Environmental factors are thought to play a significant role in the development of MS. Studies have shown an association between geographic latitude and risk, with the risk increasing from south to north [16,17]. The lowest risk in found among persons living near the equator. As such, the prevalence of MS is higher in geographic locales having less sunlight exposure (and hence diminished production of vitamin D), suggesting that low levels of vitamin D may be a risk factor [17,18,19,20]. In addition, persons who smoke have an increased relative risk compared to those who do not [17,19,21].

Conceptually, MS is now considered to be an autoimmune inflammatory disorder with complex and variable pathologic features [1,29]. Susceptible individuals are those of genetic predisposition in combination with environmental factors and possibly latent infection. The etiology is unclear, but initiation of disease appears to involve the activation of peripheral T-lymphocytes, programmed to recognize components of the CNS axonal myelin sheath. The disease is triggered by events that permit these autoactivated T-cells to breach the blood-brain barrier and cross-react with myelin components within the white matter of the brain and spinal cord [30]. This precipitates a cascade of immune-mediated inflammatory tissue injury. As seen on radiographic imaging and pathologic examination, the hallmark of the disease is this well-defined, focal zone of injury ("plaque") containing elements of inflammation, demyelination, and axon degeneration [1,6]. Such lesions may be single or multiple, and over time, they may be partially reparative, relapsing, or recurrent in new locations. The location of lesions is variable; early in the disease they appear in white matter, often clustering near the ventricles and sparing peripheral nerves [14].

Demyelination impairs nerve impulse transmission and leads to abnormal patterns of nerve conduction, which accounts in large part for the various clinical symptoms and signs of MS. Oligodendrocytes are cells that elaborate the myelin sheath that envelops the axon. During the early, remittent stage of the disease, as inflammation subsides, the number and function of these cells are sufficient to renew the myelin sheath (remyelination) and restore neurologic function. Over time, the repeated inflammatory insults associated with relapsing MS lead to a gradual depletion of functioning oligodendrocytes, and to degenerative changes marked by central scarring within the lesion and focal areas of cerebral atrophy. The clinical correlate is the gradual accumulation of fixed neurologic deficits as the patient with MS transitions to the chronic progressive stage of the disease.

The majority of patients with MS report some level of spasticity. Painful muscle spasm is experienced by approximately 15% and is often a source of debilitation [39]. Spasticity usually affects the muscles of the extremities (more prominent in the lower extremities than the upper extremities) and can impair an individual's ability to freely move his or her muscles.

MS pain is mainly neuropathic—the result of nerve damage and faulty conduction—and can include stabbing, burning, and shock-like sensations (e.g., allodynia, dysesthesias, paresthesia). Lhermitte sign is often considered a classic sign of MS and consists of a brief, electric shock-like sensation that runs down the spine and is triggered by bending the neck forward or backward.

Impaired vision is frequently present in patients with MS, most commonly unilateral optic neuritis, which is present in approximately in 66% of cases [43]. Optic neuritis usually manifests as acute or subacute unilateral eye pain that increases with eye movements [44]. It can also lead to blurring or graying of vision or blindness in one eye. However, while unilateral optic neuritis is common in MS, simultaneous bilateral optic neuritis (resulting in total blindness) is rare [43]. Approximately 90% of patients with MS regain normal vision over a period of two to six months after an acute episode of optic neuritis [43].

Relapsing-remitting multiple sclerosis (RRMS) is characterized by alternating series of clearly defined clinical relapses (or exacerbations) followed by periods of partial or complete recovery (remissions). RRMS affects young adults, is three times more common in women than men, and accounts for about 85% of all cases of MS [51]. Functional and structural impairments suffered during relapses may either resolve or leave sequelae.

Primary progressive MS (PPMS) is characterized by steady disease progression from the onset of symptoms, perhaps with occasional remissions and temporary minor improvements [47]. Approximately 10% to 15% of patients with MS carry the diagnosis of PPMS [49]. Patients diagnosed with PPMS tend to be older (mean age: 40 years) than those with RRMS, and there is no gender difference in incidence [49].

