation, the posterior fossa is anatomically small, forcing the cerebellar tonsils downward through the foramen magnum.
Posterior Fossa Decompression (PFD)
The primary surgical treatment for Chiari malformation, in which a section of the occipital bone is removed to enlarge the posterior fossa and relieve pressure on the cerebellum and brainstem. May include duraplasty.
Postprandial Hypotension
A drop in blood pressure after eating, caused by blood pooling in the gastrointestinal vasculature. Common in POTS and dysautonomia; worsened by large meals and carbohydrate-heavy foods.
Postural Orthostatic Tachycardia Syndrome (POTS)
A form of dysautonomia defined by a heart rate increase of ≥30 bpm (≥40 bpm in adolescents) within 10 minutes of standing, in the absence of orthostatic hypotension, along with symptoms of orthostatic intolerance.
Presyncope
The sensation of nearly fainting — lightheadedness, tunnel vision, nausea, diaphoresis, and weakness — without loss of consciousness. Common in POTS and vasovagal dysautonomia.
Proprioception
The sense of body position and movement provided by sensory receptors in muscles, tendons, and joints. Impaired in both EDS (connective tissue laxity) and syringomyelia (dorsal column involvement).
Prostaglandins
Lipid mediators released by mast cells during degranulation. Contribute to pain, flushing, diarrhea, and bone pain seen in systemic mastocytosis and severe MCAS.
Pseudotumor Cerebri
Another term for idiopathic intracranial hypertension (IIH); elevated CSF pressure without a discernible cause. Can co-exist with Chiari malformation and may complicate surgical planning.
R
Raynaud's Phenomenon
Episodic vasospasm of small blood vessels in the fingers and toes, triggered by cold or stress, causing color changes (white, blue, red). Common in EDS and connective tissue disorders.
Renin-Angiotensin-Aldosterone System (RAAS)
A hormonal system regulating blood pressure and fluid balance. Dysregulation in POTS contributes to inappropriate fluid loss and orthostatic hypotension despite compensatory tachycardia.
Resting Tremor
Involuntary shaking that occurs when the affected muscle is at rest. Can occur in syringomyelia due to disruption of cerebellar pathways, and in dysautonomia due to excessive sympathetic tone.
S
Salt Loading (Dietary)
Increasing dietary sodium intake to expand blood volume. A first-line non-pharmacological treatment in POTS; patients are often advised to consume 3,000–10,000 mg of sodium daily depending on severity.
Scoliosis (Syrinx-Associated)
Abnormal lateral curvature of the spine. One of the most common presenting signs of syringomyelia in children; caused by asymmetric muscle weakness or spasticity from central cord pathology.
Secondary Syringomyelia
A syrinx that develops as a consequence of another condition — most commonly Chiari malformation, spinal cord injury, tumor, arachnoiditis, or tethered cord — rather than arising independently.
Sensory Dissociation
Loss of one type of sensation (e.g., pain and temperature) while another is preserved (e.g., vibration and proprioception). The hallmark sensory finding in syringomyelia, caused by the syrinx selectively damaging crossing spinothalamic fibers.
Serum Tryptase
A blood test for mast cell activation. Baseline tryptase and acute tryptase (drawn within 1–4 hours of a reaction) are used to diagnose MCAS and systemic mastocytosis.
Shunt (Syringoperitoneal / Syringopleural)
A surgical catheter system that drains fluid directly from the syrinx cavity into the peritoneal or pleural space. Used when the primary cause of syrinx cannot be treated directly.
Small Fiber Neuropathy (SFN)
Damage specifically to small, unmyelinated nerve fibers that transmit pain, temperature, and autonomic signals. Diagnosed by skin punch biopsy. A common cause of dysautonomia and burning neuropathic pain; frequently found in POTS and EDS.
Spasticity
Increased muscle tone and stiffness caused by upper motor neuron dysfunction. Develops in syringomyelia as the syrinx grows and disrupts descending motor pathways.
Spinal Cord Tethering
Abnormal attachment of the spinal cord to surrounding tissue, restricting its normal movement within the spinal canal. A recognized cause of secondary syringomyelia; surgical untethering can halt or reverse syrinx progression.
Spinothalamic Tract
A major sensory pathway carrying pain and temperature signals from the spinal cord to the brain. The tract's decussating fibers cross through the central cord — the area first affected by syringomyelia — explaining the characteristic cape-pattern sensory loss.
Subluxation
Partial dislocation of a joint in which surfaces remain in contact but are misaligned. In EDS, subluxations occur with minimal provocation due to ligamentous laxity and are a major source of daily pain and functional impairment.
Sympathetic Nervous System
The autonomic nervous system division responsible for 'fight or flight' responses: increasing heart rate, redirecting blood flow to muscles, and mobilizing energy. Overactivation contributes to hyperadrenergic POTS and stress-triggered MCAS flares.
