Mononeuritis Multiplex

Firstly, this is not a single disease, rather a syndrome that can be caused by many different diseases.  It is characterised usually by pain that begins in the distribution of at least 2 seperate nerve regions.  It starts off affecting only a few nerves, usually asymmetrically, but will often progress to a more symmetrical picture as more nerves are involved.

Although pain is the common presenting complaint, any sensory or motor symptoms can present e.g. numbness, tingling, weakness, paralysis etc.  While weakness and loss of bladder control are rarer, these symptoms can occur.

Common causes include:

  • Diabetes
  • Vasculitides e.g. wegeners, Churg-strauss, GCA
  • Rheumatological disorders e.g. SLE, RA, Sjogrens, Sarcoid
  • Amyloidosis
  • Systemic infections (leprosy)
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Mononeuropathy

A mononeuropathy (as its name suggests) is a neuropathy of one single nerve.  Symptoms will usually be confined to the region of the body that the nerve innervates and will only affect areas below the level of the lesion (if there is a lesion)

Common causes of a mononeuropathy include:

  • Nerve compression syndromes e.g. carpal tunnel; cubital tunnel; herniated spinal disc (and sciatica) etc
  • Infection- usually herpes zoster but can be caused by other local infection e.g. bone/soft tissue
  • Vascular causes e.g. peripheral arterial disease (ischaemia of the nervous tissue supplied by the artery involved)

Guillain Barre Syndrome

This is caused by a process known as molecular mimicry.  This is when antibodies produced to defend against a real pathogen cross react with proteins in the body, initiating an autoimmune attack.  In Guillain Barre, this commonly occurs after a Campylobacter jejuni infection (GI) where antibodies cross react with myelin proteins.

There are many subtypes of GBS, the most common of which is known as Acute Inflammatory Demyelinating Polyneuropathy (AIDP).  The common features of GBS include:

  • Weakness- usually symmetrical and ascending in nature (lower limb extremities first- usually spreads up involving upper limbs too- can reach proximal muscles and respiratory muscles)
  • CN dysfunction- (commonly CNIII-VII, IX-XII) Facial droop; diploplia; dysarthria; dysphagia etc
  • Change in sensation- usually paraesthesia and/or numbness although pain can occur.  May precede weakness and usually starts at the extremities progressing upwards
  • ANS dysfunction- (both sympathetic and parasympathetic) Tachy-/brady-cardia; Facial flushing; paroxysmal hypertension; postural hypotension; diaphoresis/anhidrosis (excess/little sweating)

This is usually a clinical diagnosis but electrophysiology (to show demyelination) and CSF (high protein, low/normal WBC)/imaging (inflammation of the sacrolumbar plexus) (to exclude other causes) may be useful.

It is usually self limiting but can be disabling and can last up to a year.  Symptomatic treatment may therefore be used.

see also neurology problem: peripheral neuropathy

Neuro-embryology

I barely understand embryology as it is, but here goes…

During and not long after gastrulation, (Day 16-19), epiblasts that migrate via the primitive node to form a chordal process called the notochord.  This runs centrally and longitudinally, in the middle of the mesoderm, towards the cranial end of the embryo.  It is crucial for the induction of neural development of the ectoderm above it.  This region is called the prechordal plate (the notochord actually goes on to become the nucleus pulposus of the intervertebral discs- i.e. it defines the location of the vertebra and CNS).

Between days 18-20, the prechordal plate differentiates (through induction by the notochord) into the neural plate, which enlarges (this will become the cells of the CNS).  At day 20, rapid cell proliferation at the margin of the neural plate causes development of the neural groove, running down the centre of the plate.  As this continues, the neural folds develop and the neural groove deepends and eventually seals itself off to form the neural tube.

By day 24/25 the neural tube has closed cranially, and by 26-28 caudally.  The cavity formed by this will eventually go on to form the ventricular system.  Cells at the margins of the neural folds will becomes neural crest cells (which will become the PNS).

See also Mesodermal and Somite development

The Porphyrias

The porphyrias are a group of (mostly) hereditary conditions that cause a disruption in the body’s production of the heme molecule (used for haemoglobin).  This is usually due to an enzyme deficiency of some sort.  The result is a build up of nephrotoxic precursors.  (One precursor type is porphyrin which isn’t nephrotoxic but causes photosensitivity of the skin).

They can be acute or chronic.  Acute attacks tend to present with abdominal pain, D&V, mental disorders, photosensitivity and skin problems, and peripheral neuropathy (usually distal weakness and pain).  Chronic forms tend to also cause systemic problems e.g. liver dysfunction.

The porphyrin pathway and what can go wrong where.

There are lots of different types of porphyrias (see image), but the management for them is largely the same: a high carbohydrate diet and heme supplementation, as well as avoiding the use of drugs that are broken down by the same pathway (mainly barbiturates, anticonvulsants, progestins, and rifampin).