Malignant MS (also known as Marburg variant) is characterized by a rapidly progressive course resulting in major disability and usually death within one year of the onset. This disease course is most common in children, although older adults may be affected as well.

Early-onset MS is usually diagnosed in patients younger than 20 years of age. It accounts for approximately 0.4% to 10.5% of all MS cases [56]. Usually, the disease is characterized by a relapsing-remitting course, a high recovery rate from initial attack, and a long remission and slow progression rate. Sensory symptoms are more common than motor symptoms in these patients, and vision loss (optic neuritis) is a common initial presentation. Other functional systems are involved with a variable frequency. Seizures, malaise, irritability, and low-grade fever may also be present.

The diverse disease processes associated with the subtypes of MS can be detected by MRI as well. In PPMS, MRI will show small lesions that do not enhance with a contrast agent, indicating little or minimal inflammation. This particular characteristic is a clear differentiation from relapsing-remitting disease. The severity and extent of the physical symptoms of MS can be confirmed by visualization of the anatomic location of lesions within the CNS. For example, a lesion present in the spinal cord may result in numbness in the limbs and bladder disturbance. Lesions in the optic nerve are usually responsible for optic neuritis, leading to blurred vision and a loss of color perception.

MS lesions found in the spinal cord usually give rise to identifiable symptoms and are highly eloquent of the disease process; new spinal MS lesions are strongly correlated to new clinical manifestations. Approximately 75% of patients with MS have lesions within the spinal cord, principally the cervical cord, and most spinal cord lesions are located in the dorsal columns [68]. These lesions are usually oval or cigar-shaped and can span one or two vertebral segments (referred to as skip lesions).

Detection of oligoclonal bands in CSF by isoelectric focusing is the most sensitive laboratory test for MS and the most sensitive predictor of conversion from clinically isolated syndrome to MS. It is also the best test to show local intrathecal IgG synthesis. Patients with suspected MS who lack oligoclonal IgG bands in CSF should be investigated for other diagnoses, although it is important to remember that not all patients with MS display oligoclonal bands. Studies have demonstrated that the frequency of oligoclonal bands in the CSF of patients with MS varies in different regions of the world, with higher rates in Northern Europe and North America and lower rates in Asia [93].

Only results of visual evoked potentials are considered part of the diagnostic criteria for MS. Visual evoked potentials can detect sluggish neurotransmission along the optic nerve pathways, a finding common in individuals with asymptomatic MS. However, a positive finding on evoked potential testing is not specific to MS, and the abnormalities detected may also be present in other conditions.

Atypical clinical features that suggest an alternate diagnosis include [96]:

Treatment of the acute exacerbations seen with relapsing types of MS relies primarily on corticosteroids and adrenocorticotropic hormone (ACTH). These agents have been found to promote speedier resolution of the neurologic deficits, lessen the severity of an attack, and effectively reduce the risk of permanent residual deficits. Both corticosteroids and ACTH are capable of restoring the breakdown of the blood-brain barrier, reducing inflammation, and immunomodulating mononuclear trafficking mechanisms. Corticosteroids also promote quick recovery from disability [98,99].

The first-line treatment of MS-related exacerbations involves administration of high doses of IV corticosteroids, usually methylprednisolone (1 g daily), for five to seven days [100,101]. Alternative approaches for patients who do not tolerate large intravenous dosage or have poor venous access include:

It is now known that B-cell immunity also plays a key role in the pathogenesis of MS. Plasma exchange may be beneficial for relapsing forms of MS in which severe neurologic exacerbations prove refractory to parenteral corticosteroid therapy. It may also be beneficial for some patients with severe, rapidly progressive MS and similar disorders; however, it does not show any efficacy for SPMS or PPMS.