Syncope (Fainting)
A transient loss of consciousness caused by temporary global reduction in cerebral blood flow. In POTS and vasovagal dysautonomia, syncope results from failure to maintain adequate cerebral perfusion upon standing.
Syringohydromyelia
A combination term used when both central canal dilation (hydromyelia) and an extracanalicular cavity (syringomyelia) are present. Often used interchangeably with syringomyelia in modern clinical practice.
Syrinx
A fluid-filled cavity within the spinal cord parenchyma. Develops due to abnormal CSF dynamics, trauma, tumor, or obstruction. May expand over time, causing progressive neurological deficits including pain, weakness, and sensory loss.
Systemic Mastocytosis
A clonal mast cell disorder in which mast cells abnormally accumulate in organs — most commonly bone marrow, skin, liver, and spleen. A subtype of the MCAS spectrum; carries risk of anaphylaxis, bone loss, and organomegaly.
T
Tachycardia
A heart rate exceeding 100 beats per minute. In POTS, tachycardia is specifically triggered by orthostatic stress (standing) rather than occurring at rest, and is the defining physiological criterion of the diagnosis.
Tethered Cord Syndrome
A condition in which the spinal cord is abnormally attached to the spinal canal walls, preventing its natural movement and causing traction injury. A recognized cause of syringomyelia and neurogenic bladder.
Thermoregulation
The body's ability to maintain a stable core temperature. Impaired in syringomyelia (due to spinothalamic tract damage), dysautonomia (due to sudomotor dysfunction), and MCAS (due to aberrant mast cell-mediated vasodilation).
Tilt Table Test (TTT)
A diagnostic procedure in which the patient is strapped to a table and tilted to an upright position while heart rate and blood pressure are continuously monitored. The gold-standard test for diagnosing POTS and vasovagal syncope.
Tinnitus
Ringing, buzzing, or other phantom sounds in the ears. Associated with Chiari malformation due to brainstem compression affecting auditory pathways; may worsen with Valsalva maneuvers.
Tonsillar Herniation
The downward displacement of the cerebellar tonsils through the foramen magnum into the cervical spinal canal. The defining anatomical feature of Chiari malformation Type I.
Tryptase
See Mast Cell Tryptase. A serine protease enzyme stored in mast cell granules; used as a biomarker of mast cell activation and burden.
U
Urticaria (Hives)
Raised, itchy welts on the skin caused by histamine release. A common presentation of MCAS; may be chronic, spontaneous, or triggered by specific exposures including heat, pressure, or foods.
V
Vagus Nerve
The tenth cranial nerve, responsible for parasympathetic innervation of the heart, lungs, and gastrointestinal tract. Dysfunction of the vagus nerve contributes to gastroparesis, dysautonomia, and heart rate dysregulation in POTS.
Valsalva Maneuver
A forced expiration against a closed airway (as in straining, coughing, or sneezing) that momentarily increases intracranial and intrathoracic pressure. A key trigger for Chiari headaches and can worsen syrinx symptoms.
Vascular EDS (vEDS)
See Ehlers-Danlos Syndrome — Vascular Type. The most serious EDS subtype; associated with spontaneous arterial rupture and requires specialized management.
Vasovagal Syncope (Neurocardiogenic Syncope)
The most common cause of fainting; triggered by prolonged standing, pain, or emotional stress, causing a reflex drop in heart rate and blood pressure. Frequently the first manifestation of dysautonomia to be formally evaluated.
W
White Matter
Myelinated nerve fiber tracts in the brain and spinal cord that transmit signals between regions. Compression by a syrinx or Chiari herniation can damage white matter tracts, causing sensorimotor deficits.
Z
Zone 2 Exercise (Aerobic Exercise Therapy)
Low-intensity aerobic training in which the heart rate remains in a zone allowing comfortable nasal breathing. A cornerstone of POTS rehabilitation; builds cardiac stroke volume, blood volume, and autonomic tone over time. Also beneficial in EDS for improving proprioception and reducing subluxations.
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Once the words make sense, our patient handbooks explain each condition in full, and the Hierarchy of Needs shows where to put your energy first.
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Download the full glossary as a 22-page PDF to keep with your medical records, share with family, or hand to a provider who is new to these conditions. Free to download, always.
Disclaimer
This glossary was developed by the I AM HEARD Coalition resource team as a starting point for patient education and is updated as new research emerges. It is provided for educational purposes only and is not a substitute for individualized medical advice, diagnosis, or treatment. Always consult your qualified healthcare provider regarding any medical condition, symptoms, or treatment decisions.
Corrections
Spotted an error or an outdated definition? Email Info@IAMHEARDCoalition.org. We would rather fix it than leave it.
Source
- I AM HEARD Coalition. Medical Glossary: Syringomyelia · Chiari · EDS · POTS · MCAS · Dysautonomia. 22 pages. Open PDF
- The on-page glossary above reproduces the 161 entries of the PDF edition. Each handbook on our Patient Handbooks page carries its own peer-reviewed source list.