Glatiramer acetate reduces the risk and severity of MS attacks and reduces MRI lesions over time. Studies comparing treatment with ß-interferon 1b or glatiramer have demonstrated similar efficacy. Glatiramer acetate has fewer adverse effects compared to the ß-interferons. Good injection technique and site rotation can help to reduce post-injection site reactions, but in some cases, repetitive injection of glatiramer acetate can cause lipoatrophy [118].

Mitoxantrone, a cytostatic drug and a powerful anti-inflammatory, is used in the treatment of both RRMS and progressive forms of MS [119,120]. It is considered one of the most effective drugs in resolving relapses; however, due to the risks for leukemia and cardiotoxicity, it should only be prescribed to patients with rapidly advancing disease who are refractory to other therapies [121]. Some patients, especially with a subtype of RRMS called rapidly worsening MS, do not respond to immunomodulators and are managed with immunosuppressants, particularly mitoxantrone [122,123].

Natalizumab, a monoclonal antibody, may be used in the treatment of RRMS, and it is considered one of the most effective drugs in reducing the relapse rate (although long-term studies are lacking) [125,126,127]. Natalizumab prevents migration of autoreactive lymphocytes into the brain, which results in a profound decrease in CNS mononuclear cell trafficking that reduces MS exacerbations by 70% and disease progression by about 50% [128]. It also accelerates repair of myelin sheath lesions. Some studies have demonstrated that natalizumab can reduce new gadolinium-enhancing lesions by more than 90% [128,129].

Treatment with fingolimod, a sphingosine-1-phosphate (S1P) receptor modulator, results in reduction of the relapse rate in patients with RRMS; however, it is associated with an increased risk of opportunistic infections, which can be fatal [135,136,137,138]. Fingolimod was the first oral agent with a labeled indication for relapsing forms of MS [136]. It promotes the redistribution of lymphocytes from the circulation to the lymphoid organs and prevents the entry of lymphocytes back into circulation. Several studies have demonstrated that it significantly reduces both clinical and radiographic MS disease activity. Its side effects include first-dose bradycardia, arrhythmia, reactive airway events, macular edema, skin cancers, and increased susceptibility to infections [118]. Fingolimod is the only drug approved for the treatment of highly active (or rapidly worsening) RRMS.

Baclofen and tizanidine are commonly used to treat and reduce spasticity, and the benzodiazepines (e.g., diazepam) also have a beneficial effect. Other possible agents include gabapentin and dantrolene [102]. In general, baclofen is considered the drug of choice for spasticity in patients with MS [152]. An intrathecal baclofen pump may be indicated for patients with unilateral or bilateral phasic lower limb spasticity. Dantrolene should be used with caution because of the potential for hepatotoxicity [102].

Patients who experience nocturia or nocturnal enuresis should be advised to empty their bladder before going to bed and decrease or avoid fluid intake two to three hours prior bedtime. Caffeinated products, alcoholic beverages, and spicy and acidic foods can cause bladder irritation and urinary frequency and should be avoided. If these behavioral strategies are ineffective, treatment with oral desmopressin is indicated [157].

Type of sexual dysfunction varies. Reduced libido is the most frequent manifestation of primary sexual dysfunction for women with MS. Among ambulatory men with MS, approximately 60% experience erectile dysfunction, 50% report orgasmic dysfunction, and 40% experience reduced libido [160].

If safe, women intending to conceive should stop their MS treatment for at least three months prior to conception. A study conducted in Sweden concluded that pregnancies that were not exposed to the ß-interferon in utero for at least a two-week period prior to conception resulted in healthier infants than pregnancies with such exposure [263]. A small Canadian study found that pregnancies exposed to ß-interferon resulted in a higher number of miscarriages, low birth weight, and prematurity [264]. However, a larger study did not find a significantly higher rate of complications in pregnancies accidentally exposed to immunomodulators [265]. In general, even the higher incidence of complications observed in some studies was only slightly greater than that of the general population. If continued treatment is necessary, modifications to the prescribed regimen (with preference for lower risk options) may be necessary. Many drugs used to treat MS and its related symptoms are contraindicated during pregnancy.